CV34 Vacuum behavior - Hanging idle

Hi all,

I've been struggling to fix my hanging idle issue on my 1981 KZ750 LTD for quite some time now. The bike idles nicely, and at speed runs very well, but has a hanging idle and the rev don't drop after putting the clutch in. The problem gets worse as the engine gets warmer.

I've strongly suspected a air leak, but I have not been able to find any using the usual methods (spraying carb cleaner on the boots/linkages/vaccum ports). I just replaced the carb boots with new ones to rule that out, changed the throttle cable, and re-set the pilot screws. I also changed the jets to one size larger since it has an aftermarket exhaust and the plugs were looking a little lean. The bike idles and runs nicer from these changes, but they have made no change to the hanging idle.

Hooking up a vacuum gauge, I was expecting to find one or more of the carbs to show issues with the measured vacuum. However, they all read much the same, at idle and when revving, and letting the revs settle back to an idle. I took a video to show this:



The carb sync is OK (I synced them as close as I could after this video was taken, but made little difference). First thing, the measured value - around 5" Hg - seems pretty low. Is that correct for these bikes? And when revving the bike and letting the engine settle, what should I expect the vacuum reading to do? To me, it makes sense that the pressure will increase after shutting off the throttle, but should I expect more pressure there?

I'm beginning to think that maybe it is not the carbs, since I've had them off twice now and made no difference. The timing advance is clean and moves freely, so I don't think it's that either. Can anyone confirm what values and behavior I should be expecting for this scenario?

Seems lean. Maybe the pilot passage is restricted? How many turns open on the pilot screws and is the intake system all stock and well sealed?

I thought the same thing, which is why I cleaned and reset all my pilot screws and passages when I was replacing the boots. I blew through the passages with carb cleaner and it went through without restriction. The pilot screws are 2-1/2 turns out from seated.

Everything is stock except for the exhaust, which is why I went up one jet size. Everything appears to be well sealed, I replaced the boots as that was the last thing I could think of. The diaphragms are all good too.

Maybe try opening the pilot screws a little farther?

I had the same hanging idle issue on the TK22 carbs on my GPz 550D1. I did exactly what you did...
No improvement UNTIL I did what was suggested:
Use a steel guitar string (the one that is coiled around an inner strand) to mechanically clean out all the jets., especially the pilots.
Then apply carb cleaner again .
Perfect idle after that!

howardhb wrote: I had the same hanging idle issue on the TK22 carbs on my GPz 550D1. I did exactly what you did...
No improvement UNTIL I did what was suggested:
Use a steel guitar string (the one that is coiled around an inner strand) to mechanically clean out all the jets., especially the pilots.
Then apply carb cleaner again .
Perfect idle after that!

Thanks for the suggestion howardhb, however I've already replaced all the jets with new ones (I put a kit through the carbs which replaced the pilot jets, and replaced the main and secondary since I wanted to go up a size). Since changing the jets I have checked them again (since they were off to replace the carb boots), and they're still clean.

@Nessism I was planning to try the pilot screws again, but work got in the way. I'll try tonight, that's my next step.

The one other thought I've had is the plastic bungs that cover the pilot jets. They fit snug, but not tight. They've been seated there for a while without moving, but if the o-ring isn't sealing on those I would expect it to disrupt the vacuum on the pilot jet.

I'm not sure if those plugs could cause the problem but I do know they are supposed to be sealed tight.

When you had the carbs apart did you remove the bleed pipes and make sure the orifice is open? It's super tiny; you have to hold it up to a light to see if it's open, similar to a pilot jet.

Nessism wrote: I'm not sure if those plugs could cause the problem but I do know they are supposed to be sealed tight.

When you had the carbs apart did you remove the bleed pipes and make sure the orifice is open? It's super tiny; you have to hold it up to a light to see if it's open, similar to a pilot jet.

I didn't check the bleed pipe this time. But I did replace them last time they were apart with the new ones from the kit. Pretty much all the brass parts have been replaced, but the problem remains.

Quick update - I turned the pilots out another half and rode into work today. It seemed better - but it was also a cold day, and the issue is more pronounced when hot, so I could just be imagining it. I'll wind them out another 1/2 turn and see if it's better or worse.

The strange thing is that everything else runs quite well - it starts easily, idles nicely, and the throttle response is good - no hesitiation (there was a little hesitation before I went one size up with the jets). The only symptoms are that the revs don't drop when pulling in the clutch, there is little engine braking, and sometimes the idle can creep up.

I realized that I never concluded this post. I did end up fixing the issue, but I'm not 100% on exactly what it was that fixed it.

I took the carbs off again, took off the float bowls, and replaced the o-rings on the plastic pilot plugs. I don't think this helped - the old ones were in pretty tight, even though they didn't feel particularly tight when I re-installed them after sitting for a long time last time. They could have swelled a little with the fuel perhaps.

I also blew out the pilot passages with the compressor set as high as it goes (~120psi), but nothing came out, I think it was clean from last time. Last time I used compressed air from a can, and carb cleaner.

The thing I think might have fixed it is the o-rings on the pilot screws. I used the kit discussed in this post , and as discussed there the pilot screws were very tight, but I needed to use at least one since I had to remove one of the old ones with an extractor. But since they were so tight, when I wound it down to be seated, I must have crushed the o-ring as it was very hard to tell when it was seated. I replaced the mangled o-ring, and turned the slightly-oversized pilot screw down with a drill and a very fine we-and-dry paper.

I installed everything again, and problem solved. I'm not sure that the little o-ring could cause so many problems, but it's all I can think of.