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

[Schming]

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.

I also do all above procedures with great results only thing I wanted to add is the " bread tie " synch will get ya real close before installation of the carbs.

[jetmechmarty]

when I rebuild the carbs I always replace the throttle shaft seals.

I do the same. Those seals are likely over 40 years old!

[Forden]

I took them apart, and most of the seals practically fell out on their own, so safe to say they weren't doing their job. New OEM ones on the way. "Bread tie synch" is a new one on me, I have been using a guitar string. Same idea I reckon.

What's the consensus on whether to set the alignment of the butterflies and lock them down before or after tightening the nut securing the rest of the shaft assembly?

[Radioguylogs]

I believe it doesn't matter.

The disc is very accurately located by the attachment screws compared to to the side-to-side free play of the shaft.

[bikerphil]

IIRC, I would assemble the shaft, then insert the throttle plate, then install the screws with thread locker and tighten snugly, then back them off a bit, then snap the throttle a couple times and tighten down.

A JIS screwdriver is a must for this.

[cajun31]

IIRC, I would assemble the shaft, then insert the throttle plate, then install the screws with thread locker and tighten snugly, then back them off a bit, then snap the throttle a couple times and tighten down.

A JIS screwdriver is a must for this.

Too easy to mess up the screws. JIS is what you need to both remove and install those small screws. Phil has just described exactly how I make sure the plates are in place before tightening the screws down. Another thing that I have found very helpful is as you first remove those screws lightly tap the JIS screwdriver with a small mallet before trying to take the screw out. It serves to both seat the screwdriver and loosens some of the crud around the screw. I also take carb cleaner and spray each one to help loosen the crud.

[Schming]

Too easy to mess up the screws. JIS is what you need to both remove and install those small screws. Phil has just described exactly how I make sure the plates are in place before tightening the screws down. Another thing that I have found very helpful is as you first remove those screws lightly tap the JIS screwdriver with a small mallet before trying to take the screw out. It serves to both seat the screwdriver and loosens some of the crud around the screw. I also take carb cleaner and spray each one to help loosen the crud.

Yep ^^^exactly^^^ but one little tidbit I do is take a dremel and remove the peen from the end of those screws to lessen the chance of stripping the heads.

[Forden]

Too easy to mess up the screws. JIS is what you need to both remove and install those small screws. Phil has just described exactly how I make sure the plates are in place before tightening the screws down. Another thing that I have found very helpful is as you first remove those screws lightly tap the JIS screwdriver with a small mallet before trying to take the screw out. It serves to both seat the screwdriver and loosens some of the crud around the screw. I also take carb cleaner and spray each one to help loosen the crud.

Indeed. Those screws were soft! I did use JIS and one of the screws wouldn't budge even with heat and spraying--it stripped and I had to drill it to get it out. I bought new stainless ones and will be using those for install.

[Forden]

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.

Question about Colortune--do you use the long reach adapter? I don't understand how that works as it would appear to push the spark further up and away from the combustion chamber.

[Forden]

Still fighting this hanging throttle issue--a little exasperated. Observations and steps taken so far:

1. No evidence of vacuum leaks around (new) intake boots, or throttle shafts.
2. Throttle shaft seals are newly replaced.
3. I recentered the throttle plates when replacing the seals, but haven't ruled out that one of them may still be sticking slightly open.
4. Reconditioned/lubricated mechanical advance.
5. Throttle cable is not binding.

Next steps (I think, please tell me if I've missed anything):

1. Pull carbs again and really scrutinize whether any of them may hang up when allowed to close gradually. Did this before, but I'll do it again.
2. Failing that, try shortening one advance spring (worked on my CB).
3. Take Cajun's advice about pulling intake boot gaskets in favor of yamabond-type.

Any other thoughts? Someone suggested that tight valves could cause this (not seating properly) but I'm not sure I follow that.

thanks all.
Mike

[Forden]

Still fighting this hanging throttle issue--a little exasperated. Observations and steps taken so far:

1. No evidence of vacuum leaks around (new) intake boots, or throttle shafts.
2. Throttle shaft seals are newly replaced.
3. I recentered the throttle plates when replacing the seals, but haven't ruled out that one of them may still be sticking slightly open.
4. Reconditioned/lubricated mechanical advance.
5. Throttle cable is not binding.

