Topic: 72 CB750 stumble off idle when hot

[flybox1]

Not a needle issue....as soon as the slides close, the fuel draw is thru the idle/pilot circuits.
Stumped

[markreimer]

2.5-3 turns out? I didn't think the screws even maintained adjustability past 2 screws or so. Have you tried a 42 pilot?

For comparison, I'm running 40 pilots, 105 mains, needle in 4th clip from top. K&N Filter in stock airbox, 4-2-1 Hindle open exhaust, 1974 cb750K.


At 1 screw out, I have a bad stumble. It's hard to even ride it. If I turn the screws out 1/8 turn more, the stumble is almost gone, but not quite. That's where I'm at now. If I then turn it another 1/8 out, the stumble is entirely gone but I get some popping during engine braking.

So the range from nearly unrideable stumble to no stumble for me is 1/4 turn, maybe a hair more. But definitely under 1/2 turn. That makes me raise my eyebrows at you turning the screws 1-1.5 turns to eliminate the bog. Something is up here...


I swapped my advance last night with a spare as mine had saggy springs. The spare had snappy springs, and I have bang-on timing now.

I wonder if it could be a float issue? Too high or too low could cause a bogging issue.

You're 100% positive all the passage ways are clear in the carb?

[KeithB]

I have just completed a couple of hours around town and the plug shows it's running a tad lean.
Still need to do highway test.
I do have 42 pilots available for installation and I will take the carbs off and check...again...that all is clean and confirm...again...the float level.
What I did not check last time was needle position.
This is my 4th 750/4 and also did a 550/4 and have not had this much trouble with carbs.

[markreimer]

I dunno, those plugs look pretty good to me. Not too lean in my eyes?


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[flybox1]

For around town, theyre a bit light IMO.  Not enough carbon buildup. 
An accurate 1/2 throttle plug chop is needed to determine if clip changes are warranted

[HondaMan]

I have just completed a couple of hours around town and the plug shows it's running a tad lean.
Still need to do highway test.
I do have 42 pilots available for installation and I will take the carbs off and check...again...that all is clean and confirm...again...the float level.
What I did not check last time was needle position.
This is my 4th 750/4 and also did a 550/4 and have not had this much trouble with carbs.

If your gas has ethanol in it, those plugs are NOT too lean, but just almost perfect burn. The ethanol makes the plug lighter in appearance because it tends to wash away the carbon particles. When going inside these engines, you'll see this phenomenon a lot: the carbon layers they build today are a fraction as deep as they used to be with non-ethanol fuels. I can get non-ethanol here, and it makes more power, plus makes the plugs several shades darker, no other changes involved.

[jonda500]

Sounds like a rich pilot circuit to me, I'd try #38 pilot jets, not #42 as that will make it worse?
John

[KeithB]

[/quote]

If your gas has ethanol in it, those plugs are NOT too lean, but just almost perfect burn.
[/quote]
I can get ethanol free gas here so I will test it out.

I have ordered some 38 and 35 pilot jets.
Will report results

[KeithB]

Update...
Installed 35 pilot jets and screws are about 1.75 turns out.
Seems to work much better.
Need more time to test but so far, so good.

[markreimer]

Cool, please report back. I'm about to experiment with my 750 jetting as well, leaning it out specifically, to get rid of that last little hint of the stumble


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[HondaMan]

Cool, please report back. I'm about to experiment with my 750 jetting as well, leaning it out specifically, to get rid of that last little hint of the stumble


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Have you opened up the little aeration holes in the emulsifier tubes above the mainjets? For ethanol-laced fuels (or our modern slow-burning unleadeds), it helps to add a little more air in the mix at the midrange. Once past 0.039" they don't make any further difference, but most of the ones I see have calcium in the holes, making them smaller than normal. They are usually 0.035" in the 750 carbs K2-K6/F0, and should be .0375" or even 0.039" if you run a deep bowl (like I do).

