Topic: The Thoughts of Hondaman.

[HondaMan]

I'm sure we've all seen the posts from Hondaman that have appeared here over the last few months.

Rather than let them fade gently into the background, I thought that I'd collect them here.

If you want to resurect a topic, search on the title (I haven't changed them) and reply to the original thread.

And for those who have wondered...    I asked him for a bio:

"I Started with Hondas when I was 12 years old.

My first was a CB305 1968 SuperHawk, one of only 3800 built. They were replaced with the CB350. I toured with it, eventually built it into a 337cc with high compression pistons and started modifying things until I ended up roadracing it, circa 1970.

Bought a CB750K1 in 1970, while working for 2 Honda shops in Peoria, IL and Elmhurst, IL in 1969-1971. I Was a biker then, the Pekin Reapers.

I picked up the moniker "HondaMan" from being the only Honda rider in the group, amongst British bikes and Harleys.

Moved to western IL in 1971 and joined a team in starting up Western Wheels, a Suzuki/Honda/BMW/Harley shop. I was Service Manager and Head Tuner, working with racers from IA, IL, MO and IN tracks. I Worked a lot with Yoshimura and its representatives, specialized in Honda Fours, while owning a CB750K2, CB500, CB350, the 'Hawk and a CB450. I Raced all of them except the 350. Drooled daily over Mike Hailwood's Honda 250cc 6 inline that he raced in the late 1960s and had a picture on my shop wall for years.

Got rides for all my brothers, and warmed them all up so they wouldn't get embarrased in wheel-to-wheel 'conflicts'.

Moved to Colorado and set up my own part-time tuning business, worked a little with local shops on weekends for commissions (made more than my day job!). Started tuning for altitude and teaching others how to ride safely and fast. Quit the racing in the late 1970s, but still formed little clubs and rides here and there.

Got married for real in 1979, been ever since. Just now getting around to rebuilding my very tired "big four", well over 100k miles. Been with it so long, I can't think of anything else to ride!"

[HondaMan]

Here's a chronology for the interested, and why the changes over the years. It explains how you can tailor your 750 to match your riding style, too!

K0-K1: These were the "performance years". The carbs were jetted 120 mains, the spark advancers were quick-advance units (all in by 2000 RPM) and the pipes were straight-through with fiberglass-wrapped tube inserts. Gearing was 16T/45T on K0, 17T/48T on K1. Gas mileage was about 32 MPG on premium (95 octane). These bikes developed about 67 HP, peaking at 8500 RPM, putting about 61 HP "on the ground". The K0 had 4 cables on the carbs, while the K1 had the bellcrank system most folks recognize.

Late K1/Early K2: The cam changed, getting advanced about 3 degrees instead of  5, but the duration was the same (so the overlap moved, too!). On the K2, the straight-thru pipes were changed to a 5-chamber baffled design. The spark advancer was delayed to full advance at 2500 RPM. The main jets on the K1 were 120 (old factory bikes) or 115 (bikes after 11/70 build), on the K2 110. The K1 jet needles were dropped 1 notch from the K2 settings (middle notch, K2 was 2nd from the bottom). The sprockets changed to 18T/48T. These bikes put about 58 HP on the rear wheel at 8000 RPM in 3rd gear.

Late K2: The cam moved to 5 degrees early (compared to K0) and overlap was reduced about 4 degrees, while lift stayed the same. This was, I believe, an attempt to reduce the plug-fouling habits of the early K2 (1000 miles per set, typically). The spark advancer was given 2 degrees more "spread" with the same curve as the K2 early models. The static spark setting was reduced by that 2 degrees for better idle. The mainjets dropped to 100 or 105 (depending on the Honda test rider's evaluation). The pipes were like the early K2. Sprockets stayed at 18T/48T. The airbox inlets were narrowed a little for quieter (but more restrictive) operation. This model put about 45 HP on the ground at 8000 RPM in 4th gear. They felt, and ran, like they were over-geared, and lowering the ratios helped both MPG and rideability.

K3: Most K3 pipes had 7 chambers. A few early ones still had the 5-chamber models, probably leftover from K2 production. Sales had fallen off dramatically at this point. Some of the carbs received the "lifter collar" around the needle jet to make better atomization of the fuel and better MPG, about 40 MPG. (About this time, the U.S. entered the 55 MPH era, and gas shortages were causing Congress to cry out for better MPG on anything that moved.) The 5-chamber K3 had 105 mainjets, the 7-chamber ones had 100 or 95 (I saw both), and at the end of K3 they were 5 chambers again. The spark advancer was unchanged from late K2. These bikes had noticeably better midrange torque, from the "lifter collars", but less HP, about 59 at the crankshaft, and about 41 at the rear wheel at 8000 RPM in 3rd gear. These would also run on regular gas (octane 89). The K3 was the first 750 that would idle well, at about 1050 RPM.

K4: Pipes were like late K3. Carbs were similar, but the mainjet holders could no longer be removed after the first 4 months of production (to prevent "tampering" with "emission controls". Huh.) Then they went back to removable mainjets, like the earlier carbs. After that, the "087a" series carbs replaced them with normal removable jets again. The earliest K4 had K3 carbs, the later ones were like the K5 and later, a slightly different design (with only one fuel line from the petcock on the tank). Midrange torque, like at 55 MPH in high gear, was improved by having about .010" less exhaust cam lift and slower spark advance (full advance at 2800-3000 RPM), but 2 degrees less total advance. (We modified a lot of these advancers for more advance.) These bikes got real good MPG, even up to 50 MPG on premium, and would run on regular (87 octane). Power peak was at 7000 RPM (many were lower, like 6500 RPM), but only about 38 HP got to the ground unless you changed something. Idle was improved and very smooth, at about 1050.

K5: The only thing I really had time to "dig in to" on these was the mileage. They got real good MPG, frequently 45-50, running on regular 85 octane gas. They peaked at about 6500 RPM with noticeably less power than my K2. But, they were REAL quiet! And, they would idle endlessly at 1100 RPM.

I don't have specific notes after the K4, because I was by then only a part-time mechanic (weekends) and was getting away from the intense detail of it all. Sorry, k5-ers & later.

[HondaMan]

Okay, you've paid off Christmas and it's been too long since it was warm enough to ride, and you're planning for next season. Wanna ride faster or get better MPG this year? Here's some [higher-dollar] tips for those of you who want to "get into it"... 

1. All inline fours have this in common: crummy metal holding the cylinders together. As a result, after about 5,000 miles of running, the bores are no longer parallel, nor are they round. Power is lost, almost 10% by our 1970 rear-wheel dyno tests. To fix this, have the cylinders bored one oversize from where you are now, and tell the machine shop to make sure they are in parallel. They "wander" as much as .010", in my experience. Simply boring one oversize and refitting recovers all that power again, and it STAYS for a very, very long time after that.

