Topic: The Thoughts of Hondaman.

[HondaMan]

Here's the skinny on the Honda chambers:

You might notice there are pockets around the valves in the stock head. These are there to guide the incoming charge into a swirl, which accelerates as the piston rises. This causes centrifugal force to spin the fuel mixture toward the cylinder walls and accelerate gas movement SIDEWAYS across the head at TDC time. This makes a richer mix appear along the metal surfaces of the head and cylinder walls, which both preheats the mixture and cools the metal parts. When the spark fires, it is "spread" along a longer bit of the mixture because it is moving very quickly across the face of the spark plug at that moment. This is the patented Honda Combustion Process (HCP), popularly known as the CVCC (Controlled Velocity Combustion Chamber) engine in the U.S. Be very careful how you modify these chambers, because introducing cross-currents can quickly introduce knock that cannot be stopped with octane or timing changes.

For a clue toward the right way to change things, look up my post about Top End Tricks (Tips). You will see my modified 750 head's chambers there. This method removes significant portions of the peak edges of the pockets, turning it into more of a hemi-style head, to increase torque at lower RPM. It also lowers compression, so measure carefully when doing it. You MUST remove the center ridge from the combustion chamber before doing the following quench change, just like in my picture. Otherwise, the piston will hit the chamber, right on the ridges.

To modify the system and add quench, do it in the center of the piston. With a heliarc welder, draw a bead across the center of the piston, on axis with the crankshaft. This ridge can be as much as .125" high, but must be carefully relieved for edge-of-head clearance and tested for valve clearance. You can remove piston skirt weight to rebalance the piston to the original weight afterward, so measure carefully before and after. Be sure to smooth this ridge out, so there are no hot spots left that can hold carbon and create a heating problem. This brings your compression ratio back, too.

It's tricky, but worth every minute!

[HondaMan]

Here's my nickels' worth...

To make it simple: let's assume a standard cam, made exactly like the engineer wanted, and perfect production tolerances. 

This engine design has a max torque peak at about 30% of its redline range and a HP peak at about 80% of its redline range, just like a CB750K0-K4 engine with the 9200 RPM tach. This assumes the 28mm Keihin carbs, stock intake runners and airbox with the velocity stacks in place, stock pipes. Nothing else will change here but the cam timing.

Stock timing (we'll use only intake numbers for simplicity) is: valve opens (1mm lift) at 5 degrees BTDC, closes at 35 degrees ATDC. This is the "sweet spot" for the system.

Now, let's move the whole cam forward 5 degrees to open at 10 degrees BTDC, close at 30 BTDC. The early torque now comes to a peak at about 25% of redline, and there is more torque than before. But, the HP peak is also moved earlier, to about 75% of redline, but there is LESS HP than before. The engine is a little harder to start, idles a little less steadily. In the CB750K2, this drops about 3 HP at 6800 RPM as compared to the previous peak at about 7600 RPM.

Now, let's move the cam back 5 degrees to open at 0 degrees TDC and close at 40 degrees BTDC (like the CB750 "F" and K7-K8 cams). The engine idles better, and the torque peak comes on at about 40% of redline. The HP peak comes on at about 85% of redline, and there is MORE HP than before, by a little over 4 HP. The overall "feeling" is of increased power, because the power peaks are closer together with less "droop" in between the peaks.

Roadracers in Production racing (i.e., stock cams only are allowed) will often shim the valve springs and delay the cam 8 degrees to get more RPM with less torque into the 10,000+ RPM range. I did this a lot in the early 1970s. Gotta live by that redline, though, because another 50 RPM can mix & match the moving parts...   

Adjusting the exhaust timing in addition to the intake introduces other issues, but most of us can't regrind our cams for this. But, Honda's stated tolerance was +/-3 degrees PER CYLINDER from the target. So, when checking your cam, check ALL lobes and try to split the difference for all of them, then slot that cam sprocket and adjust from the one intake lobe you trust the most.

When you throw other things into the mix, like carbs, pipes and timing, things become more "spreadable" over the ranges you want. But, that's a different post.

To do this stuff, you will need some sort of degree wheel for the crankshaft (install it on the big nut on the points side) with some sort of pointer you can move to match TDC. Then, you need a dial indicator that touches the tip of the rocker arm (or, preferably with the cover off, the rocker foot) to watch for the movement. My "degree wheel" was a $1 plastic school protractor with an oval slot filed into the "0" hole to match the crankshaft bolt: worked perfectly. More recently, I made the one shown below, laminating it at Staples and cutting a slot in the center, so I can have 360 degrees instead of just 180 from the old one. The "pointer" is a piece of coat hanger wire that gets bolted into something convenient, then pretzelled to where I want TDC to be. 

[HondaMan]

Ever wonder what a full hemi head could do for the CB750? Me, too.   

I semi-hemi-ed mine in 1974, after I quit roadracing, because I wanted some more low-end pull for touring 2-up. It made a noticeable difference, so much so that retuning was required. I posted that picture here a few weeks ago.