[Nic]

If I can add more variety to this thread, and maybe help someone else, I've been wrestling with my K2, it has 657A carbs, needle in the mid position, 110 mains, 40 slows, stock air box and paper filter, floats at 26mm and pretty open 4/2 exhaust. I've been trying to stop the decel popping, I was having the stumble off idle from the lights etc but it's not a real problem compared to the popping.
After Mark saying that turning out the screw richens the mix because it adds more air thereby adding more fuel, I've been trying to figure out whether I need bigger or smaller pilot jets, turning the screws out does nothing to remedy the popping, right now I have them 1/4 turn out and the popping has improved unless I down shift at higher revs, then I get a bit of burbling but it's much improved. So just to help me and maybe someone else, before I shell out for new pilot jets, do I need a 42 pilot jet now or 35/38, If I listen to every body else I'm figuring 42? but according to Mark's theory, I have leaned the mix by turning in and would now want smaller pilots, it does my head in I tell ya.
Cheers.

[TwoTired]

Might be the one thing Mark and I disagree on; that air screw operation.

The pilot circuit design is the same as on the early CB550's.  I used an exhaust gas analyzer to prove that the air bleed screw leans when turned out and enrichens when turned in.  That experience also taught me to adjust the pilot circuit over rich, so as to have enough fuel for engine pick up when the slides are lifted and the vacuum drops.  If you enrich too far, you can grab more throttle, but the plugs tend to soot up at idle.


Btw, later carbs with an IMS work the opposite way.  But, those screws meter flow where it exits into the carb throat, not the air feeding the pilot circuit.

FWIW

[Jimray23]

This plug shows that you need to retard your ttiming 2 degrees for optimal performance.

[Nic]

Might be the one thing Mark and I disagree on; that air screw operation.

The pilot circuit design is the same as on the early CB550's.  I used an exhaust gas analyzer to prove that the air bleed screw leans when turned out and enrichens when turned in.  That experience also taught me to adjust the pilot circuit over rich, so as to have enough fuel for engine pick up when the slides are lifted and the vacuum drops.  If you enrich too far, you can grab more throttle, but the plugs tend to soot up at idle.


Btw, later carbs with an IMS work the opposite way.  But, those screws meter flow where it exits into the carb throat, not the air feeding the pilot circuit.

FWIW

I had a spare set of 40s and I found a drill bit that measured 42 on my calipers, I know, not exactly accurate but I just put them in and am about to go on a test ride, whether they are now 42 or 45 or whatever, this will prove to me at least what's what, I shall report back soon. Thanks.

[Nic]

Alrighty then, back from a wonderful test ride, I set the screws at 1 turn out, no popping, just a nice burble as you slow down, did some decels in 3rd gear at 6 grand n smooth as silk. I guess that answers that, out is lean in is rich.

[markreimer]

Cool, please report back. I'm about to experiment with my 750 jetting as well, leaning it out specifically, to get rid of that last little hint of the stumble


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Have you opened up the little aeration holes in the emulsifier tubes above the mainjets? For ethanol-laced fuels (or our modern slow-burning unleadeds), it helps to add a little more air in the mix at the midrange. Once past 0.039" they don't make any further difference, but most of the ones I see have calcium in the holes, making them smaller than normal. They are usually 0.035" in the 750 carbs K2-K6/F0, and should be .0375" or even 0.039" if you run a deep bowl (like I do).

I haven't tried that yet but have heard you suggest it a few times. Maybe in your book too. I don't ride with ethanol fuel though. I'm lucky that ethanol free gas is available everywhere here!

My gurgle happens only when I have just enough pressure on the throttle to be off idle. The slides will have lifted maybe 1mm or so. Imagine you're riding in a parking lot, first gear, at a steady 3-4K rpm. That's when it gurgles like mad. If I'm accelerating normally, it's smooth all the time.

It's my 'immediately off idle' setting I'm working on. Any suggestions? IMS screws don't seem to help. Thinking it's maybe running a tad rich there. I was going to drop the needle one notch to start


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[HondaMan]

Cool, please report back. I'm about to experiment with my 750 jetting as well, leaning it out specifically, to get rid of that last little hint of the stumble


Sent from my iPhone using Tapatalk

Have you opened up the little aeration holes in the emulsifier tubes above the mainjets? For ethanol-laced fuels (or our modern slow-burning unleadeds), it helps to add a little more air in the mix at the midrange. Once past 0.039" they don't make any further difference, but most of the ones I see have calcium in the holes, making them smaller than normal. They are usually 0.035" in the 750 carbs K2-K6/F0, and should be .0375" or even 0.039" if you run a deep bowl (like I do).

I haven't tried that yet but have heard you suggest it a few times. Maybe in your book too. I don't ride with ethanol fuel though. I'm lucky that ethanol free gas is available everywhere here!

