Done Everything, Still Has Hanging Idle

Bench sync is fixing all slides at physically equal heights, as starting point for later actual sync (with engine running) wherein final adjusted heights will slightly differ to best equate performance of cylinders at lower rpm.

Typical bench sync involves using a small drill bit or paperclip wire, etc. to assure equal heights. But some of us simply eyeball them. As the heights will be readjusted anyhow during final sync.

As earlier noted, final sync usually entails gradually lowering the idle with the thumbscrew (which simultaneously lowers all slides). Which requires sufficient adjustment room on the threaded portion of the idle thumbscrew. Hence the importance of having the thumbscrew at mid-way thread position during bench sync. Thereby enabling the successively lower idle positions typically needed during final syncing and fine tuning.

Please forgive some repetitiveness, as am trying to better explain.

Good Luck!

Patton wrote:

Bench sync is fixing all slides at physically equal heights, as starting point for later actual sync (with engine running) wherein final adjusted heights will slightly differ to best equate performance of cylinders at lower rpm.

Typical bench sync involves using a small drill bit or paperclip wire, etc. to assure equal heights. But some of us simply eyeball them. As the heights will be readjusted anyhow during final sync.

As earlier noted, final sync requires ability to further lower (or sometimes raise) the slides. Which requires sufficient adjustment room on the threaded portion of the idle thumbscrew. Hence the importance of having the thumbscrew at mid-way thread position during bench sync. Thereby enabling lower and lower idle positions typically needed during final syncing and fine tuning.

Please forgive some repetitiveness, as am trying to better explain.

Good Luck!

Awesome info Patton (as usual)!!!! I've got to admit, after the several carb rebuilds I've done, and feeling like I pretty well know my way around the vm series carbs I'd never thought of setting the thumb screw in like that when bench syncing. I've always been lucky that bench syncing with the thumb screw out has worked for me but I'll definitely be taking that step next time I have the carbs off and apart. Thanks!!!

If you really know what you're doing, and are sure you don't need to adjust your mixture screws again, throw a big glob of rtv in there and be done with'em. :laugh:

If you do need to change things later, the rtv just pops out, no mess, real easy. :huh:

GL...

Seriously though...

Air can leak in via many strange places on carbs, (like throught the throttle butterfly shaft passsages).
From air box snorkel tip, to the end tip of the exhaust pipe, needs to be completely air tight.

I have no o'rings on my 650 E 1s air/fuel screws and it runs just fine. Bob

Had this very same problem on my KZ750-4 last month. The throttle cable would snap back when released, and I was sure that wasn't the problem. But on a whime I rerouted the cable so there were no sharper curves in it.....and the problem stopped! Turns out that there was a spot that barely hung up when there was vacuum present in the carbs with the engine running. [Actually, I'm not entirely sure that vacuum was the problem, but it certainly seemed so]. It wasn't enough to hang up when the engine was off. After that I lubed the cable and haven't had the problem since. By the way, it happened more often when the choke was on.

First I tried a somewhat stronger return spring, but my hand felt it when the miles added up.

If it was an improper bench synch, would the problem occur all the time, and not at random times? I just got back from a trip, so I will remove the cables, see if it does it without the cables, check if the throttle crank is loose on the carbs, and then finally recheck the bench synch.

Is this a single cable throttle or a dual cable. If it is a duel than just twist it to the stop position and it will tell you if it is a hanging cable.

I have been reading this post off and on and remembered tonight that I had a similar problem a couple of years ago. Do you have those bar-end dampers on your bike because I had a set and my grip slid slightly away from the control on the right side and would hang up slightly on the damper. When I pushed the grip back into position all was better. Could be that simple?:laugh: Good luck David

Redwood, Hesitant to even mention, But. were I you, I'd be grasping at straws. I've experianced climbing idle, (mostly on test rides) where, Idle rpms would climb due to fuel starvation, either due to, a fuel line kinking when warm...an intemittant vacumn petcock, a gas cap failing to vent. does hang occur when running on a "remote source"? have also experianced "intemittant hang" where carb linkage contacted intake manifold clamps. Idle thoughts, luck g

From what I can tell, it's not hanging up anywhere. It doesn't always happen when I rev or when riding, it's intermittent. When it does start racing if I blip the throttle a couple more times it starts to come down. It's always only run with gas tank on.

Have you tried putting a stronger throttle return spring on it? ...Russ

Sometimes a difficult-to-find air leak may be identified by using a propane torch (unlit--Duh!) to envelope the entire carb area with propane gas while the engine is running. But of course, avoid the air filter and carb intake area.

Any ingestion of propane through an air leak should noticeably affect engine performance. Same idea as with the other type spray mediums.

The propane may offer better coverage than spritzing around with WD40 or carb cleaner or ether or moisture or whatever.

Good Fortune!