Now, I'm going for the full hemi, like a Kawasaki (can I say that here without starting a fight?) 900 engine. Here's some pix.

The first shows the stock CB750K2 head, HCP chamber and shrouding. Compression ratio is 9.0:1. You can see severe valve shrouding issues at the center and near the outside edge of the intake valve. The exhaust isn't much better off. This really reduces breathing in the low-end, which is where I want more this time around. The changes will increase "blow-past", which is the unburned fuel that sneaks out the exhaust valve during overlap, so I will not be advancing cam timing to go with it, lest I end up with blued pipes from the heat. If you leave that ridge in the middle intact on yours, this won't be so severe.

The second pix shows the areas that are being reduced.

The third shows the type of tool you should use: dip it in beeswax or Chapstick to reduce bit clogging.

The fourth shows the most important part: polishing. This must be done to prevent hot spots of carbon from collecting in the divots you will probably make in these soft aluminum heads. Start with 100 grit, work down to 150 and then 200 or finer. It takes a lot of Coke to cool you down during this part, so set aside some time here.

When reshaped, the compression ratio will be too low, on the order of 8:1. So, the head will have to be milled. I'll re-post here later with how much milling is needed. I intend to end up at 8.8:1 to 9.0:1, with each head CC-ed to a new spec, which I'll also post here. I want to be able to run on mid-range gas instead of premium this time, 'cuz it's gonna be my working ride.



Let's see how it goes!

[HondaMan]

About dimples: someone here said it best: it works well for lower RPM engines, and also for those that run very hot. Also, unless you've bored your 750 oversquare, there is one more dynamic to consider: cylinder swirl. The deeper-than-wider engine usually has more turbulence by default, especially when the pattern is guided by intake tract angles, like the 750. Going oversquare, as they say, "changes everything"...My favorite example of this is the 1000cc kits of the early 1970s for the CB: there were 80HP (big deal), 90 HP (that's better) and 100-125HP (ahh...) versions. The difference was in the heads, because all used the same pistons from Rocky Cycle, under private labels.

The ridged valves are, in one aspect, a production version of highly polished valves. This works because of laminar flow: the surface of the grooved valve sets up a thin bed of low-friction air, which smoothes the path for the air behind it to pass over while adding some shear mixing along the way. The valve is usally made thinner, but more expensive material is used for strength, then ridged and the seat pressures are lowered to prevent head loss. I've seen this on engines with solenoid-controlled valves and with torsion bar valve springs - also on Desmo Ducatis in the 1960s. I even saw it on a Harley 750 flattracker when he was torn down for suspected cheating. The ridges weren't illegal, but sure generated a lot of discussion, like here...these ridges also appear in high-viscosity transport piping, for similar reasons: low surface friction and drag, and reduced pipe wear in slurry situations. 

The Hemi design, taken to its limit, allows for larger port areas, like big valves or lots of them. It allows (not in this case) the spark plug to be centered for the most even burn propagation. It also evenly distributes the forces of the expanding gases more evenly, which is where the extra torque appears. The little ridges in the 750 head, for example, mainly direct intake and exhaust flow, and were born of the CVCC (aka HCP) research toward more MPG and lower emissions. In that regard, they work well. But, they cause small disturbances, like ripples crossing a pond on the 2nd or 3rd trip across, that create "null points", or areas of lower force, during the burn. The most noticeable effect is reduced low-end torque and uneven throttle response.

So, I am going to mix "cylinder swirl" of the undersquare engine with the Hemi, provided I can get enough compression afterward, using the flat-topped pistons. 
I fully expect to end up with an engine with no detectable "surge" at the higher RPMs, which is a characteristic of the "big four". I'd like to see the power come on sooner, stay longer, and more of it across the spread. More torque at lower RPM will be hard on the rods, so for now I'll stay with stock-ish bore sizes.

A bit of my "observed" history (not to be confused with official press releases):
The 750 engine was in development at the same time as the 1300cc CVCC 2-chamber engine, and some of its swirly design came, I believe, from that relationship. The CVCC was the ultimate forced-swirl design: a tiny little "precombustion" chamber with a tiny intake valve sat on top of the regular chamber. A small passage connected the two, and the 2-bbl carb had one side set to 12:1 A/F ratio, the other side at 17:1 A/F ratio. Two distributors fired 8 plugs on the 12-valve 4-cylinder like this: the pre-combustion chamber fired first (45 degrees early ! ), driving a rich whirlwind down into the main chamber (at an angle) to swirl that upcoming charge really fast. Then, the second spark (more normal advance rates) would fire the richer mixture at the outer edges of this little storm, which would then burn the leaner part for a longer push time on the piston. It was a peaky engine, so much so that many thought it had a radical cam, but that timing was close to our 750s. The result was an engine that would turn almost 9000 RPM in a car, getting 50 MPG at 80 MPH speeds, and would fit 2 Japanese or 2/3 of an American driver. It also made less than half of the emissions of a standard engine, but with no smog controls of any kind! We used to call them the "roller skate" cars, they were so small, but quick. 