My gurgle happens only when I have just enough pressure on the throttle to be off idle. The slides will have lifted maybe 1mm or so. Imagine you're riding in a parking lot, first gear, at a steady 3-4K rpm. That's when it gurgles like mad. If I'm accelerating normally, it's smooth all the time.

It's my 'immediately off idle' setting I'm working on. Any suggestions? IMS screws don't seem to help. Thinking it's maybe running a tad rich there. I was going to drop the needle one notch to start


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A couple things come to mind in this particular mix range: if the pilot jets have been replaced with the ones found in Keyster kits since 2008, there are 2 less emulsifier holes in those pilots. Also, the 657a carbs should have an air screw with the hole in the very tip of it: likewise the Keyster kits had solid screws, until a couple of years ago: now all of them have the extra hole (even though the #675b and later ones don't). Poor Keyster...

Another: if the float level in the 657a is set at 26mm, it is too low by a bit. This makes the pilot circuit unable to respond to the just-off-idle speed range steadily enough: it is still running the almost 10:1 idle mix until the airflow reaches enough speed to drop the vacuum at the tip. It should be trying to lean out to about 12:1 then, but it is too wet. This makes it "8-stroke", as we used to call it (on 2-strokers we dubbed it "4-stroking") because it only fires every other cycle until enough extra air shows up to let it burn again. Part of this is due to the natural spitback up the intake tract found in the Honda Four design at RPM below 2500 (also known as the "decel pop range" if too rich or wet). One 'fix' for this is to set the air screws leaner yet (3/4 turn), but this must be accompanied with slowing the spark advancer curve, or the bike will have a nasty slow spot (especially cold) to about 1500 RPM that ends with a lurch forward, not handy in traffic! The spitback pulse is caused during overlap when the [normally lean] mixture left in the chamber gets touched off by the waste spark while both valves are still slightly open: if the idle circuit is just rich enough to make this more than a tiny event, it burns back past the head, into the rubber tube range, which then must be filled in by the next cycle. This also slows down the air velocity across the pilot jet, which makes it mix richer than the actual engine speed, so the FOLLOWING cycle is almost 8:1 mix.

..I know that's a lot to digest, but you have to think in terms of just one cylinder, which is being dragged along by all the others, so the engine speed does not immediately change like on a Single or 360-degree Twin: those tell you much more quickly when the idle circuit you're messing with is out of whack. It is this same 4-cylinder action that confuses exhaust analyzers: when adjusting a screw inward until it reaches the point where the mixture is too lean to fire, the other cylinders drag that one along anyway, and it spits out it that unfired gas to the pipe(s), showing up as unburned hydrocarbons, which in CAR engines usually indicates a too-rich mix (as just one carb feeds all cylinders). Here in these engines, it is a little more complicated to analyze because of this phenomenon.

[markreimer]

Cool, please report back. I'm about to experiment with my 750 jetting as well, leaning it out specifically, to get rid of that last little hint of the stumble


Sent from my iPhone using Tapatalk

Have you opened up the little aeration holes in the emulsifier tubes above the mainjets? For ethanol-laced fuels (or our modern slow-burning unleadeds), it helps to add a little more air in the mix at the midrange. Once past 0.039" they don't make any further difference, but most of the ones I see have calcium in the holes, making them smaller than normal. They are usually 0.035" in the 750 carbs K2-K6/F0, and should be .0375" or even 0.039" if you run a deep bowl (like I do).

I haven't tried that yet but have heard you suggest it a few times. Maybe in your book too. I don't ride with ethanol fuel though. I'm lucky that ethanol free gas is available everywhere here!

My gurgle happens only when I have just enough pressure on the throttle to be off idle. The slides will have lifted maybe 1mm or so. Imagine you're riding in a parking lot, first gear, at a steady 3-4K rpm. That's when it gurgles like mad. If I'm accelerating normally, it's smooth all the time.

It's my 'immediately off idle' setting I'm working on. Any suggestions? IMS screws don't seem to help. Thinking it's maybe running a tad rich there. I was going to drop the needle one notch to start


Sent from my iPhone using Tapatalk

A couple things come to mind in this particular mix range: if the pilot jets have been replaced with the ones found in Keyster kits since 2008, there are 2 less emulsifier holes in those pilots. Also, the 657a carbs should have an air screw with the hole in the very tip of it: likewise the Keyster kits had solid screws, until a couple of years ago: now all of them have the extra hole (even though the #675b and later ones don't). Poor Keyster...