2. All Honda gearboxes have this in common: their teeth are hog-cut. These tiny steps on the faces of the gear teeth rob a considerable amount of power and smoothness from the crankshaft. Aside from a LOT of hand polishing work, you can do this: remove the gearshafts and spacers, bearings and shift forks. Apply lapping compound (like for valves) to the face of the teeth and set the shafts into V-blocks that are spaced like the crankshaft. Load the mainshaft shaft with a soft piece of pine wood (clamped onto the shaft) and turn the countershaft by means of the countersprocket, slipped onto the shaft in its normal place. Do this until you're bored to tears, and you'll soon see shiny spots on the teeth of the gears where they mesh. This will transfer as much as 10% more power through the gearset to the countersprocket. Now, clean them like your life depended on it, then dip everything into 20w50 oil and reassemble.

3. In the heads: all of the inline fours except the 400-4 had breathing issues. Grinding a "pocket" above the intake valve(s) and matching the ports to the carb tubes will improve them, especially at high RPM. Polishing the intake valve(s) helps, too. Don't back-cut the intake valves or you'll pop off a valve head - an expensive day, at best. (Backcutting exhaust valves will make them burn, and quickly - the CB650 malady). Trim (narrow) the intake valve guide bosses for a smaller profile, and polish it. Smooth, but don't polish, the intake tract. If you're good at heliarcing, raise a bead about .100" high along the bottom of the intake ports (CB750 only) in a half-moon shape, then smooth it off along the sides so it "restricts" (apparently) the bottom passageway about 1/3 of the way around. The floor of the port should ramp up to it, then be fairly abrupt in fall-off on the valve side. Do this at the point where the intake track bends down toward the valve (no, it's not easy). Although not obvious, this mod creates some "free turbocharging" at RPM above 6000, all the way to 16000 (lots of work), which deals more with fluid dynamics than I will put into this post.

4. Jetting. ALL of the inline fours came rich from the factory. At sea level, we started with a 10 size less than what it came with. Here in Colorado, it's not unusual to drop a 20 size main jet on a K0 when switching it to K&N air filter and HM341 pipes. The jet needles are almost always set too high (except the 500/550, too low). The cutaways on the slides were close for sea level, too low for high altitude by about .030". My 750K2 runs at 20 less than stock for general cross-country touring. Updates for ethanol-laced fuels: these "additives" cause the gas to burn slower and simulate a higher octane, while also burning cooler to reduce NOx emissions. If your carbs had, say, 110 mainjets for "real" gas and you have to run the ethanol stuff, drop down about 5 or 7.5 on the jet number and run a mid-grade octane as your starting point. If you run an ethanol-laced premium, drop a 10 number instead. Of note: ethanol-laced gasolines with nitrogen additives (like Shell and Phillips, circa 2012) will produce very white sparkplugs: this is NOT an indication of lean mixtures. So, don't use those fuels when trying to adjust your plug colors. Using them afterward WILL help keep the plugs cleaner, however!

[HondaMan]

At the risk of breaking tradition of 1 cam in this forum....I love the 450 DOHC. Honda's most magic bikes, IMO, were the 450 DOHC, the 500/550-4 and the 400-4. My personal choice of riding the "big four" these last 36 years has more to do the with durability of that bike than with its personality.

The 450-2 will deliver more wallop per pound than almost anything out there, a real "kick-butt" twin. It's nimble, has excellent brakes (if they do wear quickly) and throttle response that is the envy of anyone who has ever ridden, but not owned, one. I usually beat every British twin except the 1967-68 Bonnevilles and the Norton 750s with mine. I had it for only 3 years before my father-in-law talked me out of it. His neighbor still owns and rides it today, 35+ years later!

Rules:
#1. Keep the carbs CLEAN. The vacuum pistons have very small clearances and the slightest amount of stickiness (from old gas, usually) will make the pistons misbehave and the mixtures will be erratic or just plain bad. Cleaning is easy: just loosen the clamps and twist the carbs sideways to get the covers off and the pistons out. Clean the bores and the pistons with acetone. Put 1 drop of very lightweight oil on your finger, apply it to the bores and the pistons, and reassemble. You'll be surprised at the difference! Remove the bowls and pilot screws, then spray carb cleaner through the passages. Clean the floats with acetone, check the float levels, and reassemble.

#2. The intake valve rockers wore badly. This was from folks running too light oil (cheap 10w40 or 10w30), and running it too long. The recommended change interval is 750 miles or less, because the top end really beats up the oil. Use 20w50 if you don't mind losing about 1 HP. Use Castrol or Torco, in any case, because it survives these hot-running engines better than almost anything but synthetics. Don't use synthetics, or you might lose your clutch. When starting it hot, WAIT 45 SECONDS BEFORE DRIVING, because it takes that long for the oil to reach the intake rockers on hot startup (honest!). If your lifters are worn, get new ones. They can be replaced through the adjustment cap holes. Set the intakes at .003" and the exhausts at .004" (I know the manual says .002" and .003", but that's for quiet, not for longevity). Worn lifters both clatter and reduce the lift a lot, as much as .100". Worst case, they also damage the cams. Look inside with a flashlight for score marks. They can be resurfaced or replaced fairly easily.

#3. A common malady with these was the points advancer weights getting corroded and stuck or the pivots wearing and getting loose. This makes it surge when trying to run along at steady in-town speeds (the carbs can cause this, too). Cleaning or re-bushing fixes them up.

#4. If you have more than 20,000 miles on it, try to get a new oil pump. This will improve many things, subtle things, that make it a happier ride.

#5. Hop-ups: after you've replaced those worn lifters and cleaned up the cam, give it a nice valve job to make it seal well, bore it .25mm (1st oversize) to gain almost 2 HP, mill the head .010" and remove the resulting sharp edge around the bore, trim the valve guide bosses on the intakes (they are too big), reassemble and set the timing an extra 2 degrees advanced. Grind off an extra spline on each torsion bar (for the valves) and reinstall 1 notch "extra" tight. This will let you spin 10,500 RPM safely, and it will, very willingly. Use the gearing from the CL model (it's slightly lower) if you have a CB. Get good tires: you're gonna need them after this. Replace the worn swingarm bushings  and install tapered rollers in the steering head. Then, you'll wonder why you ever wanted to ride anything else.

The 450-2 riders' motto used to be: "After all, enough is enough!" It's hard to argue with that bike.

[HondaMan]

If you've ridden lots of other bikes, you've probably noticed a "flat spot" between about 3500-4800 RPM on the "mid-four". This is caused by several factors, all relating to certain things Honda wanted this bike to do:
1.) It was supposed to idle well and start very easily. This required an idle A/F mixture of about 13.25:1, a little rich.
2.) It was supposed to be quiet. Long intake runners (distance between the carb slide and intake valve) helped this.
3.) It was supposed to be easy to maintain and not leak fuel, so that carbs were to sit horizontally.
4.) It was supposed to be even and smooth through the whole throttle range.
5.) It was supposed to cruise easily at highway speeds 55-75MPH and get good MPG.