[HondaMan]

Here's a shot of the newly hemi-ed head. The stock chamber is 22.0cc volume, this modified version is 22.6cc. The head will have to be milled .008" to bring the compression ratio back near 9.0:1, so that's next.

Later update: I milled it back .020" that time, and this summer (2013) I milled it back another .020" for a total of 1mm (.040") less deck. This takes the compression to 9.2:1.

[HondaMan]

Here's the numbers I have for stock 750s:

CB750K0 : 12.60/102 MPH (1969, in Peoria, IL).
CB750K1 (my own) : 13.21/99 MPH (1970, Peoria)
CB750K2 (several) : 13.45, 13.66, 13.70, 96-98 MPH (Peoria)
CB750K2 with 17-tooth countersprocket: 12.99/101 MPH (Peoria)
CB750K2 with 18-tooth countersprocket (stock again) : 13.66/91 MPH (Denver, CO, 6800 ft altitude). NHRA says drop .5 sec for 6800'.
CB750K4 : 13.55/96 MPH best in Macomb, IL. (1974)

The gearing changed a lot on the CB750: K0 was 16T/45T sprockets, K1 was 17T/48T, K2 and later "K" were all 18T/48T sprockets stock. The tranny never changed, not did the primary drive ratio or tire sizes. The changes were due to wear problems with the drive chains.

Starting in 1975, the "F" series 750 came out with a closer ratio gearbox. Honda changed 2 gears' tooth counts and used fewer oiling holes in the countershaft to put more oil into the mainshaft. The tranny gear bearings in both the 1975 "K" and "F" bikes became cast iron instead of Oilite, to save costs. These bearings wear faster than the earlier bikes, but the bearings from the earlier ones can be installed into the later ones to reduce frictions a little bit.

[HondaMan]

Recently, I've read several posts where guys are having valve problems on CB750F and CB650. Here's a little history, and a fix for you, if it's not too late... 

One of the "refinements" that Honda made to the CB750 engine ends up causing valve problems in the K5/F/late K engines. This problem can be easily fixed
by retuning. The story goes like this:

In the beginning, the engine was volumetrically designed for a straight-thru exhaust pipe (it was actually a glass-pack in K0 and K1) and a very open intake tract (4 sq. in. intake holes). This meant that the exhaust valve is larger, relative to the intake valve, than most of today's engines. (Some people would say "the intake valve was smaller"...). Later bikes had more restrictive intakes and exhausts, but the engine never changed. Only the spark advance curve was modified to a slower curve to compensate, and the carbs were typically rejetted with every model change to try to cope. Close, but not as perfect as the K0/K1 tuning.

The original exhaust valve guides were longer, tapered to the top, and had no seals (picture below, after I take it, from my early K2 head, which is really a leftover K1 head from Honda's production scheme). This length improved the heat transfer from the valve to the head for cooling.

About the time of the K2, Kawasaki came out with the 900-4. Its oil change interval was 1500-2000 miles, even with a wet sump engine. Honda, not to be outdone on the "low maintenance" image, magically changed their specs from the original 900-1000 miles to 1500 miles by reprinting the manual. Their engineers scrambled to make this happen without hurting reliability, and they did it by switching the exhaust valve guides to the same type as the intakes, and added that valve seal on the exhaust to prevent the exhaust gases from entering the oil via the crankcase.

This is where the problem started. The intake guides are lots shorter and the seal makes the valve heat up more, to boot. The answer that we, the roadracers of the early 1970s, came up with was to increase the exhaust valve's lifter clearance to .004"-.005" up to 8500 RPM, and .006" to 10,000 RPM (stock = .003"). I raced mine at 10,000 to 12,000 RPM, using .006". This extra clearance lets the valve sit on the seat slightly longer, transferring a bit more heat into the head. Yes, it loses some cam duration, but on the exhaust side this doesn't hurt you like it does on the intake side.

Long term, like on a street/touring bike (or a 35-year-old bike), this translates into exhaust guides that gall and stick the valve. Then, one day the rider grabs a big handful of RPM and the piston taps the slow-moving valve and this breaks the guide: hence, the "F" model (and CB650-4 model) problem with exhaust valves burning and guides breaking.

That's what happened at Honda. To fix your "F" (or CB650) to prevent this, run .004" exhaust valve clearances. On the CB650, make it .005".

I've kept this .004" on my K2, and 112,000+ miles can't be wrong: the guides are still nearly perfect, despite an early chip on one from a missed-shift 14,000 RPM trip.


All the "F" and the later "K", like K7/K8 were affected.
Also, the "custom" series, but most often the CB650 in that group.

[HondaMan]

With any of the vintage Hondas, the "profile" of the front and rear tires must be the same type, or handling will have oddities.

