Motorcycle Questions? Ask a Mechanic.
Hello, this is usually caused by a vacuum leak, most commonly the rubber boot connecting the carb to the head. They are prone to cracking. You can check it by apraying some carb cleaner onn the boot while the enigne is running. If the speed changes the boot needs to be replaced.
If this does not solve the problem, just reply back and we can work further.
HI and thank you for your question.
I have 2 ideas right off the bat.
First, check your throttle cable free play. Make sure the thumb trigger moves without lifting the butterfly. By screwing the adjuster in it will add free play. You should have about 3/16" free play in the thumb throttle before the butterfly moves.
My next idea is you have an air leak. To check for that, start the engine and let it idle as low as possible, then spray contact cleaner or starter fluid around the intake manifold where the carb installs and where it attaches to the cylinder head.
Those would be the 2 most likely culprits. Check them out and let me know if you need more help.