Topic: CB750 K5 jetting? Picks up slightly when backing off frm WOT

[Dunk]

CB750 K5, Delkevic exhaust with ebay baffle (https://www.ebay.com/itm/357605748073), Dyna S, air box inlet baffles cut to match K1, otherwise stock engine. Carbs have 110 main jet, 40 slow jet, and I believe needle on 3rd (middle) position when carbs were last rebuilt 3.5 years ago.

Performance is somewhat sluggish at WOT, maybe since installing the eBay baffle in the Delkevic pipe. Definitely less performance and top speed than with stock Delkevic baffle (obnoxiously loud), and less than with Winning Performance (Kerker) pipe with straight through baffle. WOT issue may have existed before that, but less noticeable.

Issue is at higher RPM (maybe 6000+) I feel an increase in acceleration when backing off the throttle a little. Might this be too rich at WOT? Should I go back to stock 105 main jets? Or more likely to be too lean, go larger and if so should I lower the needle? Runs nice at lower speed and throttle opening. I have the carbs off now, just replaced valve cover gasket, so if I'm going to do anything with the needles now would be convenient. Looking for input on next steps, ideally without reinstalling carbs and popping a new plug in for a WOT pull... Yeah I know but carbs are on the bench now.

[HondaMan]

The symptoms suggest a touch of unburned fuel, but it may be from jetting OR from too-short spark duration as caused by the Dyna S above 4500 RPM. Both can cause unburned-fuel issues.

One way to try to 'test' the Dyna for this symptom involves removing the points cover from the engine and re-trying the test, all else being equal: this will cool off the Dyna's Hall-effect triggers and make them less 'leaky' (electrically speaking) at higher RPM, especially on a hot day. With the Dyna S, the spark voltage droops above 4500 RPM for lack of time for the coil(s) to fully discharge before being forced to re-charge from switching back ON before the spark is done. This gets worse above 5500 RPM and is especially noticeable in the smaller SOHC4 engines that rev higher all the time than the 750. This has been known since the 1970s, but often not believed, for some reason?

This "cover open" test used to be a way to determine if the triggers themselves were overheating, particularly when the Dyna 3-ohm coils were also installed. Switching to the Dyna 5-ohm coils could then help somewhat, but this was also in the days of the 55 MPH speed limits: that was sometimes enough 'de-loading' of the Dyna triggers to let the bike tour OK. At modern hiway speeds of 70+ MPH this won't happen, though: the coils reach oversaturation by about 5500 RPM with the Dyna S just because the triggers ore OFF for too short a time for the whole spark to discharge at that speed.

[PeWe]

"I feel an increase in acceleration when backing off the throttle a little."
This is typical for lean.
BUT, it works better with a less restrictive baffle.

The exhaust system might be too restrictive, jetting will not help then. It will not run faster, important to not run too rich.

I have done same mistake. My CB750 K6 had just got more power with 836cc, ported head, cam and a new 4-1 delivered with 2 baffles.
1 really silent I thought should be perfect for a long tour to former Jugoslavia, a little bit more than 2500km away from home.

Problem direct on  German Autobahn where the throttle had to be used.
Max speed ca 130kmh instead of +200kmh!! 
Engine sounded as it should explode.
Had to stop and ride without the baffle.

Both my CB750 have Dyna-S. My almost stock K2 runs new (2019) stock TEC coils, the K6 Dyna 5 ohm coils.
No issues at higher revs.
(The first set of 3 ohm coils drained the battery together with the headlamp on.)

My K2 had problems after 130kmh before. I tried jetting but found a fuel hose to be kinked reducing fuel flow to 1-2.
(Maybe 3-4 too)

So, check the fuel hoses close to the carbs. The connection pipe on carb might be angled to reach air screws. Hose kinked.

[Dunk]

I doubt the issue is ignition related. My K1 has the same ignition setup and is stock aside from ported head, and doesn't have this issue. That bike has 125 jets. I'm using 5 ohm Dyna coils on both. I could always swap points back in to test though.