It did all these things, and very well. But, the tradeoffs of design caused by the long intake runners (#2) and the angled direction change into the heads (#3) made #4 and #5 harder to obtain. The richer idle had to lean out at running speeds to get good MPG. To smooth all of these things out a little, the spark advance curve was made quick, quicker than the other Fours of the day. The result: between 3500-4800 RPM (or so), the mixture was slipping from richer-than-normal to normal while the spark advancer had already reached full timing. It made the torque curve flatten out in that range, where most bikes are just getting stronger.

Here's some simple things you can try to smooth over this "smoothie" feature.
A.) Raise the jet needle in the carb slides one notch. Install a 10-size smaller main jet at the same time (5 size smaller on last-year CB550s). Switch from the standard D7E (NGK) or X22E (ND) sparkplugs to the D8E (X24E) at this same time. Check the color of your plugs to make sure it does not get too lean, which can happen if you've installed individual air filters and/or headers that actually work (most did not), or longer, megaphone-type mufflers.
B.) Advance the timing 2 degrees static, but cut off one turn from the springs on the advancer and reshape the end of the next coil so the springs will fit back onto the advancer mounts. This slows the advance curve about 5%-8%, depending on the year of your bike.
C.) Add 4 teeth to the rear sprocket. This raises the RPM a little, which shifts the lower-than-normal torque curve downward a little to a point where the torque-vs-acceleration is not so noticeable.
D.) Test out your octane ratings. Start with a tank of Regular, then try Mid-Grade, then premium. You'll see how they affect this "flat spot".

Here's some harder things you can do to smooth this anomaly while increasing the power a little:
E.) Smooth the insides of the intake runners and match the ports.
F.) Polish the intake valves.
G.) Shorten the intake valve guide bosses about 2-4mm. Narrow them in the flow direction, but don't get thinner than 50% of the original thickness.
H.) Change from the stock air filter (paper) to a K&N filter. Open up the airbox intake holes about 25%.
I.) Install a cam with 7 degrees more duration and advance it 3-4 degrees. Action Fours used to make one of these in the early 1970s.

[HondaMan]

First word: GREASE! The clutch lifters on these bikes wear more because there is much more shifting going on. Grease those lifters!

We tried all kinds of clutch mods. The stock clutch would wear because of the slanted cork block faces, which were designed to let the oiled plates squish out the oil slowly and ease the engagement (part of the "smooth 4 effort" of the 500). Heavy-handed throttles then caused plate heating and warpage, making the faces engage less, then they wore quickly.

Barnett jumped in with their superior friction plates, but they were thicker, so their sets had 1 less plate pair than the Honda set. Result: same grip, less life. Even worse: the Barnett cork bits wear the oil pumps, causing low oil pressure after a while.

Solution: today, the plates are available with square-cut cork faces. Find these and use them. And, replace the steel ones, too. They're warped if you have 10,000 miles on them, believe it. Also, DON'T run Valvoline or Havoline oil. These excellent oils overlube the plates and make them slip. Instead, use Castrol (best) or Torco (next best) oils. Castrol and Honda worked together in the 1970s to get the right blend: trust 'em.

Update 2020: Bel-Ray EXL Mineral oils have come to replace the old, missing, Castrol XLR. Good stuff!

[HondaMan]

Although I know a little less about these than the "big Four", I've worked a bunch with them, and both my brother and I owned a pair for a long time. (We used to roadrace each other on 'em for fun!)

The 500 was Honda's reply to folks who were "inseam challenged" and those who did not care about raw power, but more civility in their rides. As such, it was Honda's crowning achievment in smoothness, their best before the Gold Wing series.

The quiet and smoothness came at a price in performance, though, and in handling. The quiet mufflers, surprisingly so for their short length, were restrictive to get that silence. Opening up the exhaust a little has big paybacks, but will require rejetting. The airbox was likewise restrictive, and opening up the inlet holes will solve that without buzzing your ears with intake noise. Use a K&N air filter, the one that replaces the stock unit in place, for the best performance with the least annoying noise. Then, either rejet or run D8E sparkplugs afterward.

The inlet tubes were not made for flow, but for easy production and maintenance. You may even find large mismatches in the bores, and the screws that are used for carb balancing often stuck into the passage about 1/8" or more. If you're looking for upper-end HP, match up the intakes tracts and trim those screws. Remove the rough casting marks inside the inlet tubes, too. You can actually pick up almost 10% more flow with these little things.

If you're tired of grabbing 2 handfuls of throttle to get from 0-70 MPH, try this: get and install aftermarket throttle cables, which are thinner than Honda's (but won't have that nice nylon slider coating). Then, inside the throttle grip, lay some small diameter wire in the groove where the cables run in the twistgrip: this makes a slightly larger diameter circle in the grip. Don't make it so big a wire that the thinner throttle cables won't ride in the grooves. (Alternatively, you can get a fast throttle grip, but you'll sacrifice the handy switches in the process.) Test it to make sure it glides smoothly, then reassemble.

The springs on the stock 500/550 were very soft, too soft for spirited riding, and they sagged quickly. Get 100 lb springs for the rear when you replace the shocks, and shim the front springs inside the fork tubes with at least 3 washers of the diameter that fits the tubes inside. Then add air valves to the caps to make air forks out of them: run 10 PSI without a fairing, 15 or so with a fairing.

The 500 frames usually ran straight and true, even to 110 MPH. They really like TT100 tires, if you don't carry touring loads. The Continental motorcycle tires match the lean angle of the stock 500/550 perfectly and make a real good all-around tire for these bikes, lasting a long time and gripping well in the wet (and ice!).

The swingarm bushings and steering head bearings were just like the "big four" and suffer the same maladies. Fix them the same way as the CB750K series bikes. And. if you have over 10,000 miles on the wheel bearings (original Honda), they're gone. Replace them with Timken or Bower bearings for a much more responsive and confident ride.

Hop-ups: Add 4 teeth to the rear sprocket and get a cam. Don't get high lift, get longer duration - BUT: not a LOT more duration, because it is a small-bore engine. A little goes a long way. Adding 5 degrees of overlap and 10 degrees of duration should be your limit for non-racing driving: it will REALLY wake up the "mid-four". Don't bother with bigger carbs: port it in the head instead. The stock carbs can make a lot more HP than they do: make a "pocket" above the intake valve, then raise the exhaust ports about 2-3 mm and smooth 'em out. This will add up to 10 HP to this neat engine, which is a lot.

