Topic: CB550 stalls while coasting

[SamP]

Well, I thought I had sorted my carb issues, but now I don't know. Long story short, I was having issues with uneven, inconsistent idles - hanging idles and creeping idles - and after adjusting the float levels and putting the older, softer springed float needles back in, these issues went away.
Pertinent info - 1977 cb550k, stock airbox, foam filter, PD46a carbs (45 slows and 100 mains, mixture at 1.5 turns out), mac 4into1.
After fixing the idling issue, a new one cropped up. Now I have the problem that often if I pull in the clutch to coast through a turn, or to a stop (especially after going uphill), the engine will almost immediately cut out and die. However, if I keep the transmission in gear as I go through the turn, or to the stop (not always possible), it will not cut out and continues running at the given RPM it was going at.
I've tested it by revving the engine on the center stand for a few seconds (5-8) to mimic the fuel drain that I would be experiencing while actually riding, both in neutral and in first with the clutch pulled in, and then letting off the throttle abruptly and the engine keeps idling at ~1000 (that's where it idles at stably, any lower and it feels like it wants to cut out and often does).
Any ideas what's going on? I've tried letting the mixture screw OUT to enrich the mixture to see if that would do anything, but at 2 full turns out it doesn't change anything. I've also tried raising the floats a bit, but then my idle problems came back - irregular, hanging idle, etc. I've checked the gas level in the bowls with the clear tube method, and in all 4 it reaches the idle jets no problem. It feels like the fuel is not getting to the bowls fast enough to keep the gas level high enough through regular running, but I have to admit, I am not sure how fast they should fill up (on average, after draining the bowls and then turning on the gas, they seem to fill up completely within about 5 seconds). New fuel line goes directly down to the carbs, no filter or anything (except in the tank over the petcock), so I don't think that's the issue. Gas cap vent is functional, carb drain tubes are clear. Not sure if it's pertinent, but gas is from last fall, and I put in stabilizer, so it shouldn't be bad.
Thoughts? At a loss here....

[Jimsun]

Seems like a faulty carb intermal or mechanic. Can u pull the carbs out and see the internal?

[calj737]

First, I commend you on the clarity of your post. Excellent info, details and diagnostic process. You answered every question I was about to ask until I read on.

I would ask this clarification: when testing on the center stand, will it cut out if in neutral as you scribe happens while riding? And have you bumped the idle up to 1200-1400 and re-tested? This is more for curiosity than diagnosis.

[flybox1]

Set your floats so your fuel level is correct (3mm+/- below the gasket seam) ....and then leave them alone!!!
It needs to be correct, and constant. 
Did you ever do a vacuum sync of your carbs after all your tune up and troubleshooting was done?  You didn't mention it...
We can only assume that your emulsion tube and pilot jet holes are free of debris, and all other air and fuel passages are clean and clear.

When you let of the throttle your slides quickly close, so your bike has to rely on the correct mixture and vacuum in an instant.
If your vacuum is unbalanced, or your mixture is off, your bike will starve for air or fuel and have trouble finding and maintaining idle.

IIWM, id go for a ride to get a new set of plugs, ride home, put them in, and under a box fan, let it idle for 5 minutes, and them pull the plugs.
I'd want to see the plug deposits to be sure my idle circuit mixture is correct. 

[SamP]

Seems like a faulty carb intermal or mechanic. Can u pull the carbs out and see the internal?

Pulled the carbs last fall and cleaned it all out, checked passages, etc. All clear. No gum, no gunk, nothing visibly wrong anywhere in the bodies.

First, I commend you on the clarity of your post. Excellent info, details and diagnostic process. You answered every question I was about to ask until I read on.

I would ask this clarification: when testing on the center stand, will it cut out if in neutral as you scribe happens while riding? And have you bumped the idle up to 1200-1400 and re-tested? This is more for curiosity than diagnosis.

Thanks, I tried to get all the important stuff there so the forum doesn't need to ask a million questions to make an actual assessment. I get frustrated when I read posts that only list the problem and nothing else, so I try not to be that guy (doesn't always work out).
 Answer is no, it does not cut out on the center stand, nor the side stand if memory serves. I have not tried bumping up the idle and retesting, but I can try that. I know the spec idle is 950, so I try to keep it as close to that as I can, but I'll give that a shot. Given how quickly and firmly it stalls out (within a second or less, usually), I am not sure this will change anything, but I'm open.