If it's the restrictive baffle creating this problem I was considering making a straight through baffle with fiberglass wrap like a traditional baffle. I can test switching back to the original Delkevic baffle for a day.

I noticed one of the slow jets had larger emulsion holes. I dug trough spare carbs and found a set of Keihin with the small holes, cleaned them all up and installed that set of 40 slow jets. I wonder if that was contributing to the clutch rattle this bike developed at some point and I can't get to quiet down.

I also verified all floats are at 25mm (new style needles with stronger spring) and swapped to 108 main jets (from 110), the smallest I had on hand. I'll see if I notice any difference in WOT performance. I can swap to larger main jets to compare as well.

Not sure if fuel line is run ideally, bend is a bit tight to the carbs. I reworked it drilling another hole in the mounting plate some years ago as it was definitely getting pinched. WOT with restricted fuel flow was different from what it does now. It would perform normally at or near WOT for a few seconds then start to lose power at same throttle opening. WOT performance would briefly return after a lower throttle opening for at least a few seconds, then taper off again as bowls drained.

[CB750R]

From what I understand if you go from WOT and roll off and it pull hard its a sign your a little lean wide open. as the air gets chopped but the fuel has momentum and continues to be pulled up for that brief moment.

go up a main and re test In same scenario until it doesn't "surge" when you roll off.

[HondaMan]

Not sure if fuel line is run ideally, bend is a bit tight to the carbs. I reworked it drilling another hole in the mounting plate some years ago as it was definitely getting pinched. WOT with restricted fuel flow was different from what it does now. It would perform normally at or near WOT for a few seconds then start to lose power at same throttle opening. WOT performance would briefly return after a lower throttle opening for at least a few seconds, then taper off again as bowls drained.

You may be "on" to something there, with the K5 fueling arrangement. I have often fought that battle, too, with that sharp angle of the (now even thicker) 5.5mm fuel lines trying to get through that hole in the carb bracket. Some of the things I've variously done to help it is/was:

1. if the bike is not running a fairing (which makes too much heat behind the engine for this "fix" to work right) I have moved the fuel lines to in FRONT OF the carb's mounting bracket, installing the tee in between the #1-#2 carbs to put that short hose through only to the 1-2 carbs. Then I ran the hose across in front of the carb bracket to reach the 3-4 side. There are 2 different carb mounting brackets on the K5: one of them has holes for fuel lines on BOTH sides of the bracket, the other has only 1 hole on the left side. Th latter makes the hose into a sort of "j"-shaped tun to get to the 3-4 carb's side.

2. shorten the "drop" hose from the tee by the 1-2 carbs to be only as long as the face-to-face spigot length. This removes the widened spot in between the upper and lower hoses, which can "pump" the fuel upward a little bit when the engine is hot and the area around that tee heats the fuel up. This action can be much reduced by adding a couple of ounces of oil to our modern gasolines with their butane and ethanol blends, which can boil at temperatures found there during in-traffic rides at low speeds. It takes about 3-4 miles at speed to cool the area back off afterward, at which point the symptom can magically disappear.

3. make a 90-degree bend from copper or brass tubing, and use slightly larger 1/4" fuel line between the petcock and this elbow to pass the 5.5mm hose through the hole in the carb's front bracket. This increases the local hydrostatic head slightly while introducing some cooling to the fuel as is exits the hot tank. This was used as the "fix" on CB750K5 bikes behind raced (stock class) on tight tracks on hot midwestern summer days that suffered this boiling-fuel problem where track speeds were low and engine RPM was high. Sometimes I saw the copper/brass line run all the way to in front of the 2-3 carbs where it teed off to go back to the 1-2 carbs and continued across to reach the 3-4 side. The larger 1/4" fuel line (usually clear so the fuel could be observed) was used, too.

One thing that absolutely did NOT work was: installing an inline fuel filter. The wrench was thinking the extra "pressure" of having more fuel sitting there would improve the fuel feed. It had exactly the opposite effect...

P.S.: you're lucky to have all those carb racks to choose from!

[Dunk]

CB750R, that makes sense maybe it is lean, if it does the same I believe I have a set of 115 jets I can swap to see if it improves. If so I may pull the rack and drop the needles a position or two to keep part throttle economy, given my ~80 mile/day commute.

