Good evening, my mates had his dads cbr1000 1989 re commissioned recently, a reputable mechanic has cleaned and rebuilt the cabs inducing balancing them in the bike , tank cleaned and treated and other bits and returned it running. It sat for 3 months while he did his test and waited for warmer wether but now it’s got problems.
so once it’s warmed up it rides fine for few miles then the revs slowly climb up to around 6k on their own (once you turn it off and leave it for 5-10 mins it’s fine again) , we know its not the throttle cables , we have taken carbs off and inspected the rubber hoses that go into the head and they are fine and definitely sealed. Visually inspected all the vac lines yet still can’t find the source of the problem.
before we ask the mechanic to look into it again any one have any ideas?
thank you for reading

 

Obviously air is entering the combustion chamber through a gap somewhere after expansion from heating has take place.

My first thought is, are you sure the carb intake rubbers are fitted correctly and that the carbs are correctly seated into those rubbers?




If there's some misalignment there, it could cause an air leak when warm.

Good luck fixing this little problem.

Dennis

 

One super easy way to check for a vacuum leak is to use a propane or mapp gas torch, like the kind that is used for soldering. While the engine is running, take the torch (unlit) and open its valve. Place it up near all the carb boots and other places where a leak could occur.
Since the gas is combustible, and the vacuum leak will pull the gas into the carb, you will see a marked increase in RPMs. That will show you the area of the leak.
Some people use aerosol cans of some sort of combustible, but I found that while they work, they can leave a mess or residue. Propane leaves no trace.

 

Good evening , so no air leak. we have located carb no 4 ( outside carb throttle side ) to be the problem , sprayed wd40 and brake clean and it’s no different.
the bike idols but soon as you rev the bike it won’t return to idle .

reved it so it held the revs and then covered each carb with cardboard to see what one made the revs drop enough to remove the increase in engine rpm.

next question is what to look for , the diaphragm is new and have double check it’s fitted correctly and sealed, the needle valve definitely closes fully ( manually closed it and blew into the fuel line ) and the float definitely floats.

any advice appreciated

 

Sounds like the cam between carbs 3 and 4 has become disconnected and 4 is not springing back to the close state?