Set your floats so your fuel level is correct (3mm+/- below the gasket seam) ....and then leave them alone!!!
It needs to be correct, and constant. 
Did you ever do a vacuum sync of your carbs after all your tune up and troubleshooting was done?  You didn't mention it...
We can only assume that your emulsion tube and pilot jet holes are free of debris, and all other air and fuel passages are clean and clear.

When you let of the throttle your slides quickly close, so your bike has to rely on the correct mixture and vacuum in an instant.
If your vacuum is unbalanced, or your mixture is off, your bike will starve for air or fuel and have trouble finding and maintaining idle.

IIWM, id go for a ride to get a new set of plugs, ride home, put them in, and under a box fan, let it idle for 5 minutes, and them pull the plugs.
I'd want to see the plug deposits to be sure my idle circuit mixture is correct. 

When I re-set the fuel level at that height, the idle issues came back - running rich, hanging idle, flooding the engine leading to stalls when on the center stand and pivoting the front tire (or any other sort of movement). I may have set them too high, but it was difficult to gauge since the fuel level rises ever so slightly with the floats sitting in the gas. According to the clear tube method, it's not far off now, but any higher and those issues crop back up. Can take a photo if needed. I can work on dialing those in as close to 3mm as possible, but if idle suffers, I'll just have to drop them again.

I did vac sync them, and they're all even now. I checked all the passages last fall and then last time I took them off (month ago) and all was clear. Can/will video where carbs are synced at if it helps.

Plugs are already new (<20 miles on them, any deposits would be from that, idling, and syncing), as are caps.

Forgot to mention, the clutch discs are new, as are springs. Plates are old, but were in good condition. Discs sat in oil for a month or more before install. Could sticky clutch discs be a culprit in stalling in gear with clutch pulled? Thing is, I don't think they're sticky, and the clutch doesn't drag.

[flybox1]

show us what your plug tips look like please.  line them up 1-2-3-4
dragging clutch plates shouldnt have an effect.  you will however, see rear wheel spinning while in N on the center stand.

[SamP]

Will do, but out of time for more tests today. Cold as-is or after a bit of idling?

[flybox1]

Will do, but out of time for more tests today. Cold as-is or after a bit of idling?

if the majority of the deposits on the plugs are from driveway idling, as-is will do.
if you've been riding, the deposits will be cumulative from those throttle positions.

[SamP]

So do you mean to say the clutch should move the back wheel in neutral, or should NOT? I do get some back wheel movement, but nothing I can't stop with my hands, and it doesn't affect idle if I do. Mechanic friend told me some movement is normal, but I couldn't tell if you mean to say that this indicated a dragging clutch.

[flybox1]

Yes, a little spin is ok.
It should be minimal once the bike has been ridden to operating temps.
If its still whirling after a good ride, there are a few things needing attention.

[harisuluv]

I haven't read through everything in detail, but seeing as how problems are coming and going away in relation to setting the float height (fuel level) and your hypotheses that the bowls seem to not be "filling up" fast enough, it's possible your float height might be correct, but there is an additional tang on your floats:  the other one dictates how far they will hang down, IE how "open".  This is the tang that contacts one of the float posts on one side and acts as a "stop."  You can set each float to only open up a negligible amount, all the way to where the float seems like it will fall out (not as relevant on your pd carbs with the valve clip).  Granted, there isn't going to be much difference between about to fall out and 60% less open if any.  But it's something to check, it wouldn't be crazy for some PO to have misadjusted them.

Note:  CB350F and CB400F floats do not have these tangs for "float drop"

[SamP]

Well, I got a few minutes in before the sun set, so I got the chance to try some stuff.
I tried upping my idle to ~1600, and then sped around my street, drove up my driveway and pulled in the clutch. RPMs dropped to just under 1000, and I thought it was going to stall, but after that dip it recovered without any throttle blips.

Re-set the idle back to 1000-1100 where it's stable, and tried CLOSING the mixture screw down to one turn open, thinking that possibly my mixture is too rich. Down the street, back up the driveway, pulled the clutch and it stalled.

Pulled the plugs. They all look pretty black (tough to see in the photos since it was getting dark). Took a few to compare, #3 seemed wet with gas, though it could have been oil (may or may not have installed an old ring upside down when I put the top end back together and was unable to verify the UP on the ring, but I'm 90% sure it was facing the right direction - that's another possible issue for another day).