HondaMan, I like the 90* bend in hard line idea. I may try that with nickel-copper brake line, unfortunately I only have 5/16 and 3/8 on hand that I've used for fuel line on cars. Maybe I can buy a short length of 1/4" to use for this. It bends smooth and easy and is easy to flare a nipple on the ends to receive rubber hose. This K5 bracket originally had only one hole, I added the second to avoid tight bends and up/down loops, seemed to have helped over how it was ran when I got the bike in 2016 but I have no idea if that was "correct" or just what last guy thought worked best. I like the K1 duel feed petcock better, seems to give room for less sharp bends and even if pinched a little there's twice the area for fuel to flow.

Did the late model left side petcock bieks have carb racks with idle screw on right side? It's a pain to reach the idle stop screw with left side petcock and left side screw. I think I recall seeing some with right side idle stop screw, though all my carbs are set up for left side.

I had to check the slow jets I stole from one of those racks as it had 140 main jets and was on a cammed screamer of an engine. Was worried they may have been drilled, but no they're the same ~.016" as the Keihin #40 jets that were in the K5 carbs.

Been thinking for years I should rebuild these three spare racks and sell them over winter, maybe I'll get to it eventually. I think I have another set of carbs somewhere, I found the mounting plate but not the carbs. No idea how I ended up with so many!

[HondaMan]

CB750R, that makes sense maybe it is lean, if it does the same I believe I have a set of 115 jets I can swap to see if it improves. If so I may pull the rack and drop the needles a position or two to keep part throttle economy, given my ~80 mile/day commute.

HondaMan, I like the 90* bend in hard line idea. I may try that with nickel-copper brake line, unfortunately I only have 5/16 and 3/8 on hand that I've used for fuel line on cars. Maybe I can buy a short length of 1/4" to use for this. It bends smooth and easy and is easy to flare a nipple on the ends to receive rubber hose. This K5 bracket originally had only one hole, I added the second to avoid tight bends and up/down loops, seemed to have helped over how it was ran when I got the bike in 2016 but I have no idea if that was "correct" or just what last guy thought worked best. I like the K1 duel feed petcock better, seems to give room for less sharp bends and even if pinched a little there's twice the area for fuel to flow.

Did the late model left side petcock bieks have carb racks with idle screw on right side? It's a pain to reach the idle stop screw with left side petcock and left side screw. I think I recall seeing some with right side idle stop screw, though all my carbs are set up for left side.

I had to check the slow jets I stole from one of those racks as it had 140 main jets and was on a cammed screamer of an engine. Was worried they may have been drilled, but no they're the same ~.016" as the Keihin #40 jets that were in the K5 carbs.

Been thinking for years I should rebuild these three spare racks and sell them over winter, maybe I'll get to it eventually. I think I have another set of carbs somewhere, I found the mounting plate but not the carbs. No idea how I ended up with so many!

It's good you had one of those petcocks where you can add the 2nd pipe: even that casting disappeared after a while (like on the F0/1). The early version of that casting had 2 pipes on it: they were found on the K4 bikes with the "657b" series carbs. When the "7A" and the "087a" carbs came out, the 1-outlet petcock also appeared. Not all: I have seen K5 bikes with the later "087a" carbs and 2-outlet petcocks, though I don't know if someone swapped them out, or ?.

And, yes, there are both RH and LH idle screw mounting plates, and BOTH sides! The 'both' version appeared during the transition from the K4 to the K5, and I have seen them on F0/1 bikes as well (same disclaimer: don't know if it was OEM or not, it was in this century that I've seen them). I think I still have a "both" rack around here somewhere, but it also has a broken tine on one of the lifter fingers, so it would take a bunch of work to swap in some from another set. The RH side versions were on the K1/K2/K3 bikes with the 657a/657b series carbs. I [personally] think the both-sides screw versions came out because of loud griping from users of the 1-outlet petcocks who got burnt fingers from tinkering with the idle: if you were out on the interstate steadily and pulled off for gas, the bikes would sometimes not idle because the float bowls were too low. Then the riders would try to increase the idle while hot, with predictable annoyance. My answer to this one, short of recasting the mounting plate, was to deepen the float bowls to 25mm or even 24mm so the bowls wouldn't run so low at constant hiway speeds. When doing this to a K5 it also increased acceleration when passing, as they seemed to be the most sensitive to low-level bowls. It's interesting to note these were the same bikes Honda advertised could get "50 MPG" when we were suffering the big gasoline shortage from the Oil Embargo by Carter: those were the under-tuned 750 tricks Honda used to promote sales in those days.