For example, on a CB750 K0-K5, the stock front 3.25x19 matches the rear 4.00x18. If you substitue a low-profile rear tire like a 4.00-85x18, the rear sidewall will "lean over' at a different rate from the front: specifically, it will lower the rear axle centerline about .250" (1/4") compared to the front tire at full lean angle. This causes an increase in trail as the bike leans over, which makes the rear feel like it is "washing out" from underneath you. In contrast, increasing the rear tire size to a 130/90 will cause the rear to "rise up" at the axles centerline during this lean, which reduces trail. This makes the front end feel like it is trying to turn wider than you expected, and you end up re-correcting as soon as the turn starts, then re-re-correcting that correction, etc...

Either situation cuases you to feel insecure as you dive into turns. In stock tires, there are a couple of other concerns: edge traction and vertical load capability vs. side load capability.

A round-profile tire has a very even load transfer rating when going from vertical to angled, but the contact patch shrinks slightly on round tires at lean, because the tire rolls away from the rim's edge. The trigonometric tires (like TT100 profile) compensate for this by having incredibly strong beads and a smaller tire patch surface while vertical, so the G-forces are consistent all the time (important especially when swapping from one lean to the other at high speed, where G-forces come into play at the vertical point). BUT, "trigo" tires carry less weight for a given size, and require considerably higher tire pressures.

The shape of the tire pattern at the edges has more to do with side-loading than you might think. If there are "open vertical crossbars", i.e., openings that go STRAIGHT ACROSS the tire from the edge inward, there will be a "drift" capability. This accomplishes 2 things: 1.) it lets the tire "warn" you that the edge of traction is approaching by drifting slightly off of the anticipated line and 2.) it increases the forward drive and rearward braking traction by applying sideways edges to the concrete, relative to the bike's direction. Some tires will angle these "open vertical crossbars" to reduce the drift, but this also reduces corner brake/drive traction and grip, and removes the "warning" that you are almost out of sideways traction. The CB750 has more lean angle than most bikes (MUCH more than, say, a 500/550), so sideways traction issues are important on these heavier bikes.

Front tires' tread pattern plays another role: directional stability. If the grooves go mostly AROUND the tire's circumference, it will be more stable in the pointed direction than if a block-profile tire is used. But, it will have less braking traction, because the block profile puts more "edge to the concrete" whne the binders are on.

Lastly, size makes a large difference to handling. If the front tire is larger than stock and the rear is stock (or smaller than stock), the bike will have more trail and the handling will slow down. If the front is smaller than stock and the rear is stock (or larger than stock), then trail is reduced and the bike tends to become "twitchy" and very quick-steering. At the extreme, the latter causes wobbling and a tendency to track every groove in the road.

So, for best all-around results, keep the sizes near to stock and match the profiles front and rear. If you switch to metric, match the second number (profile number) to the original for best all-around results, and use the SAME profile front and rear (like 110/90 front, 120/90 rear). For hard cornering, switch both to trigonometric: but remember, for example, the 3.25 round profile equals the 4.10 trigo profile: that's an equivalent switch. If you undersize the tires, or use trigo tires, raise tire pressures. If you oversize the tires, you must experiment to find the "sweet spot" where the pressures don't change too much from tire flex.

A trick to remember from Continental Tire's motorcycle tire engineer: measure your tire pressure cold, then ride for at least 10 miles until the tire is well warmed, then measure the pressure again. If it rises 2 PSI: you're spot on your required pressure. If it rises more than 2 PSI, your PSI is too low. If it does not rise 2 PSI, your PSI is too high, and your tire patch size will be reduced. When riding 2-up or heavily loaded, add 2 PSI to your "normal" PSI as a start, but check again when warmed up.

As a reference: I run my (CB750) TT100 (4.10x19 front, 4.75x18 rear) tires at 34 PSI single-rider. I run my Avon touring tires (120/90 rear, 110/90 front) at 30 PSI.

Update for Avons after 2008: the Avon tires now use 35 PSI as their minimum, and I am running my new ones at 38 PSI front, 38-40 PSI rear, to make the corners smoother and ore 'even' feeling.

[HondaMan]

Note: Mark has submitted an updated edit of this post to address some recent issues (Oct-09)

Jaknight and several others have asked about this, so here's my history with them.

There are some Japanese-made "kits" available for the steering head, with typically use NKK, KOYO, DAIDO, or other Nippon-related bearings. Most come in a green box with a spacer washer that fits on the bottom of the triple tree's post, UNDER the dust seal (if you still have one!) so as to push the dust seal up into the steering head collar upon assembly. These "kits" usually have a top bearing that sits too high in the race, for the purpose of providing an extra .040" of clearance to nestle into the bottom of the steering crown (there's a cast-in recession under there for the old ball race). (I think M3Racing and PartsNMore carry these kits, among others).