Pulled the plugs.


Plugs 1 (left) and 2


Plugs 3 (left) and 4


Checked fuel level in the bowls. Here's #1


Here's #2


#3


And #4


For what it's worth, this all started with me chasing down some weird clunking/stuttering coming from the engine on the left side. Split the cases, swapped out the countershaft and bearing in the transmission, primary chain, cam chain, tensioner slipper and then putting it all back together. Of course that weird clunking is still there. Hard to describe, but the engine sounds like it's laboring to crank somewhere/somehow - very pronounced when the idle gets low. I'll try and upload an updated video of how it sounds tomorrow.

[harisuluv]

looks awfully rich, maybe even an oil leak too? 

I saw that your slow jets are 45, no reason to go higher than stock with your setup, too rich.  42 should be stock.  maybe go back to a 90 main.  with just a 4-1 you might not need any jetting changes.

Also looks like your airbox boots are really sagging, something going on there, probably aren't optimally sealing in that condition.

PS:  kudos for the details and pictures of your setup!

[SamP]

Totally possible on the oil leak, but I was gonna wait until the new rings set before I jumped to any conclusions.

Anyway, one issue at a time. I've get my plate full as is.

Airbox boots are new and tight, no leaks. Looks worse than it is, but I've got new screws on them so they tighten nicely. As for the jets, I went by the carb jetting equation - foam filter, mac exhaust are my only additions. But if it's still running rich, I suppose it wouldn't hurt to try the old jets and see if that solves the mixture. I just wonder if that will also solve the stalling while coasting issue. No chance closing the mixture screw more could replace swapping out the jets, is there? Not that I'm being lazy, but I noticed no change in the coasting stall out when I turned the screws in a half turn.

[HondaMan]

Those plugs are fouled, need new ones (or sandblast those clean?).
Consider this: the O-rings in the carb castings that bolt the hoses to the head, if you have not changed them, are rock-hard now and leak. Vacuum leaks with these pulse-feed carbs make the mixture very rich (unlike a constant-vacuum car engine) because the airflow across the jet(s) slows down if there is vacuum leak downstream. Since these carbs idle at 12:1 mix ratio, leaning out toward 14:1 at about 1/4 throttle, low speeds are always rich.

Another point of vacuum leak: the hose clamps on the rubber boots. If you removed the carbs and disturbed their years-old seal, chances are GREAT they don't seal now. There several ways to fix it: tighter clamps, new hoses, soak your exisitng ones in Xylol with wintergreen oil added (for extended life), or better clamps like the Oettikers.

[SamP]

Those plugs are fouled, need new ones (or sandblast those clean?).
Consider this: the O-rings in the carb castings that bolt the hoses to the head, if you have not changed them, are rock-hard now and leak. Vacuum leaks with these pulse-feed carbs make the mixture very rich (unlike a constant-vacuum car engine) because the airflow across the jet(s) slows down if there is vacuum leak downstream. Since these carbs idle at 12:1 mix ratio, leaning out toward 14:1 at about 1/4 throttle, low speeds are always rich.

Another point of vacuum leak: the hose clamps on the rubber boots. If you removed the carbs and disturbed their years-old seal, chances are GREAT they don't seal now. There several ways to fix it: tighter clamps, new hoses, soak your exisitng ones in Xylol with wintergreen oil added (for extended life), or better clamps like the Oettikers.

Noted on the plugs. I'll replace those once I get her running smoothly and cleanly. They have probably about an hour running currently, so that will wait until I've got the rest of the bike optimal before I replace them (...again...).

I replaced all four gaskets under the intake manifold when I did the top end, carb boots are new, and carb-airbox connectors are new. Screws on the clamps are new, clamps are old, but seal well. Sprayed some carb cleaner around all connections on #4, and nothing. Haven't sprayed the others, because I was able to get an even sync, and nothing else leads me to believe I have a vac leak anywhere, especially since the plugs are saying that it runs rich. What's making you think vacuum leak?

[harisuluv]

What mark is saying that a rich condition on our "pulse type" carbs can actually lead to a rich condition, contrary to what you might think (vacuum leak = lean condition). 

Also, I know you want to replace the plugs after it gets running smoothly, however in order to get it running smoothly it's going to need new plugs.  Might want to get a couple sets.