[PeWe]

My K6 087A carbs can have idle screw on left or right side.
I changed to fit an earlier tank with petcock on the right. Idle screw moved to the left side where a threaded hole is done for it.

The 064A carbs can also alter the  side for the idle screw.

Do not miss an eventual underpressure in the fuel tank due to blocked air ventilation hole in tank cap. I have seen both rust and a too wide aftermarket rubber gasket.

I had to drill hole plus file star pattern on a new aftermarket tank cap.
I heard the underpressure when opened the cap.

[Dunk]

50 MPG sounds great for this commuter bike. It seems to get in the 36-42 MPG range now they few times I've measured.

I had to lower one float a hair from 25 as it was leaking out the overflow and into the air box. I'll set them all to 26 and see how it goes.

Jet change from 110 to 108 drastically improved WOT. It feels more powerful and more crisp at or near WOT where before it would slow down and be down on power. I ordered 105 jets to see if there's any further improvement. I'm also considering dropping the needle one clip position to see if part throttle performance is acceptable slightly leaner. MPG is primary concern with this bike.

[HondaMan]

These bikes can run very lean, and for a long time, with no ill effects on the engine.  You'll know it is getting toward the limit when there is a slight 'stumble' when the throttle is rolled heavily ON while in the upper gears (3, 4) and above 4000 RPM. It won't hurt the engine, no way, no how: there were some [brand X] bikes out there in the CB750's day that could cook the pistons or burn the valves when doing this, but the 750 isn't one of them. In truth, I have run these with #95 mainjets when tinkering with this back in the 1970s, and with the needles all the way 'down' to the #1 clip site, and all that happened was: the top end wouldn't exceed 95 MPG on a flat road. Oddly enough, the float bowl level had/has more effect on the top speed than the jetting: I think that's because the last 1/8 slide opening doesn't change the speed of the engine any (the carb's already FWO by then).

I DID finally get the sparkplugs to stay white after the usual 10-mile test run out to the lake and back, though: it also felt kind of cold-blooded with the #95, not wanting to increase speed readily in 5th gear at hiway speeds. I had to wind it up to at least 6500 RPM in 4th to get it over 5000 RPM in 5th gear, lest it fall back down. That was about the only "ill effect" I had with the experiment. I did all the test runs with the needles in the top position (only wanted to pull the carbs twice overall, to start it and to end it).

At the time, I was trying to figure out how Honda was getting their advertised 50 MPG, though I was using a K2 to do it (mine), since I get closer to 35-40 MPG. The K4-5 cam is slightly later-opening at the intake valve and the #2 and #3 intake valves open 1-2 degrees later than the #1-#4 intakes on those unique cams (I have one of these in a box downstairs, came from a K5 that got a Megacycle in the rebuild). This is the "marque" of one of those so-advertised bikes, and they run silky smooth. But they won't get 100 MPH top end in normal tune. Since our 'world' was at 55 MPH then, it didn't much matter?

[Dunk]

Most engines will tolerate a very lean mixture at light load, short of a lean stumble that gives driveability issues. I put a wideband in my 64 Falcon and while testing carb tuning surprised to see the wideband go max lean at 17:1 (who knows how much leaner it was) and still hum along just fine on the highway at 80+ MPH. For fear of melted pistons I richened it slightly.

Got me thinking a wideband on the bike would be interesting and useful, if not every pipe maybe in the collector. not sure the alternator has enough chooch for that though.

