First 1981 750 DOHC restore.. HIGH HANGING IDLE ISSUE

[Bruce Drapeau]

Picked this bike up in 2015 and haven't had time to get at it till this year. License plate on it is from 1989. Replaced four bent valves, ground all valves and seats, re-shimmed, all valves, reassembled and installed engine, and did cranking compression check. (145ish all 4)... Front brakes calipers froze , return port in master cylinder plugged , still has original exhaust and in very nice shape... etc. Had carbs off three times, last time disassembled rack installed all new fuel tube o-rings plus everything that couldn't be installed with carb rack together. Started bike and it sounded almost perfect,.. did carb synchronization, and idle mixture procedure. seemed to run Perfect.
Time for test ride.....everything went good until it warmed up with a load and Idle was hanging about 2500-3000 rpm. I have done every check in this forum, (that I could find) except spaying throttle shafts with flammable carb cleaner to check for vacuum leak. I did run a wire through the Pilot jets when putting rebuild kit in, but I did not remove them. If nothing concise on throttle shafts found., Next step is to remove carb rack and pull pilot jets.
Evidently I am missing something?? Any input would be appreciated.

 

[brettp]

Hanging idle usually due from air leak between the carbs and engine. Do the spray test

 

[Bruce Drapeau]

Finally got around to checking for vacuum leaks again and decided to recheck all points of interest plus throttle shafts that I missed.
First when warming up engine, it started and ran good, idle stayed stable and came down to idle very well when throttle was cracked on stand, engine warmed up. ( 15minutes ). When spraying carb cleaner on #3 carb shaft engine immediately increased RPM. Leak #1.
Rechecked all rubber flange boots, found #2 boot leak at head, we briefly loading engine with brake, while cracking throttle thus getting a slower return to idle rpm, It would immediately return to idle when sprayed with carb cleaner, Leak #2.
tightened clamp to max, Thought at first problem was solved , engine came down to idle perfectly every time throttle was cracked on the stand again.
Decide to load the engine by driving it, idle hanging problem came right back. Not as bad as before, lower rpm and shorter duration hang. recheck # 2 boot and it had no leak.
Betting on throttle shafts must increases clearance after engine is loaded, and bad shaft seals.
I am speculating when loading engine it produces more heat and warms the outside housing of the carburetors, also at the same time throttles are opening more and vaporizing more fuel creating a temperature drop, (of as much as 60 degrees) which cools the throttle shafts creating larger clearances between the shaft and bore.
Next project is throttle shaft seals.

 

[okdaley]

On my 1979 DOHC CB750 also had the same issue that persisted and demonstrated the same behavior. It is not a vacuum leak. I added a small washer to shim the needles of the Keihin Carb pack. Now Idle remains where set at about 1200 rpm - in my case, I had to replace the original pipes which I loved. With Headers, there was backfire and stuck with High Idle once warmed up. I could get it to come down, but sliding the clutch to drag on the engine. Eventually I rebuilt the Carbs. I replaced the Mains from 68 to 102 jets. Added Pods that did not cover up Vacuum intake and air port. Improvements but still High Idle would show up. Demo on Instagram

Btw: - In my case, to get at the Bell Caps of the Carbs, I do not need to pull the Bank. That is also worked out to be smooth these days, but once that Tank is off, they are exposed and accessible. Most importantly, Turn the Gas Tank's Petcock Valve to off, and run the engine till it stalls. This assures that there is no fuel in the line between the carbs and the tank. You do not need to drain the fule bowls as you are just working on the vacuum Pistons only at that point. First time I over shimmed and it did not work. 1 thin washer only per Needle -

 

[NormK]

I was also thinking it could be a needle problem, where did you put the shim? I am open to all suggestions And another question, how do I remove these needles, do I have to prize the black piece out?

 

[mrtwowheel]

I was also thinking it could be a needle problem, where did you put the shim? I am open to all suggestions And another question, how do I remove these needles, do I have to prize the black piece out?

Don't pry anything, should pull out with your fingers.