TOPIC:

1981 KZ750H2 LTD Keihin CV34, stock airbox, 4-1 exhaust. 

I recently developed an issue with my bike where it wants to rev to 4.5k RPM and stay there. It’s only really noticeable when I am slowing down or stopped and the bike wants to rev at those RPM at a stoplight. Kinda attracts unwanted attention. I know the valve clearance is set because I did it 300 miles ago. Floats are balanced. It is definitely because of a lean condition in the pilot circuit because I can’t sit at a stoplight for 20 second before the bike will start to visibly smoke because it is so hot. 
today I pulled the carbs and cleaned everything out. Everything. Blew out all the passages, jets, jet holders, pilot screws, all the pilot gas circuits as well as main and secondary fuel circuits, the slides are not frozen and spring back to closed fine. Put it all back together and set the pilots to 2.5 turns out. Started up fine, rode it for 10 minutes and the issue is gone. I will clarify that there is no issue after 4.5k rpm. Pulls wonderfully all the way to 9k RPM. Well after my successful test, I come home and let it sit for a while, probably an hour. Go back and start it up to go run an errand and it’s doing it again, not immediately though, like halfway on my trip there it just starts to rev back to 4.5K just like before. I am able to lower the rpm by releasing the clutch at lower speeds which forces the rpm to come back down, once I do that it will maintain the rpm that I have pulled it down to. It’s definitely a lean issue with my pilot circuit but I’m not sure as to why. It’s not because I’m not getting enough gas in the carbs because it will pull without an issues after 4.5k, if the gas was lacking then it wouldn’t be fine at the higher rpm. What issue could I be facing that returns shortly after a carb cleaning that makes it run so lean?

Sounds like air leaks.  Have you replace the carb boots that attach to the head?  If not, that's what I'd do.  The rubber gets hard and can easily crack, allowing false air to enter.  Some people spray junk on the boots and listen for a change in rpm, but in my experience that only finds the gross leaks, not the smaller ones which cause idle creep.  

Nessism, would a carb cleaning fix the issue only for it to return? I have not checked the boots but I did consider it. I did check the vacuum/recirculation system and it held a sustained vacuum so I ruled that out. It is currently dark here so the neighbors wouldn’t appreciate a sustained 4.5k rpm rev so I could try to spray something. But I will definitely try that in the morning! 

Well sir, you are correct. Carb boot on #4 is leaking. That’s the only one that reacted to the carb cleaner. Maybe I can tighten the screws and see if that helps. New boots are $20 so I may just bite the bullet and go ahead and get them. 

wrote:

Sounds like air leaks.  Have you replace the carb boots that attach to the head?  If not, that's what I'd do.  The rubber gets hard and can easily crack, allowing false air to enter.  Some people spray junk on the boots and listen for a change in rpm, but in my experience that only finds the gross leaks, not the smaller ones which cause idle creep.  

I would tend to agree with you ref using WD40 etc but there are specifically designed " leak detection" sprays available that do work well and doesn't leave the oily deposits that WD40 etc does

wrote:

wrote:

Sounds like air leaks.  Have you replace the carb boots that attach to the head?  If not, that's what I'd do.  The rubber gets hard and can easily crack, allowing false air to enter.  Some people spray junk on the boots and listen for a change in rpm, but in my experience that only finds the gross leaks, not the smaller ones which cause idle creep.  

I would tend to agree with you ref using WD40 etc but there are specifically designed " leak detection" sprays available that do work well and doesn't leave the oily deposits that WD40 etc does

I don't know the answer.  I just know that I've seen and read about several instances where spraying junk around the intake didn't fine a leak, when there was a leak there regardless.  Most of the time I just replace the boots and call it done.  40 year old rubber is not good.

I have done this method to find a intake leak on my truck before, it is fuel injected too. Much harder to hear the difference in a modern fuel injected motor but I did find it regardless. A standard can of carburetor cleaner works just fine, no need for WD40 or any specialized spray. I also just went ahead and ordered the boots, they should be here Friday. 

wrote:

wrote:

wrote:

Sounds like air leaks.  Have you replace the carb boots that attach to the head?  If not, that's what I'd do.  The rubber gets hard and can easily crack, allowing false air to enter.  Some people spray junk on the boots and listen for a change in rpm, but in my experience that only finds the gross leaks, not the smaller ones which cause idle creep.  

I would tend to agree with you ref using WD40 etc but there are specifically designed " leak detection" sprays available that do work well and doesn't leave the oily deposits that WD40 etc does

I don't know the answer.  I just know that I've seen and read about several instances where spraying junk around the intake didn't fine a leak, when there was a leak there regardless.  Most of the time I just replace the boots and call it done.  40 year old rubber is not good.

Nessism: I don't claim this method to be full proof (like so many diagnosis methods) but it will work in the majority of cases and is both cheap and a good indicator before ordering parts that you may not need. I completely agree that in the case of these old bikes replacing perishable items like seals, boots and manifolds is good practice anyway as you say due to aging (same with wiring which has hardened and cracked) With regards air leaks there are a variety of methods to diagnose but are too expensive for the DIY enthusiast (an exhaust gas analyser will show a high Hc reading, etc) Due to the complexity of vacum systems on modern vehicles and the environmental issues of spraying chemicals a lot of dealerships these days use smoke testers to check the integrity of intake systems (again not full proof)

The package arrived, Of course it comes in on the day I have to leave town for a week so I am unable to install it. It’s also gonna be perfect weather the entire week I’m away. Tis life…

Back in town today and I got a chance to work on the bike. Changed all the boots and the issue still persists. But now the bike reacts to 3/4 cylinders for the carb cleaner test. Is there a special procedure/gasket maker I need to apply to the boots? All clamps are tightened and only seems to react at the base where they mount to the head. All screws are tightened to German standard, gutentight. The old boots appeared to be in good condition but I did have to massacre one bolt in order to get a stripped screw out. Anyone?

Are you saying that the air leak is between the inlet manifolds and the head ? If so I would suggest taking the manifolds back off and put a "straight edge" across the face of the manifold and also the mounting face on the head (I believe that the steel insert in the manifolds is "stamped" so a good chance that it isn't flat) is there a gasket or an "O" ring as a seal ? Once you know what the scale of the issue is you can look at how best to resolve it
 

There is no separate/loose oring, but there is a raised rubber ring formed into the carb holder that acts as an oring to seal the holder the the head. Did you buy a quality set. The cheap $25.00 sets from evilbay are not recommended.