[flybox1]

If you re-read hondamans first paragraph, he explains the possible richness causes.   
Per harisuluv, swap out pilot jets, and add in new plugs.
Noticing all the oil on the threads of your plugs, especially #2, would lead me to think oil leak as well...

Fuel levels in #3/4 are a bit low.  Lower both floats by 1mm when you're in there next.

[calj737]

If you re-read hondamans first paragraph, he explains the possible richness causes.   
Per harisuluv, swap out pilot jets, and add in new plugs.
Noticing all the oil on the threads of your plugs, especially #2, would lead me to think oil leak as well...

Fuel levels in #3/4 are a bit low.  Lower both floats by 1mm when you're in there next.

You mean lower 1/2 to be equal to 3/4, right Fly?

Samp- you replaced the jets with new ones. What is the brand and source for those jets? If they are not true Keihin jets, this may be an issue too. Replace the factory jets, install new plugs (theyre cheap and required) and equalize the fuel level for all 4 carbs. Replacing the plugs is required so the effect of the adjustments can be accurately interpreted.

[HondaMan]

Samp- you replaced the jets with new ones. What is the brand and source for those jets? If they are not true Keihin jets, this may be an issue too.

+1 !
I ran across this recently in some troublesome PD42 (750F) carbs. Grr...

[flybox1]

If you re-read hondamans first paragraph, he explains the possible richness causes.   
Per harisuluv, swap out pilot jets, and add in new plugs.
Noticing all the oil on the threads of your plugs, especially #2, would lead me to think oil leak as well...

Fuel levels in #3/4 are a bit low.  Lower both floats by 1mm when you're in there next.

You mean lower 1/2 to be equal to 3/4, right Fly?

Samp- you replaced the jets with new ones. What is the brand and source for those jets? If they are not true Keihin jets, this may be an issue too. Replace the factory jets, install new plugs (theyre cheap and required) and equalize the fuel level for all 4 carbs. Replacing the plugs is required so the effect of the adjustments can be accurately interpreted.

I guess it all depends on how the measurement is taken     I think the fuel levels in #1&2 are correct.  Lowering the float heights in these (i.e.  21mm -> 20mm as measured) would only make the fuel discrepancy greater.  I still suggest lowering the float height in #3&4 by 1mm (i.e.  21mm -> 20mm as measured), thus raising their fuel level to match #1/2.  Sorry if I wasn't crystal clear that my 'lowering' is towards the carb body, and not towards the bottom of the bowl. 

[SamP]

Honestly can't recall the make on the jets, but I got them from 4into1. Most likely not OEM, but I vaguely recall having difficulty sourcing OEM - this was last fall, and I was just happy to have some jets.

Should have a chance to lower the floats in 3 and 4, and install the old jets and some new plugs on Saturday.

[flybox1]

aftermarket 'kit' jets are notorious for complicating tuning issues. 
try jetsrus.com  They sell both Keihin, and aftermarket main and pilot jets. 
Their aftermarket jets are of a higher quality than other online kits, and while i typically buy OEM jets, I currently am using jetsrus aftermarket pilot jets, without issue.

[SamP]

aftermarket 'kit' jets are notorious for complicating tuning issues. 
try jetsrus.com  They sell both Keihin, and aftermarket main and pilot jets. 
Their aftermarket jets are of a higher quality than other online kits, and while i typically buy OEM jets, I currently am using jetsrus aftermarket pilot jets, without issue.

Thanks, I'll keep that in mind if I need to order new jets. Kinda bummed the ones I ordered aren't gonna cut it, but that's been the story of this rebuild thus far. Since it's running so rich I'm gonna see if the stock jets work, since they should lean out the mixture. I'll run it with the old plugs to see if that solves the stalling issue, and then with new plugs to see what color they turn and check for stalls - maybe get some good, ethanol free gas? There's a place by me that I can get it.

Weird to consider I might not need to rejet, with the current, albeit minor, mods I've made to it.

[calj737]

Your mods never indicated a need for jet changes. There's not much that's really breather a great deal more air into your system. I'd suspect that all along the issue of "rich" was maladjusted carbs, not incorrect jets.

It would also be really advisable to replace the plugs with clean plugs every time you foul them. Or at the least, clean the fouled plugs. Using a "weak link" trying to troubleshoot that system is making things much harder on your diagnosis.