This extra height causes some troubles, which in the long run can result in shorter life and not-as-solid front end stability. Biggest trouble: the top bearing, when tightened as proper, distorts the roller cage and the bearings are forced into a slightly unnatural location, high in the race. The result is a hard-to-feel torque-in point when you're reassembling, and some users don't get them set right as a result.

There are 2 solutions. First step for both is GREASE THE BEARINGS. Don't forget that, wheel bearing grease is fine. Also, the races get driven into the steering head, the bearings sit on the post.

1. If you use the Japanese "kits", then you must follow this procedure to "set" the bearings, and do it again after 100-200 miles of riding. You might find you need to do it again next year, but then it will settle in:
A. C-stand the bike, then either have someone sit on and balance back the bike so the front tire just rests on the ground, or jack up the engine to the same point. There should be about 50 lbs or so on the front wheel.
B. With the upper steering crown raised about 1/4" (or removed altogether) so you can get to the spanner nut, tighten the nut while constantly turning the forks, stop-to-stop. Tighten until the drag is real heavy. It probably won't stop altogether, but it will be close to that when tight.
C. Back off the spanner nut about 1/2 turn from this point and wiggle stop-to-stop several more times to redistribute the grease. See how the "drag" feels. If it is real smooth, tighten it back down about 1/4 turn until it starts to drag again. You want some drag at this point, which will dissipate with some riding.

The "hard tightening" here will distort the roller cage to match the partial-race mesh, without your having to ride & retighten for the next 12 months while it settles itself in.

This is my preferred method:
2. Use Timken (update 2010: or KML) bearings. You may need to install a spacer washer for the bottom, and another one for the top (dimensions below). [NOTE: circa 2002, you must grind Timkens' bearings to fit: probably not practical now - they stopped making the exact size we need. Edited 10/2009]
A. Install as noted above, big washer on the bottom, etc. On the top, the bearing will not stick up high enough to engage the recess in the bottom of the steering crown, because these are correctly-matched bearing sets. This is where the other washer comes in (if Timken): put it on last, then the crown.
B. Tighten as above, but then just back off 1/4 turn when done. It will be butter-smooth and fully meshed. Retighten in 100-200 miles, because you probably didn't drive in the lower race like you thought you did, and it will move a bit and loosen the stack.

The Timken bearing numbers are: (Note: not in production after 2009)
Lower: 07100
Upper: L45449
The lower washer is: O.D.: 1.750", I.D.: 1.245" (1.250" will do nicely, too). Thickness can range from 0.100" to 0.125" .
The upper washer is: O.D.: 1.500", I.D.: 1.000" to 1.020". The thickness can range from 0.040" to 0.060" and it will fit.

The Timken numbers I have are circa 1972, just removed from my bike. If they have changed, and I can find the new numbers, I will edit this post later. I will be trying to locate some of these bearings, because I just had to install one of those "green kits" in mine, if only to get it back on the road before Fall...  Undecided

Update 10/2009:
Here's the 'word' from Timken: circa 2002, these bearings are only available when ordered in large quantities, like 500 or more sets, at which time they will grind the OD and ID of these two to any spec. The O7100 and L45449 are now only available in 3 standard sizes (each), none of which will drop into the SOHC4 bikes.

Currently, KML is marketing properly-ground bearings to fill in the market's void. These are made of harder materials than the "green box" bearings mentioned above. I found that I can  engrave them with my buzzbox engraver, which is VERY difficult to do on the Timkens, so they are not quite as hard. But, the "green box" bearings I have can be scratched with a hard drag of a file, which makes them considerably softer than either the KML or the Timkens.

[HondaMan]

The "run" is a vertical tube that feeds one port of the switchable valve, the "reserve" is just the top open end of the valve, and it feeds another port. So, when the fuel drops below the tube's height, the "run" is empty. Honda tried to balance the hydrostatic head a little by making the "reserve" feed hole bigger, but there's little head to work with on "reserve".

The petcock has 2 outlet hoses: one goes to the left 2 carbs in a "T" between them, and the other to the right 2 carbs, same "T" style. The lean side of the petcock is the pipe that is furthest from the little "reserve" opening in the shutoff valve: the fuel has a little bit of difficulty passing over the "bouncing" fluid column in the nearer one to get to the other one. So, it tends to feed more from one side than the other.

If you have a spare petcock, pull it apart and you'll see why. If there's more interest, I could pix one up and make an FAQ of it.

Update 2013: to reduce confusion from those who have asked about the post-1975 bikes: in late 1974 models of the 750, Honda switched to using one fuel hose to a common fuel pipe that feeds all 4carbs. The reasons are many, but the result is: if your carb bowl vents are even the least bit dirty or clogged, the fuel will "fall behind" at hiway speeds and the bike will lose power. If you are trying to use pod air filters (my advice: don't...), this problem will become much worse on these carbs, to the point where you will try to install larger mainjets (which makes the trouble worse) to prevent the too-white sparkplugs. So, be sure the air vents are perfectly clear, and consider raising the float levels to 25mm or even 24mm on these carbs, to improve the feed at 80+ MPH constant riding.

