Sigh! Let's try this again. Yet another rebuilt carb issue.

It seems as though every time I fix a problem, another takes its place.

This is new it wasn't happening earlier in the day.

Carbs rebuilt. Bench synced. Idle/air screws at 3 turns. Floats seem to be causing lowish fuel level in the bowls.

The bike fires easy with choke. As it warms, I reduce choke until it is off completely. Bike idles about 1700. Great.

Now, most (not all) of time, If I just blip the throttle, it revs to about 3-4k. Just as the revs start to come down, the thing turns the other way and revs up to 6-7k and won't come back down. I have to use the kill switch (which sometimes causes a backfire) to shut it down. Restarting the engine starts the process over from 1700k idle.

I haven't let it run this way very long for fear of breaking something. It doesn't do this if the choke is on. Throttle and linkage all return to closed as they should. I haven't re-checked the diphragm piston since installation but they all passed the clunk test and I polished the bores until I could see mirror images in them.

What's left to look for? Once the carb hats are on and I lift the diaphragm piston and release it, how long should it take to hit bottom? I know the whole diaphragm setup will cause the piston to drop slower than the clunk test but how slow? Since it is forcing its way up from the 3-4k rpm range, could the diaphragm spring be so weak as to be allowing the piston to be forced up?

I'm gonna pull the airbox side boots and see what I can see.

 

1700 rpm is not an idle, it should be at 1050 rpm.
I had a run away idle on my bike, fixed it with new throttle shaft seals.

 

Sorry I misquoted the RPM I was seeing. Did yours behave normally other wise like mine does? By that, I mean it would sit at a normal idle then jump like mad and and stay there when you touch the throttle? Holy cow it's nerve wracking when the thing takes off like that.

 

Re: Sigh! Let's try this again. Yet another rebuilt carb iss

This sounds very similar to my problem - Cleaned and rebuilt my carbs with new throttle shaft seals from Chacal. Carbs were spotless with new mixture screw o-rings and a diaphragm for the #1 carb. Diaphragm bores polished, clunk test good, enrichment valve seats polished, emulsion tubes and jets all cleaned and the idle jet shooting fluid for miles.

Bench sync'ed, mixture screws set at 3 turns. Floats heights checked and all good. Like you the bike fired easy on the coke and as it warmed up I could reduce the choke until it was off completely, unfortunately it wanted to idle at 1700 and if I blipped the throttle about 3000-4000. Tried sync'ing, first attempt the revs went up to 6000-7000 rpm and wouldn't come back down, I'd have to use the kill switch as well.

Rechecked the "clunk test" - good. Disconnected choke cable and slackened off the throttle cable - definetly not either of those. Carb boots checked out good with no leaks. Compression checke - OK and valve clearance checked and reshimmed.

For the RPM to get to where it was it must have had enough air to be on the main jet, so I removed the carbs again (fourth time, first two just for cleaning). This time I decided to have a really good look.

First I noticed wear in the #3 throttle body/throttle shaft. This is because unlike all the others this TB has the pulling (upward) motion of the throttle cable as well as the rotating motion. A friend helped and we bored, reamed and rebushed (he machined brass bushes) for this TB. Next we looked at the butterflys, with the diaphragms removed and against a strong light, centralised the butterflys (best done with all the carb individually, not assembled in the rack so the sync screws don't interfere). This I beleive was the main issue.

I don't think there's been enough emphasis on centralising the butterflys after a rebuild, the tiniest of gaps becomes a larger gap the moment a sync screw is touched. The butterflys as symetrical so can be flipped over to provide a better fit if necessary. Carbs the assembled in the rack and bench syn'ed.

After this last "job" the carb idles at 1050 on the idle adjuster, I can also adjust this up and down (on the idle adjuster) as you would expect, something I couldn't do before. They also accelerate and come down to idle quickly and other than a slight hesitation off idle (yet to carry out the dynamic sync and mixture setting) they are finally running well.

 

Throttle shaft and fuel pipe seals ordered from Chacal today. I shot compressed air inside the TB at the point where shaft exits the body and the seals should be. All but one leaked pretty seriously.

 

Kiwi:

You're close. Very close.
Now, just adjust each individual Carb as needed. No need to do them all any more.
Get each Plug showing you a nice coloration and you'll be done!