Next steps (I think, please tell me if I've missed anything):

1. Pull carbs again and really scrutinize whether any of them may hang up when allowed to close gradually. Did this before, but I'll do it again.
2. Failing that, try shortening one advance spring (worked on my CB).
3. Take Cajun's advice about pulling intake boot gaskets in favor of yamabond-type.

Any other thoughts? Someone suggested that tight valves could cause this (not seating properly) but I'm not sure I follow that.

thanks all.
Mike

Getting back to this after a busy work season and other projects. I have done #'s 1 and 2 of my next steps--rechecked the throttle plates, not hanging up and are centered as they should be; and tried shortening one advance spring to no effect.

I haven't yet tried #3, ditching gaskets for yamabond, but will try that. If that doesn't work, I'll be ready to light the bike on fire, so if anyone has further ideas, PLEASE post, I'm listening...

Thanks as always for the help.

Cheers
Mike

[DiverRay]

You NEED to verify the valve adjustment is good BEFORE trying anything else! I always set valves on the large side of the gap, IE if it's .2 to .3, set to .3 or .35 as the close up as they wear.
Second is VACUUM SYNC after setting mixture. Carbtune will NOT do the same as a vacuum sync, and these four carbs like working together. You can carbtune after the sync if you feel you need to.
DO NOT play with advance other than clean and oil. If it doesn't work well, you will not notice except while riding or doing a timing check with a light over the RPM range.

[Forden]

You NEED to verify the valve adjustment is good BEFORE trying anything else! I always set valves on the large side of the gap, IE if it's .2 to .3, set to .3 or .35 as the close up as they wear.
Second is VACUUM SYNC after setting mixture. Carbtune will NOT do the same as a vacuum sync, and these four carbs like working together. You can carbtune after the sync if you feel you need to.
DO NOT play with advance other than clean and oil. If it doesn't work well, you will not notice except while riding or doing a timing check with a light over the RPM range.

Thanks Ray, I did the valve adjustment after the head went back on. Carbtune is a vacuum sync, and I did that as well--do you mean colortune maybe?

[DEEBS11]

The first think that comes to mind is # 3 cylinder has a terrible air leak. But the sync has been done and that did not show up on the meter. Slide diaphragm problem? Blown head gasket? Leaky rubber manifold? How did you sync the carbs when the motor was racing over 5K?. The sync has to take place at about 1100 rpms. That is not possible on a racing engine to a degree......this one is a thinker.

[Forden]

The first think that comes to mind is # 3 cylinder has a terrible air leak. But the sync has been done and that did not show up on the meter. Slide diaphragm problem? Blown head gasket? Leaky rubber manifold? How did you sync the carbs when the motor was racing over 5K?. The sync has to take place at about 1100 rpms. That is not possible on a racing engine to a degree......this one is a thinker.

Thanks. I thought the same thing about #3, but no change when ether is sprayed around the manifold joints, throttle shaft seals (or anywhere for that matter). I suspect the exhaust valve is leaking, causing the higher temp on that header. I may have to pull the head again and check that to find out. I did lap them but don't have a lot of experience with the process. I checked for leaks before putting the head back on.

It was idling correctly when I did the sync. It is maddening--I have gotten it to idle and rev perfectly well on the stand several times, though it has also showed the racing problem with the throttle cable disconnected, so it isn't that the cable is sticking. The one condition where it has manifested every time is on the road and under load.

Slide diaphrahm problem maybe. I must confess being a little hazy on the CV carb function as it's my first bout with them--I did inspect the diaphragms closely and they looked sound to me. Blown head gasket absolutely not.

The hunt goes on... I'm waiting for the moment when the simple and obvious cause reveals itself, so can bang my head for half an hour, then breathe a huge sigh and go ride her properly.