I'd think regardless of needle clip position WOT (and near to) should be governed by the main jet. Maybe appreciable economy to be gained with a very lean part throttle for light load cruise but a bit fat on main jet.

The other think I wonder about it total timing. Cars have vacuum advance in addition to mechanical, increasing advance at light load. More timing advance requires less fuel so long as not into detonation or pushing down on the piston while it's still coming up. Can these engines benefit from more total timing? How much initial vs total? I have a dial back timing light so can do some testing to see in actual degree if the timing marks are accurate.

[M 750K6]

The vacuum advance on my car ('59 TR3A) retards the ignition when you first stamp on the throttle. Should have been called a 'vacuum retard'.

[HondaMan]

Most engines will tolerate a very lean mixture at light load, short of a lean stumble that gives driveability issues. I put a wideband in my 64 Falcon and while testing carb tuning surprised to see the wideband go max lean at 17:1 (who knows how much leaner it was) and still hum along just fine on the highway at 80+ MPH. For fear of melted pistons I richened it slightly.

Got me thinking a wideband on the bike would be interesting and useful, if not every pipe maybe in the collector. not sure the alternator has enough chooch for that though.

I'd think regardless of needle clip position WOT (and near to) should be governed by the main jet. Maybe appreciable economy to be gained with a very lean part throttle for light load cruise but a bit fat on main jet.

The other think I wonder about it total timing. Cars have vacuum advance in addition to mechanical, increasing advance at light load. More timing advance requires less fuel so long as not into detonation or pushing down on the piston while it's still coming up. Can these engines benefit from more total timing? How much initial vs total? I have a dial back timing light so can do some testing to see in actual degree if the timing marks are accurate.

You're spot-on in your thinking, Dunk: watercooled engines can (and do, in cars like my Ford Escape) run well to 22:1 mixtures with light loads. It is acceleration that requires some fatness in the mix. With carbs like the later 750K7/8/F2/3, Honda solved this with accelerator pumps, running the engines at close to 16:1 at 1/2 throttle if the mainjetting and float bowl levels were correct (#105 mainjets typical, or #100 at my altitude near 6000 feet). One can improve on the throttle response with deeper floatbowls in our 'static mix' situations of plain ol' carbs instead of computer-monitored combustion, which is something I do when rebuilding these carbs, as a rule (particularly the roundtops of the K6/F1 and earlier bikes). The PD carbs are more tightly metered (and leaner), hence the complexity of adding the pump.

Honda had the needles in the roundtop carbs at their middle notch for the K0/1 bikes with the HM300 free-flowing exhausts. When they switched to the more-restrictive HM341 pies they had to raise the needles 1 notch to #4 to smooth out a flat spot in the performance right around 3k-4k RPM, most noticed when touring interstates here in 5th gear at 3950 RPM. It was a lean spot in the mixture when the needles were in the middle slot because the airflow was slightly less at steady-state speed (like freeway riding) right at the bottom range of the slide-needle mix authority. At first they used the #110 mainjet with this setup (until somewhere between 1/1972 and 3/1972 builds) but switched to #105 mainjets because of dark sparkplugs when ridden in commuter traffic. This change also matched when they switched from the 657A carbs to the 657B carbs, the latter being more accurately cast in improved molds.

if your commute has lots of idle/stop/low-speed activity, you might want to switch the pilot jet down a step to #37.5 instead of the normal #40, if the plugs tend to run dark. (This can be the Keihin idle jet stamped "38" or "37.5" (rarer) or the aftermarket K&L/Keyster versions stamped "40".) The low-speed mixture from the roundtop carbs is a bit "clumpy" and unevenly aerated in the 2000-3000 RPM range because the mainjet is almost working, but it 'pulses' the fuel up & down when riding along at those speeds: you can feel it when riding these bikes. I minimize this activity by retarding the spark advancer with slightly stiffer springs (cutting off one/half or one coil from both springs) on most of the bikes I build these days, to make it smoother and match up better with modern gasolines. Full advance then comes on around 3000 RPM instead of 2000 RPM, which most of them (with their heat-annealed springs) do today.