In response to a question about pod/stacks and 4-1 exhausts:

I dislike open airboxes for noise reasons. Unless they are fitted with velocity stacks, the improvement is usually overrun by the annoyance, with maybe a 5% change. You can get almost 5% with just matching the ports. 4-into-1: I have never seen them make any difference. 4-into-2: these make quite a difference on the 500/550, if you can get good ones. Unfortunately, the good ones also block the oil filter....

On a 500/550, for about the same $$$ as buying 4 K&N filters with velocity stacks, buying 4-into-something pipes and fighting with the resulting tuning, you can get a cam with more overlap and install it, right in the frame. For performance, that's the first place I'd go with the 500/550. Don't get more lift, just longer duration (this will cause more overlap, by design), maybe 8-10 degrees. This can add 10 HP to the under-aired 500. I don't know if Yosh still makes the street cam, but look around - maybe someone has one. Update 2020: use the 126-00 from Megacycle, good all-around and increased power.

[HondaMan]

Several of you asked me for pix of "how to modify the clutch for smoother action and longer life". I'll attempt to show something here (I haven't posted pix here before, hope this works)...   

Below is a picture of the oiling holes in the "recessed" area of the hub splines, like the ones you want to add to your hub. These are the original "inner" holes, as Honda called them. Please note, this hub has not been modified yet, but look at the hole on the farthest right, down in the spline.

The image shows both an "inner" and an "outer" hole set: don't add "outers" to your hub, at the "top" of the teeth. This weakens the hub a bit where the plates engage the center.

The hub you see here is from a 135 HP CB750, so the marks from the clutch plates are quite pronounced. It ran a stock Honda clutch with stock oil holes, which gave short life.

If you look around your hub, you will see "sets" of oiling holes, alternating 1-hole and 2-hole patterns. They also alternate "inner" and "outer" around the hub. Again, don't add "outer" holes, as this weakens the tooth in the hub. Estimate an "in-between position" from the existing hole locations, then drill 1/8" holes. Add as Honda did, 2-after1 and 1-after2. I typically add about 50% more holes than the stock hub had: different year models varied these holes, with the K8 having the most. (My K2 had 12 holes stock, now has 18 holes.) Also, bevel (some call it "countersink") the holes, all of them, on the inside of the hub. This improves oil flow and is the only "official" modification that Honda would approve, because they were so afraid of warranty issues with heavy-handed American riders.

Beginning in 2008 I add "oil sipes" for lack of a better term: this is a shallow groove (2 or 3) ground into the surface of the hub near the holes. It gathers a little more oil to the hole.

The holes at the back of the hub are often blocked by the web of the hub. Grind that away for more oil to reach the plates, too.

[HondaMan]

If you're running a TT100 on one end, run it on both ends.

The V-profile of the TT series is designed for increasing patch contact with the road as the lean angle increases. The trade-off for this is a slight instability at about a 5-10 degree lean angle if you have round-profile on one end and V-profile on the other. If the TT100 is on just the front, then the bike feels like it "dives" into the corner "ahead of schedule" (compared to what you are telling it to do). Then, as the tire loads in the turn, the caster changes (reduces) suddenly because the rear tire does not match the "squiggle" on the ground. The result is wobbly lines in corners, especially sweeping turns.
If the V-profile is on the rear only, and a round-profile tire on the front, the caster goes the other way as you start to lean over. The increased caster then tries to make the bike "climb" to the outside of the turn. When you straighten back up, it tends to overcorrect, making you "wobble" the other way.

Not that I'm against TT tires, you understand - they are my favorites (except when installing those steel beads) for overall mountain riding and wet traction. But, with touring loads, they wander all over the lane because they don't hold a heavy load vertically as well as round-profile tires. And, the block pattern on the front accentuates the deceleration "head shake" (K2 and later) unless you fix that problem (well, if you're lucky enough to have a K0, so yours probably doesn't shake, anyway!).

The "big four" is one of the most top-heavy bikes in the world. The gas tank "feels" like it's solid lead when compared to other bikes. This requires a different riding sense, but one that will reward you with far better riding skills than, say, someone who only rides CB500 or BMW, etc. This high center requires you to "cross-steer" into a turn, then correct when you hit the angle you want. You don't realize you're already doing this, but since you rode lower-center bikes before, it feels different to you. To understand what I mean, go into a nice, empty parking lot and try this: ride about 5-10 MPH and push the bars slightly to the right. The bike will immediately go into a left turn. "Catch" the turn at the angle you want, then steer INTO that turn to stand the bike back up. Do LITTLE turns at first, then you'll get it. Once you get the hang of this, you'll ride everything else (especially bikes with lower center masses) with much greater ease and confidence.

I used to teach this in Motorcycle Safety classes. We saw 2 customers get killed and one seriously hurt when they jumped onto big bikes after riding 100cc-class putters. So, I talked the boss into sponsoring a Safety class one Saturday per month during the Spring and Fall, for free. We always had a big class, and no one got killed for clumsiness in their riding in the following 3 years. The most important thing I could teach was the "cross-steer" concept, because it makes your brain stop thinking about what you're doing and start reacting according to how the bike "feels" under you. Very important, especially with tall bikes.

[HondaMan]

I've often been asked, by those who've ridden mine, why my 750 "feels" like a much smaller bike. Yours can be made to "feel" like the hotrod CB450 with 750 power by doing these things.

#1. Replace the steering head bearings (little balls and tracks) with VERY HIGH QUALITY tapered roller bearings. DO NOT use the Japanese or other oriental ones: use Timken. There IS a significant performance difference. And, set them up slightly tight, as they will wear in after about 500 miles. (Update 2010: All Balls has KML bearings now, almost as good as Timken's discontinued ones).
#2. Replace the rear swingarm bushings with tight-fitting, oil-impregnated bronze bushings (or send it to me for a rebuild with custom-fitted Oilite bushings). Change the oriental-style grease zerk fitting(s) with US-style zerks so the grease can actually be forced into the bushings. Lube those bushings generously when assembling them, and use new felt seals when you do.
#3. Get Timken, SKW, or Renolds wheel bearings for the front wheel (2 each) and the rear (3 bearings back there). They cost nearly 3 times as much as the oriental ones, and they are worth every cent.
#4. All of the CB750K0-K6 models I have seen have the "optional" steering head damper mounts. Get one from Fox or a BMW shop and install it. Set the damping to medium for startup, then adjust to suit when you get used to it.
#5. Tires. Use ONLY ribbed tire pattern in the front of a CB750K2- or later model. You can get away with a blocked pattern on a K0 or K1 model, because they don't "shake their heads" during deceleration. A ribbed front tire pattern helps make the bike run more stably under most street conditions, and it does not "wash out" suddenly in corners. The rear must be a symmetric pattern, preferably a block pattern. The new "sideslash" patterns on today's crotch rockets will cause sideways drift under heavy acceleration on wet (or slick) surfaces with the 750 geometry. Sizes: K6 and earlier: front must be 3.25 or 3.50 by 19" (or metric equivalent). Rear is usually 4.00x18 (metric is 120/90, roughly), some later ones had 17" rears and should be 4.50x17. IF YOU USE TT tires, use the 4.10x19 front and 4.50x18 (or 5.10x17) rear, AND ON BOTH ENDS. DON'T use a TT on one end only, and DON'T use them for heavy touring loads, they'll be all over the lane. These tire size combinations preserve the already-short 3.25" trail on the front end of these bikes, as well as the load rating. Larger rear tires will cause wobble, every time, because they shorten the trail.
#6. For those of you who have short inseams: your first choice should be to install 12" length rear shocks. Get 110-lb straight-wound or 90/120-lb progressive springs. If you install "lowering blocks", you will get a VERY stiff rear suspension, because the increased cant of the rear shocks will not allow them to compress over bumps. Shy away from those blocks. After installing these shocks, lower the front triple clamps about 1/2" on the front forks.
#7. Use teflon-mixed oil in the front forks. Get teflon-coated seals that are slighty shorter in length (1/4" instead of the stock 5/16" to 3/8" thick stock units). These will "float" up and down slightly between the top of the fork retainer and the upper C-clip, which makes them seal better and respond to minor road irregularities MUCH better. Install the optional steel washer ABOVE the seal (good kits include these washers). Drill and tap the fork caps and install threaded Shraeder-type air valves (found at tires stores, used on mags), one in each fork. Run about 5-10 PSI pressure if riding single, 15-20 if carrying heavy loads.

