1980 KZ750 Low Carb Vacuum

Need some help - After trying to sync the carbs on my 750 E1, I've noticed cylinder one is running very low vacuum and cannot be sync'd with cylinder 2 (2 to 3 inches/hg), cylinders 2 ,3 and 4 are fine (8.66 inches which equals 22cm/hg), I've done a compression check and cylinder 1 is running at 100psi (manual states 109 to 171 is the spec), all others are around 130 psi I've also checked valve adjusted and all cylinders are fine. Will being 9lbs below spec cause the low vacuum or should I be looking into the carb or valve timing, I do have some cam chain noise and was thinking about going to a manual adjuster. One other thing I tried was putting a teaspoon of 30 weight oil into cylinder one to see if the compression increased which it did, I know this indicates worn or a stuck ring but not sure if it's causing my vacuum issues. I thought I had the bike fully restored and ready for the road and now this. Any help or suggestions would be appreciated. I don't want to take the head and cylinder off if I don't have to.:S

Chec for vacume leaks at the intake. Also check to see if the valves are opening all the way. Exhaust restriction will cause the same problem too.

I did check for vacuum leaks at the intake and checked the boot beween the intake and carbs for cracking, all seem to be okay. How would I know if the valves on cylinder one are opening all the way I do see the cam opening and closing the exhaust & intake valve, but with out taking off the head I have no idea if their opening or closing properly. Also if I had an exhaust restriction wouldn't I expect to have the same problem with carb two given the exhaust is combined after the header? unless the header was plugged or the exhaust valve was opening.

I moved this thread from the Engine forum to the Carb forum, and removed the duplicate thread with no replies that was already here. Sorry I can't actually provide any helpful information, though!

*Edit*

I guess we all make mistakes, and after reading through this post more carefully, I realized it is actually more engine related than anything and moved it back.

-Johnno

Post edited by: Johnno, at: 2005/10/16 23:42

You might be onto something with the piston rings. If you've found no valve seating issues and no leaks between the carb and engine, that may be the place to look. I wouldn't think that 8 psi difference from one cylinder to the next would make it impossible to synch the carbs...

I burned a small hole in a piston once too... I guess that would have made it difficult to synch the carbs OTOH, the performance hit was huge, so I could tell that something was really wrong. Do you have the same symptom?

Post edited by: Idaho_Spud, at: 2005/10/17 01:20

glenns wrote:

I did check for vacuum leaks at the intake and checked the boot beween the intake and carbs for cracking, all seem to be okay.

My #2 is 30 psi down, rings suspected, and I could sync my carbs better than that. Did you check for vacuum leaks using wd40? I found leaks I couldn't see or feel. Did you check for air leaks in the carb body, like from the mixture screw? My gut feeling is you're missing some air somewhere.

Plan for the weekend is to put the cam shaft cover back on (pulled off to check valve clearance) then spray WD-40 or Carb Cleaner around the intake and carb to see if the RPM's change, if not next step is to pull the carbs and take a look, with the carbs off I'm going to put new needle and seats in and check the float levels given cylinder 4 is running lean. Time to get back to basic's for me.

Well after getting back to basic's, cleaning, rebuilding the carb's and setting the float levels the correct way (clear tubing attached to carb drain), and putting alittle high temp silicon on the carb boot flanges where they meet the engine, I now have great vacuum and was able to sync the carbs, she's purring like a kitten at 1000rpm, the other thing I added was a manual cam chain tensioner made by APE and no more cement mixer noise. On to replacing the steering bearings with aftermaket roller bearings and she's ready to go.

nice job, dude! good to hear positive progress.