[HondaMan]

I have a question about the Master Cylinder.

Disc brakes are "self-retracting" in that they use the vacuum effect of the returning piston sucking the pads away from the disc when the lever (or pedal) is released.  My pads don't feel like they are retracting well enough when I release the lever, but they can be pushed back easily by pressing on the caliper.  I think it is a lack of vacuum in the lines that are not allowing the pads to fully retract.

Actually, the disc pad is retracted by the O-ring in the caliper. When the brake is applied, it deforms (stretches a little), then it pulls the pad away when you let go by realigning itself. When the O-ring gets old, it lets the caliper slip past it instead of holding on with rubbery friction, and the caliper/pad doesn't return well.

Also, it takes very little crud in the edges and crevices around the pad to make them stick OUT. I usually polish the outside circumference of my pads with FINE sandpaper and ScotchBrite, then apply grease around it. This both makes the brake smoother and prevents ingress of crud later. Honda's earliest manuals recommended this procedure, the later ones just said "clean well around the pads".

[HondaMan]

You might need to align the front forks, but first, check to make sure you got the front axle & nut on the right way (some other post here noted they got the nut on the wrong side, an old error I've even forgotten, but common...). Also, make sure the axle caps (on the ends of the forks) are installed properly. They should be flat and even at the back side with a (parallel) gap on the front side (except K7-8 and F2-3 bikes, they are the opposite direction). Tighten the "closed" sides first, then the fronts, so the cap is sitting parallel to the end of the fork. The caps are DIRECTIONAL: they have a HI and LOW side to them. The gap side must be in front, until after 1976, in general. Look on your caps: if they are the ones that fit the gap at the back, they will have a little "F" on the bottom-front side to indicate this orientation.

To align: loosen the lower fork clamp (triple tree) bolts just enough to be loose, then finger-tighten them. The top ones should be tight, as per normal, so the tubes don't slip up during this next part. Loosen the bolts to finger-tight on one side of the fork brace (usually the non-brake side is easier). The next step is easier with a friend standing by with a wrench: get on the bike, on both wheels, and pump hard down & up several times on the front end. Try to hold it down on the last "bounce" (just don't let it all the way up), then have your friend tighten the lower triple tree bolts. Then let up and tighten the fork brace. This relieves little stresses all over, providing your tubes are straight.

[HondaMan]

Recently I've had several e-mails asking about shock-related handling issues, so I thought I'd add a couple of things to my other post(s?).

How to tell if your shocks are worn out: go through a sweeping turn, like a freeway onramp, at about 45 MPH and see if you tend to wander about the line you want. Then, do the same trip again, but increasing the throttle as you go (try to avoid 100 MPH). If you can hold the line better while slowly accelerating, the rear shocks have insufficient damping, whether from wear or from the damping setting (or oil). If the line declines while accelerating (i.e., you "slide" toward the inside of the turn) and your tires are good, then the front shocks are overdamping (aka "drooping" in height) and either need air assist, lighter oil, new springs or some combination of these three items.

Honda seemed undecided about trail on the 750 and did not give it enough, at least on the "K" models, for a self-steering ride. Stock trail was 3.75", while 4.00" works much better above 35 MPH. I guess they couldn't decide whether America was going to take the 750 to the Steak-n-Shake or across the country, and settled somewhere in between quickness and cruise stability. For this reason, when you replace rear shocks, shorter ones will improve high-speed handling while longer ones will make quicker steering at low speeds and more twitchy-ness at speeds over 45 MPH. Stock length was 13.0" on the K0-K1 and 13.125" on the K2-K4 bikes that I measured. I like 12.75" and am currently looking for a pair of these, as mine are goners.

If you tour heavy a lot, use longer shocks, like 13.125", with 120 lb springs. Set damping high with heavy loads. If you cafe', try to get the damping to be the same front and rear, no easy trick, and go for the 3.75" trail. If you dragrace or stoplite race or ride fast, lower the rear a little, because the increased trail will help keep you straight when shifting hard.

[HondaMan]

Removal:
The old bearings are best removed with a pair of hacksaw cuts. Install the hacksaw (12" is better than my 10" shown in the picture) through the swingarm and cut through the bearings in 2 places, about 40 degrees apart or so. Then, break out the little piece and the big piece will tap right out. Don't sweat the little bit of cutting marks you'll get in the steel tube of the swingarm, but don't cut through the swingarm, either..