[Dunk]

My K5 has 086A carbs, my K1 has 657B carbs. Not sure what's correct or better for each.

I do have 38 and 42 Keihin main jets, I was considering trying to 38 since a lot of lower speed light throttle is on that circuit. I spend a fair amount of time in the 2500-3000 RPM traffic range. My commute is 80 miles/day round trip, depending on route it could be mostly highway or half or more around town stop and go back roads. I can do almost entirely highway at 90+ MPH if I'm lucky enough to be a light traffic day or leave at the right times, but today I spent almost 2 hours etting home in largely around town and traffic and grateful for every red light or traffic jam to warm my hands. Maybe a bit more initial/total timing but not coming to full advance to near 3000 RPM would be better?

After changing 110 to 108 jets and lowering floats from 25mm to 26mm I noticed when leaving work, temps mid 30s, that letting the engine warm a minute or two I felt lean spots accelerating onto two lane blacktop, had to roll into the throttle slower for good manners. A couple minutes more with more temperature in the engine the lean spots and hesitation were gone. Bike lives in the garage at 55*, no such issues in the AM leaving for work. To be expected, but maybe tells me I'm getting closer on mixture if it acts lean when cold.

For extra fun my headlight low beam died on the way into work this morning, another biker flashed high beam which made me check when I got to work. On the way home my last 10 miles were without a clutch, the cable snapped. I made good use of side streets and sidewalks to wait for gaps in traffic nad double back. I did not feel like pushing and hopping on while layered up like for freezing temps.

[HondaMan]

My K5 has 086A carbs, my K1 has 657B carbs. Not sure what's correct or better for each.

I do have 38 and 42 Keihin main jets, I was considering trying to 38 since a lot of lower speed light throttle is on that circuit. I spend a fair amount of time in the 2500-3000 RPM traffic range. My commute is 80 miles/day round trip, depending on route it could be mostly highway or half or more around town stop and go back roads. I can do almost entirely highway at 90+ MPH if I'm lucky enough to be a light traffic day or leave at the right times, but today I spent almost 2 hours etting home in largely around town and traffic and grateful for every red light or traffic jam to warm my hands. Maybe a bit more initial/total timing but not coming to full advance to near 3000 RPM would be better?

After changing 110 to 108 jets and lowering floats from 25mm to 26mm I noticed when leaving work, temps mid 30s, that letting the engine warm a minute or two I felt lean spots accelerating onto two lane blacktop, had to roll into the throttle slower for good manners. A couple minutes more with more temperature in the engine the lean spots and hesitation were gone. Bike lives in the garage at 55*, no such issues in the AM leaving for work. To be expected, but maybe tells me I'm getting closer on mixture if it acts lean when cold.

For extra fun my headlight low beam died on the way into work this morning, another biker flashed high beam which made me check when I got to work. On the way home my last 10 miles were without a clutch, the cable snapped. I made good use of side streets and sidewalks to wait for gaps in traffic nad double back. I did not feel like pushing and hopping on while layered up like for freezing temps.

Wow! Clutch-cable breaks are REAL annoying: it happened to mine, too, in 1973 on a trip through Utah, and the next day the pull-side throttle cable also broke. That first day I was about 2 blocks from a Honda shop near St. George, pushed up to and fixed right there. Next day, I swapped the pull-push cables at the carbs and throttled backwards until I could find a Honda shop nearer Salt Lake City, and they had one (whew!). I've tried to avoid Utah on the bike since then: there's a cable gremlin living there, I think?

It does sound like you're right about on the numbers with the #108 mains: if the bowls were at 25mm instead of 26mm then that temporary leanness when warming up will likely go away. The #40 idle jets were (and still are) a little bit rich, especially when the springs in the spark advancers get too soft and advance too soon: this makes the engine spit back toward the carbs in the 2000-ish RPM range, making them momentarily even more too rich until about 3000 RPM. I solve this by both slowing the spark curve (tightening the springs) and by using the #38 pilot jets, because to get to that mix ratio with the air screws puts it at the end of their authority (3/4 turn) and things get erratic, especially when cold. The #38 jet (#37.5 in Keihin, usually stamped "K38") lets you move more toward 1-turn-out territory, making the adjustment more linear with modern gasolines that burn so very slowly now.