Another: if the float level in the 657a is set at 26mm, it is too low by a bit. This makes the pilot circuit unable to respond to the just-off-idle speed range steadily enough: it is still running the almost 10:1 idle mix until the airflow reaches enough speed to drop the vacuum at the tip. It should be trying to lean out to about 12:1 then, but it is too wet. This makes it "8-stroke", as we used to call it (on 2-strokers we dubbed it "4-stroking") because it only fires every other cycle until enough extra air shows up to let it burn again. Part of this is due to the natural spitback up the intake tract found in the Honda Four design at RPM below 2500 (also known as the "decel pop range" if too rich or wet). One 'fix' for this is to set the air screws leaner yet (3/4 turn), but this must be accompanied with slowing the spark advancer curve, or the bike will have a nasty slow spot (especially cold) to about 1500 RPM that ends with a lurch forward, not handy in traffic! The spitback pulse is caused during overlap when the [normally lean] mixture left in the chamber gets touched off by the waste spark while both valves are still slightly open: if the idle circuit is just rich enough to make this more than a tiny event, it burns back past the head, into the rubber tube range, which then must be filled in by the next cycle. This also slows down the air velocity across the pilot jet, which makes it mix richer than the actual engine speed, so the FOLLOWING cycle is almost 8:1 mix.

..I know that's a lot to digest, but you have to think in terms of just one cylinder, which is being dragged along by all the others, so the engine speed does not immediately change like on a Single or 360-degree Twin: those tell you much more quickly when the idle circuit you're messing with is out of whack. It is this same 4-cylinder action that confuses exhaust analyzers: when adjusting a screw inward until it reaches the point where the mixture is too lean to fire, the other cylinders drag that one along anyway, and it spits out it that unfired gas to the pipe(s), showing up as unburned hydrocarbons, which in CAR engines usually indicates a too-rich mix (as just one carb feeds all cylinders). Here in these engines, it is a little more complicated to analyze because of this phenomenon.

Very interesting bit of advice here. Thanks! My entire carb is built with genuine keihin bits. Learned that lesson early thankfully.

I had recently confirmed floats at 26mm. Outside of the advance 'mod' you referenced, is there a better float height? I was going to stick with the usual 26 base setting followed with the clear tune method to tweak till I was 1-2 mm under the gasket surface


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[TwoTired]

Guess I need to be more specific.  I did the exhaust gas analyzer test on a CB550 with a 4 into 4 exhaust.  When I turned out the air screws, I did so on all the carbs.  There was no dragging along of one cylinder.  They were all still firing evenly, with even heat on the exhaust.

I actually turned all 4 air screws out in equal steps until the Analyzer read lowest hydrocarbons, and that was 5-6 turns out from seated.  I expect this was the limit of the adjustment range of the air screw.  I did have to compensate the main idle knob to get the idle speed back down to 1K when turning the air screws outward.  This was done interactively each time I opened the air screws farther.   Engine idled beautifully on the centerstand.  Eager to take the bike out for my super tuned test ride. I tried to set off.  Just about any application of throttle under load, just made the engine wheeze with no power.  It was just unrideable.  I nursed it back to the garage slipping the clutch, where I began turning all the air screws inward equally and then testing the driving result. Each inward turn made the throttle response under load better.  After several adjust and try cycles, I had the bike able to accept 1/2 throttle twist under load in any gear and accelerate smoothly and predictably.  I could ride again.

Checking where the final air screw setting ended up.  I counted 1 and 1/2 turns out from seated.  Exactly what the Honda shop manual said to set them to nominally. (raises eyebrows)

I rechecked the exhaust gas analyzer reading in each exhaust, and they were the same across the pipes.  And way more hydrocarbons than what I was able to tweak out at 5-6 turns.

The following is an excerpt from publication "Honda Motorcycle Carburetion - Tech Series" Copyright 1975 , American Honda Motor Co. Inc. Motorcycle Service Department.

Seems pretty clear to me how to adjust air screws and how they function.

FWIW

[HondaMan]

The following is an excerpt from publication "Honda Motorcycle Carburetion - Tech Series" Copyright 1975 , American Honda Motor Co. Inc. Motorcycle Service Department.