If you do ALL of these things, you will not believe the difference the next time you hit a corner. It will feel like the bike lost 100 lbs somewhere... 

[HondaMan]

When I helped guys modify the CBs into midget racers, here's what we did (circa 1973-4):
1. Cut off the tranny, made a new plate for the open side to seal it.
2. Drove the oil pump off of the right side of the camshaft, direct drive. We made an adapter for it. Then, it produced about 30% more flow and pressure.
3. Bored to 836cc, 11:1 CR, running alcohol and in demonstartions, nitro with gasoline.
4. The valves (intakes) were changed to 30 degree faces, the intake ports opened into a "pocket" above the valve as deeply as possible (too deep once, had to heliarc it closed and start over), passages left UNPOLISHED for better mixing, but valves polished like a mirror. The cams were real radical, but made by welding up and regrinding the stock ones. Roller rockers were fitted for the lifters, and they were lightened considerably. Overlaps were in the 40 degree range, which made them a real bear to start.
5. Using stock points, but with doubled springs and a hole drilled into the ground-side pad, then slight mods to the opening ramp on the points cam and extending the advance angle to about 48 degrees, we ran 16,000 RPM redlines. (i.e., no electronic ignitions were allowed).
6. Yes, it can be done reliably to a bike engine, if you consider 3000 miles between rerings reliable. But, the tranny mass tries to limit the redline. With a LOT of tranmission work and clutch work, the bike version can reach 16k redline, but if you limit your aspirations to 14k redline you will have much more streetable power. You need a 7 or 8-speed gearbox to cope with the powerband above the 14000 range, because it drops to less than 1500 RPM of useable bandwidth. The midgets used a torque converter and 6-speed gearbox (1/2 midget size) to cope with this.

It's radical. I don't remember having any crank problems with the 1971-73 engines we used (K1-K3 series). Oil was critical, though, because synthetics were almost unheard of back then....

[HondaMan]

Check your wheel alignment like this:
1. prop the bike upright (not on centerstand), as vertical as possible - or, get a friend....
2. Adjust the rear wheel so that the marks on both sides are dead equal (the marks are good on these swingarms).
3. Tie 2 strings onto the rear wheel. Rotate it until they are as high to the rear as you can get it, while pulling the strings to the front wheel, without hitting something underneath. You are going to measure the parallellism of front against rear wheels.
4. I use 2 wooden blocks for this part: set the strings on each side of the rear tire so they touch the tire on the front sides, then anchor them to the wooden block (nails?).
5. Set the front wheel straight. You'll probably find that the wheel is offset to one side. So, measure how much, then start adjusting spokes. TAKE YOUR TIME. You'll find it to be either 3mm off or 5mm off center, if someone else hasn't already tried this.

The wobble is caused by the front and rear wheels being parallel, not in line. When you're done, you may notice the odd-looking difference of wheel spacing between the front fork tubes. Remember that "look", then look for it when you see your next K0 model...it's a secret...but it's out in plain sight!

[HondaMan]

Things that make ratty idle:

#1: float bowl level. The adjustment of the idle bleed screw will not cure this one. One low float will cause that cylinder to barely run. Don't overlook the "air equalizer" tubes, the ones that vent the bowls. If one is pinched shut or plugged, that float level will always be low. Pulling off the hoses to check is a good start, but run with them attached to prevent fires from splashing fuel on rough roads.

#2: If all else fails, swap the coil wires and the points wires, 1-4 to 2-3 and vice versa. (I have seen one side weak on too many Honda dual coils.) See if the problem follows the coil.

#3: you've done this one: bad plug.

#4: carbon buildup on the intake valve guide and stem. When the assembly warms up and the guide opens up, the thing runs OK. Try running BG44K (1 ounce per tank of fuel) for a while, or pull the head to check it out.

#5: this is often overlooked: valve clearances. The exhaust should be .003" unless you wind it up a lot or have low-restriction exhausts, then it should be .004". Intakes should be .003" (yeah, I know, Honda says .002", but that's good for noise reduction, not RPM), especially if you are running velocity stacks or a K & N intake filter.

[HondaMan]

In the late CB750K3 Keihin carbs, Honda added a "lifter" to the needle jet to improve fuel atomization in the 3500 RPM range and reduce the tendency these engines had of fouling the plugs in city driving. However, these items don't fit the earlier carbs, but there is a fix for them, too: remove the "emulsifier tube", also known as the "main jet holder", and you will see tiny holes cross-drilled in the part where it screws into the carb body. These holes range in size from .025" in the "K" model to .033" in the "K1" to .035" in the "K2" and early "K3" setups.

Drill half these emulsifier holes out to .040" and reinstall, driving it a while to see the improvement. If it's not enough, drill the rest out, too. If you live at altitude, like me (Denver, CO), drill ALL of them out to .040" to start, and consider drilling at least 2 EXTRA .025" holes as well. I finally drilled the original ones to .044" and added the 2 extra .025" above that. At lower altitudes, it won't make much more difference with the larger holes being any bigger tan 0.039" size, though.

You'll be real glad you did.

[HondaMan]

Here's a tip for those of you who like the stock pipe style (4-pipe models), but want a little more midrange pep: find the pipes from the CB750K and CB750K1 models  (HM300). They had a straight-thru design with glass-packed baffles, needed repacking about every 10k miles or so. These will add almost 10 ft-lbs of torque at 5000 RPM if you rejet properly, which is: whatever size mainjet you now have, raise it by 20 and lift the needle 1 notch, unless you're running K&N air filters, then leave the needle alone.