I just went to #35 pilot jets a month ago as an experiment against butane being added to the gasoline here. But as luck would have it, my wife suddenly needed medical attention and my chauffeur services, so I have only ridden 1 tank of gas since then, haven't had much time to tune things.

[Dunk]

I changed air box boots, noticed two were loose and letting air past them last weekend when I changed the valve cover gasket. While I had easy access I swapped in the Keihin 38 jets (from 40). The 38 jets are the 32mm long style, the 40 jets I removed are shorter at 28mm long. Not sure how much difference this makes?

It's more sensitive to throttle stop opening. I initially left air screws at the same ~1 turn out, on a quick test ride to warm it up I notice crisper throttle response at lower RPM, very smooth and feels stronger rolling into it from ~2000 RPM. An improvement I think.

Idle was a little higher when I got home, very sensitive to idle stop screw position. RPM increases a bit with air screws further in, I left them at about 7/8 turn out for now. I could open them a bit more to lean it out but the sensitivity to throttle stop screw position gets worse and it almost hangs, with a slower return to idle as I lean it out.

I'll commute a bit with these pilot jets and see how it acts or if after totally hot like getting off a long highway run it wants different air screw settings.

[HondaMan]

I changed air box boots, noticed two were loose and letting air past them last weekend when I changed the valve cover gasket. While I had easy access I swapped in the Keihin 38 jets (from 40). The 38 jets are the 32mm long style, the 40 jets I removed are shorter at 28mm long. Not sure how much difference this makes?

It's more sensitive to throttle stop opening. I initially left air screws at the same ~1 turn out, on a quick test ride to warm it up I notice crisper throttle response at lower RPM, very smooth and feels stronger rolling into it from ~2000 RPM. An improvement I think.

Idle was a little higher when I got home, very sensitive to idle stop screw position. RPM increases a bit with air screws further in, I left them at about 7/8 turn out for now. I could open them a bit more to lean it out but the sensitivity to throttle stop screw position gets worse and it almost hangs, with a slower return to idle as I lean it out.

I'll commute a bit with these pilot jets and see how it acts or if after totally hot like getting off a long highway run it wants different air screw settings.

Those slightly longer pilot jets put the venturi (for the fuel in the metering hole) down further in the fuel. Normally, the 'pinch point' where this venturi sits is right about the same as the depth of the fuel in the float bowl so (while slightly slow in action) the fuel level is "visible", so to speak, to the tuner. The longer ones that I have (from JetsRus) put this venturi under water, so to speak: it is slightly below the float bowl's level. This will make the pilot opening in the carb always slightly wetter, which will make it "drool" (that's what we old racers called it, anyway) some extra fuel after each intake stroke. This slightly wets the intake a little bit more than normal below 2500-ish RPM (or lower).

I've used those jets in the past when someone asked me to "fix" his chopper. The front end had a very long springer, so the engine sat tilted upwards a bit and the carbs were dead level instead of inclined slightly downward. This made the idle mix always very lean, and raising the floats deep enough to keep them wetted resulted in always-wet gaskets and weepage of fuel on the outsides of the bowls, albeit slowly. There was no way to make this right with stock equipment: those extra-long idle jets actually solved it because their venturi was within 1mm of the float bowl's depth at 24mm (mighty deep, like racer level) float setting. This kept the outsides of the bowls from being weepy wet all the time, and the bonus in the end was that it quit drooling this extra fuel back into the intake air filter (which the owner was smart to enough to retain after his 'pod' filters had totally failed to make reasonable performance in traffic). He was using a foam element in the filter because he had gone through several paper ones, soaking them all in fuel in less than 10 miles' ride time(!).

I went 'round the horn here with this explanation/experience because while I think your extra-long idle jets aren't a terrible choice, you'll probably find they require a change to the float depth (larger number, like 26-27mm) to make the transition from those idle jets to the mainjets smooth. This will, in turn, likely mean a change in the needle position in the slides: my offhand guess would be to the 4th or even 5th (lowest) notch to avoid a flat spot just off idle, before 2500 RPM happens. It might feel "flat" otherwise between about 1800 and 2500-ish RPM as the extra richness of the idle jet transitions to the leaner slide mixture that occurs there.