Seems pretty clear to me how to adjust air screws and how they function.

FWIW

Yep, this US Honda manual, and an excerpt from their Singles manual (50/70/90/100/125 engines) also showed this same statement, but it was mistranslated from the Jinglish found in their original manuals. The Jinglish statement read like "...for richness, turn screw inward..", which, when taken in the rest of the context of the Japanese, was meant as a remedy, like: "..if too rich, turn screw inward..". This statement was a common recommendation based on the cause: the Singles in particular tended to vibrate their air screws outward over time (the Fours, not so much), so the bikes would start fouling plugs from slowing points timing (gaps wearing down) and unscrewing air screws. We saw it a LOT on the bikes that I worked on from 1968 thru 1978 (when I quit working in shops 'cuz I got married and needed a better-paying job, and went to engineering). Our Honda reps confirmed this at the time, too: both the one from Peoria, IL and the ones who visited here in the Denver metro area in those days, as there was confusion then, too.

[PeWe]

That plug look really black inside as well as the air/fuel ring.
- This does not matter?  I'm curious to know since I have plugs with black inside, isolator is OK and the strap too.
- The strap (ground electrode) seems to have the color change in the corner as it should have (maybe a little bit up to the tip) which indicate ignition correctly set, right?
- About the air screw... it work like a paint spray gun? Less air, less paint coming out. No air-no paint at all...

I've changed jets and use the butt dyno, listening for lean take off's and not smelling rich at idle. Check throttle at high speed if bike will wake up when release a little from WOT which for me is too small mains.....

One important carb/jet check that is easy to forget is normal cruising speed for a longer period of time. 70-120km/h for at least 100km. The throttles are almost not opened, mostly pilot circuit. Check plugs after that.

I have had way too lean carbs that had good result on dyno at WOT while it almost could cause a meltdown in the most used throttle lift.

[TwoTired]

Yep, this US Honda manual, and an excerpt from their Singles manual (50/70/90/100/125 engines) also showed this same statement, but it was mistranslated from the Jinglish found in their original manuals. The Jinglish statement read like "...for richness, turn screw inward..", which, when taken in the rest of the context of the Japanese, was meant as a remedy, like: "..if too rich, turn screw inward..". This statement was a common recommendation based on the cause: the Singles in particular tended to vibrate their air screws outward over time (the Fours, not so much), so the bikes would start fouling plugs from slowing points timing (gaps wearing down) and unscrewing air screws. We saw it a LOT on the bikes that I worked on from 1968 thru 1978 (when I quit working in shops 'cuz I got married and needed a better-paying job, and went to engineering). Our Honda reps confirmed this at the time, too: both the one from Peoria, IL and the ones who visited here in the Denver metro area in those days, as there was confusion then, too.

Very interesting story.  But, it's STILL in direct conflict with personal experience and testing with the CB550.  I will defer to those with direct experience with the 750's early carbs, since I have no direct interactive experience with those carbs, (and I'm glad all my 750's have PD carbs with a different pilot circuit).    But, apparently personal lore trumps the books, and books are all wrong about the carbs operating on the same principles as the CB550 carbs.  I'll stand fast on CB550 carbs 627B, 022A, 069A, and 087a Carbs, which all behave the same regarding the air screw, inward for richer outward for leaner.  I've got hands on tuning experience with those.  Further, I have been tuning carburetors all my life, and I can hear when and engine cries for more fuel, needs it to accelerate, or shows color on their spark plugs, and what it means for air screw adjustment.   Sometimes, all you have to do is add a bit of choke butterfly, which richens the mix (in my experience).  If the engine pickup improves the mix from that without choke, it needs more fuel.  Turn the air screw to achieve that.  If it burbles on acceleration (engine clearing the over rich condition) with added choke, then turn the air screw to reduce fuel input.

Very sad that many will still be confused about air screw adjustment, and not be able to predict what turning that screw does for engine operation.

I just cannot buy the confluence of events that reads like an interesting SCi-FI novel regarding air screw operation on early CB750 carbs.  I remain in denial.

Caveat Emptor!

[Nic]

Yes well my recent experience also confirms in for rich.

[jonda500]

100% I have to agree, I am now absolutely, completely convinced that turning my air screws in makes the mixture RICHER and turning them out makes it LEANER,
John