If you have the K2-K6 pipes (HM341 model), these have 5 chambers in them. Drilling 4 holes of 1/2" diameter in the last baffle (remove the plug piece at the end first, then reinsert the plug piece after the mod), will raise almost 2 HP at 6500 RPM, no other changes needed. Make sure to keep the rubber interconnects between the pipes in place, these help more than you realize at higher RPM.

If you have the K3-K5 pipes, these may have 7 chambers in them. They can be modified as the K2-K3 pipes, but the holes must be drilled through the last 2 chambers, which can be tricky because the exit in each chamber is spaced 180 degrees from the previous chamber. There is slightly less to be gained from the 7-chamber types because the chambers are shorter.

I like the stock, voluptuous look, myself...I like the quiet, too!

[HondaMan]

If the CB750's sudden clutch engagement bugs you, here's a fix that both smoothes it out and makes the plates last a LONG time, much longer than normal:

Remove the center hub from the clutch assembly and add 2 oil holes in between each oil hole already there. The holes go into the BOTTOM of the splines, where the "teeth" from the clutch steel plates engage the hub, or else you'll weaken the hub. Oil every plate and reassemble. You won't believe the difference! I went through 4 clutch sets in the first 40,000 miles (some of them Barnetts), and the last ones have gone over 50k miles and are still fine. (Update 2014: they went over 90,000 miles and still had 20% wear left available.)

[HondaMan]

While most people know the long points of the Honda ignition, they are very easy to "fix" on the side of the highway in the middle of nowhere (been there) with only the NEUTRAL SWITCH light bulb for timing....I am tired of changing them. I have well over 100,000 miles on my 2nd CB750, bought it new in 1972, rebuilt it for roadracing in 1973 for a couple of years, then turned it into a (extra-easy-breathing) tourer for the next 90k+ miles' worth. Now, it's time for the Dyna III with the higher voltage coils. - UPDATE in 2009: after this Dyna also failed (not my first one...), I quit the Dyna "S" stuff and created the now-popular Transistorized Ignition, specifically for this forum. Many of you know this story, now!

The CB750K, prior to the K5 model, had problems with the spark plugs: they would not stay clean. This was due to 2 factors: 1.) the coils generate about 8K volts with new points at stock dwell, about 7K volts when the points have "dirtied up" a bit, about 1000 miles later. 2.) The late intake valve opening, necessary for the 9500+ RPM redline, causes rich mixtures from the stock Keihin carbs right in the 3k-5k RPM range where it "feels" good.

So, from some VERY long experience with these bikes (and the 110+ that I have built, modified or raced with other guys over the years), here's some advice: regardless of E-points or stock, get the advancer springs from the 1972 K2 version, which are smaller in diameter (by 1.5mm) and slightly longer overall (by 3.5mm), then cut off 1/2 coil from one end and reshape the end into a normal loop. Then replace your K/K1/K4/K5 versions with these units. This retards the beginning of the advance curve to 1500-1600 RPM and makes it "top out" at about 2700 RPM. This gives MUCH better around-town driveability, better gas mileage, and allows use of the "7" series NGK plugs (or "22" series ND plugs) for everything but long highway trips - without burning the exhaust valve guides to a crisp. Oh- and, you can run lesser grade fuels with no trouble, too - the original grades required were about 94 by today's standards, which is hard to find. This mod lets you run with 89 octane.

[HondaMan]

The CB750 was also not always 736cc. I saw 2 bikes, one at Mannheim Honda in 1970 and another at a Michigan shop (name escapes me at the moment) that had a 4mm SMALLER bore and the crank was 2mm LONGER throw. The little "nameplate" (736cc) on the front of the cylinders had NO NUMBER on them. I don't know what the displacement was, but these were 2 kick-ass 750s that had radical cams and dominated the local street crowds, stock from those dealers. They were labelled 750 and the owners (and their shops) had no idea where they came from: I only got involved at Mannheim because I had some valves that would replace a damaged intake on it: the valve was the same size as the SuperHawk's Yosh intakes (!) - definitely NOT the stock 750 valve.

[HondaMan]

If you have a K0 or K1, use this rule-of-thumb if the pipes are original type (straight-thru with fiberglass) and the airbox is intact: reduce the jet size on the mains 1 number per 1000 feet above sea level.
Drill out the emulsifier tubes to .040" holes (see that post around here, somewhere).
Also, on K0, lower the jet needle 1 notch, especially if you have the 4-cable throttle type: on those, you might need to go 2 notches if it's an early one (i.e., aim toward the middle notch).
All of these, taken together, makes the off-line start a little smoother (helpful on greasy streets) and the midrange throttle response quicker.

On K2 and later with chambered mufflers, reduce the main 1/2 number per 1000 feet, or just drop it by 5 to start.

[HondaMan]

Until the late 1990s, the CB750 was, to my knowledge, the only "big displacement" undersquare inline four production bike built. "Undersquare" means the bore is narrower than the stroke length. "Conventional" hotrodders complain that "undersquare engines can't breathe" like square or oversquare engines. This only illustrates that they misunderstand the great advantage of 'long holes'.

At low engine speeds, the longer bore gives greater torque for a given displacement, period. In most cases, long-bore engines were used for torquing applications, for this reason. Honda went it one better, though, by making a long-bore engine that could rev, REALLY rev. The rod ratio is actually over 2:1 ! When the revs begin, a special effect known as 'ram tuning' rears its pretty head, which forces more charge into the cylinder than one can get with a shorter stroke. This occurs because the vacuum is deeper at the midstroke, it lasts longer, and can be controlled more easily through the cam timing. Astute mechanics may notice that the 750 cam is a little late on intake closing compared to, say, a 350 twin or "modern" touring bike. This means that the power range starts later and stays longer than a larger-bore engine, and the resulting 'turbocharging' is free.

Next, the long-bore multi that revs has the extra low-end torque that makes it driveable while you're on your way to the powerband. This is no small thing, as riders of the CB500/CB550 know. The 750 has no "flat spot", like all other multis made. It never did. The long intake tubes, longer than the carb bore diameter, make up the midrange that otherwise starts to fall off in most fours before the powerband comes up. Then, the combination of ram-tuning in the cylinders and the long intake tract come together at about 6500 RPM to make the show start. The cylinders actually run richer at this RPM than below it, no matter how you tune the air inlet system.  This is one of the great secrets of this engine, and why it is just as much at home with a touring load as it is between stoplights, once one understand the advantages. Add to this the higher controllability of mixing fuel for small bores, rather than large ones, and the smallest of changes come together in the biggest way.