On the plus side: this will probably help keep the sparkplugs cleaner! 

[Dunk]

After a commute today I think I'll be changing these jets back to the 40s, I have both short and long style 40 though. Maybe I'll tinker with air screw settings a bit more though. Once at temp the idle is not always perfect, wants to be richer to be smooth. Takes forever to warm up to a fairly normal idle and smooth part throttle, at least today when leaving work below freezing. 20-30 mins to get to temp for good part throttle and decent idle. I stopped for 10 mins at the hardware store and it was almost like starting all over again. These leaner jets might work okay in summer but definitely not looking at 2-3 more months of these cold temps. I'm at sea level though, my work is right on a tidal river, and I recall you're at elevation. I bet an accelerator pump would make the leaner pilot jetting work well.

If the choice is a bit rich at idle but better manners I think I'll prefer that. Whether I end up with 105 or 108 mains in final state I can adjust needle height after that to get best throttle response and manners when fully warmed up. After starting it below freezing today and dealing with the idle and part throttle for 20-30 mins I know it'll be a bear in Jan/Feb single digit cold snaps.

[scottly]

Cooler temps make an engine run leaner, as the fuel doesn't vaporize as readily, which is why carburetors have chokes for cold starts. WFO is dominated by the main jet, while the idle jets and mixture screws are for idle to 1/4 throttle, and the needle from 1/4 to 3/4 throttle.
At the temperatures you're riding in, you may want to consider blocking off some of the airflow over the cylinder fins, and also perhaps ducting warm air into the air-box.   

[HondaMan]

It does sound like you're getting the 'hang' of the jetting, though! You're right on all the counts. I live at 6000 feet altitude, so everything is "off" by a factor of about 5%-ish overall. With almost #100 mainjets, though, it is easy to guess at the next size, in percentages.

[Dunk]

Yesterday was colder yet around 14* when I left for work, but I turned mixture screws ~1/8 turn in to 3/4 turn out. Huge different in how it runs when cold and much shorter warmup to decent manners time. The bike had more power at highway speed like this, didn't think slow jets would affect it much but I was hitting a wall around 80 MPH full tuck and hard to get past it with the leaner air screw setting but after turning in 1/8 turn it get to 85+. Maybe it means main jets are close, or maybe mains are slightly rich and colder temp helped for more power.

Heat wave today on account of rain, a bit above freezing and was fine. I think I'll go back to #40 and try leaning out air screws as far as I reasonably can, the see whether 105 or 108 work best at WOT.

If I could figure an easy way to get warmer air to the carbs it would probably help this time of year. I always get milkshake sludge on the air cap. I was wondering about venting the crankcase and oil tank or at least valve cover to the air box. That blowby gas should be warm, and sourcing fresh air through a warmer and filtered air box may help with any sludge from cold ambient temps, not sure. I wonder if there's a fitting I can bolt or rivet to the air box similar to the 77-78 bikes to plumb it that way?

[Don R]

 There should be some warm air behind the cylinders, I wonder if a temporary duct could be fabbed from car pre-heater hose?

[newday777]

A Vetter fairing and the lowers would help getting some heat to the airbox.

[scottly]

There should be some warm air behind the cylinders, I wonder if a temporary duct could be fabbed from car pre-heater hose?

I was thinking maybe air-cooled VW heater hose, the form-able corrugated stuff that's about 2.5" in diameter?   

[HondaMan]

A Vetter fairing and the lowers would help getting some heat to the airbox.

You have NO IDEA how well this works - unless you've done it!
I have both, and you are absolutely right. In cold weather I screw on the Lowers and the bike runs better than in the summer, every time. Once the temps get above 80 degrees or so, the carbs can boil [modern ethanol-laced] fuel in the carbs with the Lowers aboard, though, not so good. So, the Lowers come off in Spring.