Installation:
Put the new (bronze) bushings and shaft in the freezer for at least an hour. Put the swingarm in the oven at about 200 or so for that hour (in Colorado, put it out in the hot sun). Then, grease the outside of the (cold) bushings and tap them into the (warm) swingarm with a wood block and small hammer. They will go in easily, provided you cleaned the swingarm out well before all this started.... Grease the shaft and slide it in, then the felt washers, the plastic end caps and the cover (cup) washer. The inside of this cover washer must be flush against the swingarm when all is assembled correctly. Put it in the bike, torque it down (80 ft-lbs or so) and grease it a lot, until you're sure it is full, while moving the swingarm up & down to help pump the grease through the bushings. NOTE: if the bushings don't tap in easily, stop. Get some allthread rod, a socket that fits over the bushing, but inside the swingarm, and pull it in with the allthread and nuts as noted below:

Notes:
Use a long piece of 1/2" or 3/4" allthread rod and large nuts and washers to press in the bushings. Use an old bushing to press them into their recessed position, about .200" deep or so from teh outside edge. Some bushings are very soft, and the tapping can turn to pounding too easily if something is wrong, ruining the bushings.
Many have pointed out that the swingarm bolt should end up at 40-60 ft-lbs, depending on which manual you are reading (Honda does not say, in my manuals). The 80 value is probably too high for these aging swingarm bolts (it was OK for racing, when they were young), so try 40-60, waiting for that "stopped" feeling, and call that good. If your clearances in the bushings are good, the performance will still be good at 40 ft-lbs.

[HondaMan]

A bunch of you have asked me about pictures for the "emulsifier tube mods" I've mentioned in some posts. Here's a close-up shot of some modified ones from my hi-altitude CB750K2.

The upper 4 thru-holes (8 total holes) in this one have been drilled out to .039". The lower 2 thru-holes are still stock (about .021" or so). This particular setup works well with my modified airbox (bigger inlet slots than stock, K&N filter), reducing the K0-K2 tendency to gas-foul plugs and (K3-early K4) "bogginess" in low midrange throttle runs.

[HondaMan]

The SOHC4 bikes all came with "resistor caps" for the sparkplug connections.
On the bikes prior to 1976, the ohms value of the resistor inside was 7500 (7.5k) ohms from the factory. Over time, they slowly burn out and increase in value: at 9500 (9.5k) ohms, they are considered to be "burned out" and can cause several problems, including hard starting and fouled sparkplugs. This can also make the spark jump to the engine or frame (or your leg, in the rain), making for some pretty night effects. 

From 1976 until the end of the SOHC4 series, the ohms value was 10,000 (10k) from the factory, and the coils came with thicker spark wires because of this higher resistance (to prevent pretty night effects!). This later design was intended to reduce AM radio interference for cars next to your bike, and for riders who were sporting fairings with built-in radios for touring. It also made the plug cap life shorter, as the "burnout limit" was (is) only 11,000 (11k) ohms. These coils can be retrofitted to the earlier plug cap resistances with improved spark being the result, as the internal parts of the coils did not change.

Today, we can get plug caps from NGK in 5000 (5k) ohm values, which will slowly burn out over time to the 9500 (9.5k) limit over a much longer period. The newer plastic and rubber boots are superior quality: a bargain for their $5 price tag. They come in the straight, 90 degree, and 135 degree angled shapes for all of the SOHC4 bikes. Use these as replacements for superior performance on your bike.

NOTE: NGK makes caps in 0 ohms, 5000 (5k) ohms, and 10,000 (10k) ohms types. Pick the right one!

An alternate approach for the CB750, CB350F/400F and the CB650 bikes, and touring with the CB500/550, is to use the 0 ohm caps from NGK and the resistor sparkplug DR8ES-L from NGK. This heatrange is ideal for today's gasolines that are laced with ethanol: it is halfway between the colder D8EA that fouls easily and the D7EA that fits the CB500/550 engines. There must be at least 5000 ohms in this circuit, whether you use Honda or Dyna or Accel coils, to make things work properly.

Below are some pictures of a cap I broke open so you can see a resistor from inside, and a shot of how to measure the caps with a digital ohmmeter. The cap shown is a burned-out CB750K4 cap (1974 build), registering 10.5K ohms.

Set your ohmmeter's range switch to register in the appropriate ohms range first. Then touch one meter probe to the little screw tip inside the wire end of the cap, and the other probe to the spring socket where the sparkplug snaps in to the cap. Polarity makes no difference, here.

Update 2013 for Dyna 3-ohm coil users:
You can stretch out the short spark duration of these coils by adding more plug and cap resistance. The SOHC4 engines crave long-duration sparks. So, if you can find them, use the 10k ohm caps with these coils. If not, try the 5k ohm caps PLUS the resistor plugs (2k ohms) for a near-OEM pre-1975 setup of 7000 ohms. The resistor plugs in NGK are DR8ES-L, in ND (better, IMO) they are XR24ES-U.

[HondaMan]

Here's a couple of shots of the difference between "old factory" and "new factory" head castings. The rougher one, around the edges of the fins, is the K3 head casting, the smoother one is a K6/F0/F1 head. The K3 was typical of the "old factory" methods, where the castings were cleaned (or not) by hurried technicians during the heyday of the 750's production in that factory, when Honda struggled to even try to approach the demand for the bike. The ports also suffered many irregularities (at best) during the K2-K4 era until the New Factory molds replaced the old, much-used-and-repaired molds.