Keep that in mind for your tuning efforts: you are working with small displacements, so don't listen to the V-8 guys. A lot of their stuff does not work with these bikes. Headers on small bore engines are highly overrated.

To be sure, Honda struggled with this design. It went from a 600+cc to a 700cc to the 736cc version in R&D. They got a lot of experience, though, from their famous 90cc engine family, which was frequently stroked to 105cc size. Their primary "engineering" reason for undersquare was publicly stated as "to control the width of the engine", but my Yoshimura friends assured me this was public camoflage to Kawasaki (who, by the way, lost out to Honda by 2 hours of being the first to introduce an inline 4 at the New York expo in 1969, but that's the "New York Steak" story, for another day). The CB750 was essentially a multi-cylinder Honda 90 with better metals and a very risky gearbox design (that proved out well, fortunately).

So, when you hear old Cycle magazine readers quoting that rag as saying "this is the bike that changed motorcycling", smile. That rag was speaking about riding to the drive-in on Saturday nite and coast-to-coast the next week. In truth, it changed the way all motorcycles were comparatively built from then on.

[HondaMan]

When I roadraced, I did it at 15k-16k RPM peaks (yes, the 750 will do this).

It ran with (now you'll think I'm lying) 28mm Keihin carbs. They didn't look like the ones in your bike on the insides, but the outsides were stock. Had to be for the class of racing.

Part of the tricks: serious "pocket porting" above the intake and exhaust valves. And, yes, we went too far on 2 heads, reaching air in the process on #2 once and #3 another time. Hemi head. We removed all of the shrouding around the valves. This system directs the flow into a circular path so the compression stroke pushes the richer part of the mix to the walls for better cooling in a stock engine. Limits the flow almost 25% though...Compression: the bores were stock except for 1st oversize pistons (undersquare engine), head milled .010", .020" and .040" in various versions. We heliarc-ed little ridges across the center of the piston crowns, them ground them to clear the valves while reducing volume as much as possible. The quench area was likewise modified down to .007". The intake seats were narrowed and the floor above it narrowed as much as we could. Then, like Ford did on the legendary 427, we added an apparent "restriction" on the floor of the intake ports to cause the air to compress itself in the "pocket" over the intake valve. The result was free turbocharging above 8000 RPM.

Cam chain issues were a bear. We finally used Yosh chains with case-hardened steel rollers on tensioners and idlers - not rubber. The rockers were ground and ground again (yeah, we bent and broke a few) to get lighter, then case-hardened for strength. The valve stems were narrowed in those portions where they did not contact guides. The valves looked like mirrors, all the way down the shafts, but the ports were just mildly hand-sanded for a smooth finish. Brass inserts were used in the guides. Stock valve keepers were used for lack of something lighter. Yosh made lightweight retainers and rocker clearance adjuster screws. The opening ramps on the cams were gentle start, then agressive lift, then gentle beginning-of-close and almost snapped shut afterward. Stock lift or .010" over was used, but we saw little difference between them. We sandblasted the valve springs until they started to weigh less than stock by a few grains.

The carb bodies were bored and slides from 30mm Keihins (specially welded and modified) were installed. We used the standard 4-carb throttle mechanics. Jetting was an obsession....the rubber hoses to the heads were hand-sanded to match passages all the way in. The port inlets on the head were polished and widened only a little.

The real trick was the hand-built spark advancers. They went to full advance at 3000 RPM, then a second device that fit over the (modified) "normal" unit had tapered pins that moved down toward the weights on the "normal" unit. At about 12,500 RPM, these tapered pins engaged and pushed back the advance weights about 4 degrees or so to get past a "wall" we kept hitting right there, due to volumetric inefficiencies. This retard let the heat build up a bit and it would then "slip" into the 13,500 powerband start range. The tapered pins then retracted because of their advance weights and the full advance returned. It was 48 degrees, by the way... Today, this would be a piece of cake with computerized timing, but all we had was the hand-built (and designed, by me) transistorized ignition, following Ford's lead from the 427 again, that was switched by the points. Double-springed Nippon Denso points. Honest! Although, we did ease the opening ramps on the points cam, and welded up a higher ridge in the open portion. (That's something I learned from the CB450, running it to 12000 RPM).

Oil had to be preheated, then during the race it was too hot. So, we got some power steering coolers from some full-sized Fords, they look like a U with radiator fins. Then we cut into the return hose from the engine and piped up to the front, just below the oil filter, where the cooler was mounted on the frame. We made copper head gaskets. And, we cross-drilled 1/8" holes in the fins on the oil filter housing: lots of them, for better cooling.

The pistons: we shortened the skirts about 5mm, then cut the piston pins' retaining grooves for a slightly larger circlip (the "button" versions weren't reality then). We dimpled the higher faces of the piston skirts with a round-ground 1/8" drill bit, .005" to .010" deep (deeper up higher) in a fairly random pattern, using this to also help match the piston weights. The skirts looked like mirrors otherwise.

The rods: Yosh made some real nice ones, but they required crankshaft rebalancing. We lightened the rods wherever possible (less than 1/2 ounce overall, though), then shot-peened them with glass beads. Same thing on the crank weights. Then we trimmed the alternator so it weighed about 1/3 as much, and it only generated about 130 watts, just enough to keep the slightly-oversized battery alive thru a race. The whole assembly then had to be rebalanced, an expensive deal. Oh- the piston pins were Yosh high-strength units, very light. They cost $8 each in 1972! Must have been titanium or something, but were really light and never broke (or wore!).

Big surprise: the stock oil pump was up to the task. We just beveled the intake holes and matched passages all thru the engine to allow free flow.

By 1974, when I was getting out of the semi-pro thing, someone had started making roller-rocker kits. I wish I'd had some of those.

[HondaMan]

In response to the following question from Commando750:

"Correct me if I am wrong but most exhaust systems pair up opposing cylinders (that is, cylinders that fire apart, not next to each other, in the firing order). For instance, in order to increase exhaust flow on a 1-2-4-3 firing order (the firing order on a SOHC 750), a 4 into 2 system needs to link header pipes from cylinders 1 & 4 together, and 2 & 3 together. This would create a vaccum that each exhaust pulse could follow, making for better flow.

However, when I look at aftermarket exhaust systems I find that they pair up 1&2, 3&4. Why is this? Is my information on the firing order wrong? Or are they designing these pipes for form over function? Are there any advantages to pairing adjacent pipes together other than convienence or a unique exhaust note?The pipe question: above 5000 RPM, it makes no difference which 2 pipes join in a 4-2 pipe system, because the distance is wrong, anyway. Period. If anyone wishes to see the evidence, look back at Hooker, Action Fours, RC Engineering and the rest: all different combinations of 4-2 were used, and the only thing they really did was improve ground clearance or appearance, one over the other."



You're on the right track, though: the 4-1 pipes improve things for stoplight-to-stoplight racing. The 4-2 versions, with proper jetting, all work so much the same that I'd go for looks or ground clearance before worrying about the power.

