Post-top end rebuild issues: racing idle, hot cylinder

[Forden]

Hi All,

Two issues on the table for your consideration. One i think I have a good prospect to correct, one I'm still speculating.

1. With bike warmed up at idle, after revving throttle, RPM's race up and keep going. won't return to idle speed.
Suspected cause: centrifugal advance. Had this problem with my CB. Tightening springs (which were weak) fixed the issue in that case. Springs seemed good in this case, but advance assembly was void of lubrication and definitely needed service and didn't move that freely. Did that and will report back after I get a chance to test (need to replace broken circlip). No sign of throttle cable binding, snaps back as it should.

2. #3 cylinder is running significantly hotter than the others, by 100-150 degrees.
Suspected cause: uncertain. First obvious thought was a too lean mixture, seems like too much temperature difference for just air pilot screw adjustment to correct (all were set at 1 1/4 turns out to start after rebuild). I've checked for air leaks at manifolds (which are new) and throttle shafts, starting fluid squirted in and around with no effect on RPM. Still need to tune idle/air screws for highest idle and set timing, and sync carbs (bench sync done), but this big of a temperature difference indicates something needs addressing in #3 before running it too much, I think.

Carb/fuel notes:

• stock carbs with stock jets fully cleaned and rebuilt, OEM parts used for anything needing replacement (needle valves/seats, pilot jets)
• stock airbox
• diaphragms looked supple with no tears
• bike idles pretty smoothly and seems to run well even pre-tuning/carb sync, except for racing high after throttle has been opened/revved (ie. #1).

Please add to my list of things to check after considering above, and thank you! Getting closer every day to putting this girl on the road.

Cheers,
Mike

[bikerphil]

Have you properly synced with a set of gauges? Bench sync may not be good enough.

[motoman]

Have you properly synced with a set of gauges? Bench sync may not be good enough.

100percent Agree with bikerphil. Carbs HAVE to be synced in order to solve the issue it's having. All four cyls. doing the same 'work' is highly important.

[Forden]

Thanks, I'll sync them with my carbtune as soon as I get the racing throttle sorted. Hopefully the reconditioned advance will cure that...

[jkorn]

Don’t think it’s the advance. Years ago I removed one of the springs, playing around with acceleration. Don’t remember it causing your issue. But then that was back in the 80’s……….

[bikerphil]

Thanks, I'll sync them with my carbtune as soon as I get the racing throttle sorted.

The carbs being out of sync may be the cause of the racing throttle.

Question: When rebuilding the carbs, were the throttle plates removed from the carbs? If they were, they may be misaligned upon installation preventing them from fully closing.

[Forden]

The carbs being out of sync may be the cause of the racing throttle.

Question: When rebuilding the carbs, were the throttle plates removed from the carbs? If they were, they may be misaligned upon installation preventing them from fully closing.

It is racing up to 5K and climbing before I shut it down, and only after being warmed up and throttle given--I didn't think sync could cause that, but we'll see. I did not remove the throttle plates, no.

[Forden]

Don’t think it’s the advance. Years ago I removed one of the springs, playing around with acceleration. Don’t remember it causing your issue. But then that was back in the 80’s……….

I think the mechanism could have stuck after advancing. And shortening the spring on my Honda fixed an almost identical issue (well documented on those bikes). But we shall see! Hoping to check tonight.
Thanks all.

[jkorn]

Throttle cable? You mentioned after “throttle given”. With engine off do they open and close with no restriction? Nice strong return spring

[Forden]

Throttle cable? You mentioned after “throttle given”. With engine off do they open and close with no restriction? Nice strong return spring

yep, snaps shut strongly. newly serviced advance didn't solve it either, as you predicted. i'll get the carbs synced today and see where I'm at. meantime it's almost warm enough here to be comfortable, so I'm going for a ride!

[jkorn]

So, throttle controls butterfly gates between carb and intake. Once the are almost closed I don’t see how you could get a run away idle with limited passage. During rebuild did you breakdown the linkage between all carbs? Can you see that all four carb controls move the same? Hummmm

[LAB3]

I'm following jkorns thinking on the misaligned butterflies, if one or more of those is off a tad it could very easily bind up in the bore of the carb and since you seem to have addressed the other possible issues it makes a good bit of sense.

[Forden]

Thanks guys, I follow that logic, and yes, carbs were completely disassembled--linkage should be fine, and I didn't notice any of the butterflies hanging up when they were apart. But doesn't mean it isn't happening now. Loving the prospect of the PITA of getting the airbox out (and back in again after checking on that)... I'll recheck everything including vacuum/sync before going that route.

[Forden]

Carbs are off and butterflies don't seem to be hanging up at all, but I'll re-center them anyway. found another useful post suggesting a strong light through the bore to be sure when closed no light is getting through.

I'm considering throttle shaft seals as the next likely culprit. I read that when they're old they seal fine initially but as the carb bodies warm and expand, the seals don't do their job and voila, vacuum leak.

[cajun31]

Carbs are off and butterflies don't seem to be hanging up at all, but I'll re-center them anyway. found another useful post suggesting a strong light through the bore to be sure when closed no light is getting through.

I'm considering throttle shaft seals as the next likely culprit. I read that when they're old they seal fine initially but as the carb bodies warm and expand, the seals don't do their job and voila, vacuum leak.

when I rebuild the carbs I always replace the throttle shaft seals. Just gives me piece of mind. Quick tip to make sure the butterflies are seated properly is to take the carbs off the bike and lightly loosen all of the butterfly screws. Then snap them open and closed a few times. I then take the screws out one at a time and put a small amount of loctite blue on each screw before reseating them. I would then re bench synch them. One other thing I do if you are not real sure concerning the idle mixture screw setting is to start with them all set at about 2.5 turns out from lightly seated. This always gives me a good starting point when I remount them to the bike. Generally I don't have to worry about racing carbs etc... unless I missed something else. I then synch them, followed by colortune on each carb and then another synch. One other thing you might want to do or at least check while you have the carbs off the bike are the intake manifolds. Do you have gaskets under the manifolds against the engine. If you do, then remove them and use a small amount of Yama bond or something similar on the face of the manifold before mounting them. Will solve any leaks you might be having. Just something I do. Yeah I have chased that one before. Sounds like you are very close.