The last 2 pix show how the exhaust-side fins tended to end up from being rushed out of their molds and into the next machine steps, still hot. The ends of the fins get distorted and full of flash, as do the inner passages through the head. Cleaning these away really improves the performance of cooling on the head, which keeps it from losing power when hot. It also helps reduce oil leaks from gasket and sealant wear.

[HondaMan]

I've been getting a bunch of inquiries about this one, so here's the details: making CB750 DOHC pistons work in the post-1975 engines (F0/1/2/3, K7/8 bikes):

All that is needed to make these pistons from the 1979-1982 DOHC750 engine fit is: trim off 1.5mm of the shoulder, above the top ring, to about 3/8" in from the outside edge. Then gently file off the sharp edges around the outside of the crown and the edges of the remaining dome. The stock DOHC piston is the same as a 4th oversize (+1mm) for the SOHC4 engine. Everything else fits. And, you can use Honda rings, because they still sell them!

The compression ratio on the K7 I just did this way last Spring came out to about 9.4:1 CR.

In the F2/3 you can avoid trimming off some of the shoulder (only make it about 1/4" in from the outside edge) and round off the sides of the dome with a file a little bit, to fit the rounded F2/3 chamber. Clay-test it with no base gasket under the cylinders and no head gasket: you need 1mm of clearance from the valves in this configuration. The final compression will be slightly over 9.1:1 this way, very close to the original 9:1 of the F2/3 engines.

[HondaMan]

This one is becoming more common with the carb kits we are getting for the CB750K1-6/F0 these days: if your carb kit came with Keyster's needles (#D17) and you had no choice but to use them (DON'T, IMHO, if you can avoid it...the OEM needles don't "wear out"), then you are gonna need a bigger mainjet, and deeper float bowls, lest you have lackluster performance right at the freeway speeds where you need it. NOTE: these comments apply to bikes with OEM airboxes: if you have 'pod' air filters, none of this is valid.

If your original mainjet was #105 (like in #657B, 07A, 087a carbs), you will want to raise the Keyster needle to the 4th or possibly bottom (5th) needle slot, and increase the mainjet to #115. Your OEM needle was #27201, the Keyster needle is #D17, and too thick, acting even leaner than the ones found in the F0 bikes' #087a carbs (i.e., emission-controlled carbs).

If your original mainjet was #110 (like in the #657A carbs), you will need to raise the needle to the last (5th) slot and increase the mainjet to #115 at least, and maybe #120. Watch the sparkplug color to see if the #120 is needed (if they stay white, use the #120).

The K0 carbs (with 4 separate cables, not sandcast carbs) came with either #120 or #115 jets. Try the #120 with the Keyster needles set on notch #5. You will likely find these carbs to have staggered floats, with one side at 24mm and one side at 25mm, as measured from the tiny sliver of a reference post inside the gasket area. Most of the K1 (#657A) carbs also have this staggered float arrangement.

In any case, raise the (non-K0) float bowl level to 25mm, or if you have the extra-stiff Keyster float valves (another issue), try 24mm. These things will help keep the carb throats wetted at lower engine speeds, making starting and low-speed pull-away smoother.

If you installed a Megacycle cam (like my favorite, the 125-00), try changing the pilot jet to #38 (down from #40) to get the proper adjustment range working on the air screw again. This will put it back into the 7/8 to 1-1/8 range of adjustment again, and make it linear. This same trick should help with other cams that open early, too.

[HondaMan]

Here's some pictures of how to install the Transistor Ignition on some of these Fours, and a CB350 Twin. Some have been done by members of our forums who were kind enough to share their pictures, some by me. The one called "Grnd1.jpg" shows the Green wire's connection to the points baseplate: the supplied ring terminal fits snugly on the condensor screws, so mounting it to the lower condensor's screw makes a clean and simple installation. This wire MUST connect directly to the points plate.

The second-to-last last one shows the Black wire wye connection, this one using the Blue coil wire and the Black/White feeder to the left coil (1-4 coil, CB750 shown).

The bottom one shows the junction site near the top right rear of the engine(s) where the points plate's wires enter the main wire harness on these bikes. This is where the Transistor Ignition's wires plug into the coils and points: Yellow-to-Yellow (points 2-3), Blue-to-Blue (Points 1-4), Orange-to-Yellow (2-3 coil), and Purple-to-Blue (1-4 coil). The connectors are already installed to match the stock wire harness, so it is plug-and-ride with all the Honda wire colors. Kawis, Suzys, and Yammies vary the colors, though.

[HondaMan]

Here's another Transistor Ignition installation, on a CB500-4. This box has the Security Switch, and John wanted to be able to reach the switch without opening the seat.