If you want the best performance overall, with the excitement of the full-throttle rocket these "big fours" have, here's the full poop:
Use 4 pipes, joining together the two on each side near the ends (Honda wasn't kidding when they did this, honest). Inside the engine, make this change: get a cam with 5 degrees more duration than stock, and advance it 3-4 degrees from the stock hole. Modify the spark advancer so that it can get 4 degrees more spark, but add some tension to the springs for a later advance (I have a post about this somewhere...). Richen the main jets about 10 or 15 to start, unless you're running velocity stacks, then make it 5-10. Drop the countersprocket to 17 teeth, raise the rear to 48 to 52. Make sure you have a good tires and an empty road, 'cause you're gonna need both....

In 1973 we tested several exhaust pipes on a 1972 K2 model, running on a rear-wheel car dyno (dropped the dang bike twice doing it). NONE of the 4-2 pipes increased the on-ground HP over the stock Honda pipes, and these were the baffled stock pipes, not the glass-packs. They DID make more output at lower RPM, though. We jetted and jetted, to no avail. The on-ground HP, stock, was 49 with the Honda pipes (at 7500 RPM) and 49 with Hooker, RC, Action fours and one from JC Whitney that a local liar said gave him 11-second ETs in the quarter mile. They all peaked out at around 6000-6500 RPM. Gearing was: 18-tooth counter and 48-tooth rear.
For stoplight racing, I'd recommend a 17-tooth front and 50 rear with these pipes.

We tried the RC 4-1 pipe and got 50 HP at 5500 RPM, but only 46 at 9000 (2nd gear, all tests).  Same gearing.

Then, we tried the Yoshimura 4-pipe tapered megaphones. They required rejetting from the stock 110 mains to 135 jets, but put down 61 HP at 9500 RPM with no other changes. The 5500 RPM point was over 50 HP, but my notes don't show exactly whether it was 51 or 55, cause I was too excited: just "50+".

In a later test of a stock K4, we got 41 HP at the rear wheel, but did not change pipes. (Didn't drop it, either.   )  This had the same gearing as above, but with about 5K miles on the chain and sprockets.

[HondaMan]

From my archive notes:

1. The CB750 K0 spark advancer angle was 25 degrees, with 10 degrees static timing. (35 total).
2. The K1 (late model) through early K4 was 29.5 degrees advance, with 10 degrees static (39.5 total).
3. The late K4 (ran on regular unleaded gas) was 24.5 degrees advance, 10 static (34.5 degrees total).
(All degress are plus or minus 2 by Honda's spec.)
4. The K-K6 & K-77 was the same as the K4 late model.
5. Starting advance: 1200-1400 RPM (later = better in-town driveability, less suddeness). Full advance at 2400-2600 RPM.
If you want to run today's regular, bend in the "stops" on the advancer to yield K4 specs. For extra power, bend them outward to match the K1/K2/K3 advance, run premium gas only. For extra smoothness and better MPG, cut off 1 turn of the advance springs and bend the next coil out to reinstall, run mid-grade gas. This delays total advance to about 2600-2800 RPM and more closely matches the carb mix.

1. Cams: After K0, most K models were intake: 5 BTDC open, 30 ABDC close.
                                                       exhaust: 35 BBDC open, 5 ATDC close.
             K-76, 77, 78 models were intake: 0 BTDC open, 40 ABDC close.
                                                  exhaust: 40 BBDC open, 0 ATDC close.
( the later cams idle better, reduce emissions - if you care...)
2. The earliest K0 cams had more duration: some I measured at 5 degrees, some at 7 degrees more than the later K0 models and all following models.
3. All cams (Honda) I measured had same lift. Even R/C Engineering's drag cams and Yoshimura's roadrace cams only added .008" to .0012" more lift: lift doesn't add much.

[HondaMan]

No, this is not about hi-class callgirls...this is about NOT losing HP to Honda's "improvements" to the CB750K series engines.

Below, there are 2 pictures. Study them closely and you'll notice there is tape on one rocker shaft and not the other....oh, sorry, that's to remind me which is which.. The one WITHOUT the tape is the rocker shaft setup from a K4. The other one is from a K1/early K2. Notice, in particular, that there are extra holes drilled (and small bolts inserted through) the rocker housing and rocker shaft(s) on the K4.

Here's what this is about: the K0/K1/most K2 and some K3 engines had the (taped) boltless version. This allowed the rocker shaft to rotate freely under rocker loads so as to reduce friction losses. However, it makes a little bit of noise and if you ran light oil (10w40) in hot weather, it wore out the non-oiled ends of the shaft and the supports: they were not intended to act as full bearings. By the time Honda was making the K4, the CB750 was the touring bike of choice on U.S. highways: you could not take a putt without seeing hundreds of them, everywhere. Most had fairings: most of those were Vetter Windjammers. These fairings did a good job of funneling engine noise right up into your face, which Honda did not miss. They decided to quiet these bikes down (I even have a letter from American Honda about this), so they did all sorts of things to the pipes, airbox, carbs and inside the engine: this was one of those things.

But, this mod immediately lost HP. It also caused the bottom side of the shafts to wear much faster: as you can see in the photos, the K2 version has 112,000 miles on it and the K4 version has 20,000 miles on it. The shaft is MORE worn on the K4 than on the K2, even in total diametric wear.

The good news: you can remove the little bolts and run without, and pick up most of a full HP. I recommend replacing the shafts, though, when you do. Otherwise you will never get the valve lash set exactly right, because the worn side will keep rotating underneath the rocker.

[HondaMan]

The CB750K0-K5 was claimed to only have 1 cam, at least by American Honda. Although this was never true, there are several ways to modify yours to get the tuning you want.

Below is a picture of 3 different cam sprockets. The one on the left is a stock sprocket, but modified by RC Engineering and sold with their hop-up kits. It allows +/-5 degrees, +/-8 degrees or stock timing, depending on which way you install it.

The one in the center is a roadrace sprocket from Yoshimura, circa 1971. It is lightened and dynamically balanced for high RPM: it weighs 55% of the stock sprocket's weight. It is made for a fixed degree timing and you buy the timing you want, then install and measure it to make sure. The K0, K1, K2 and post-1976 "F" bikes have ones with similar lightening holes.

The one on the right is a stock sprocket for most of the 750s. This is suitable for slotting, and the best choice for that sort of thing, for dialing in the +/-3 degrees of production tolerance that the CB750K series endured. With nothing more than a rat-tail file, you can elongate the holes forward and back about 1/4" (both ways) for that 3 degrees and use a washer under the bolt heads to secure at the right location.

When dialing in your cam, remember this: American engineers use the lift value of .050" to determine the lift has started. Honda uses 1.0mm, which is about .040" instead. Keep that in mind when watching your dial indicator.