9 times out of 10 - this is caused by the ISS(idle set scew) not being adjusted properly.

u can check to see if this is the proble, by removing the air filter, and applying the brakes. the carb should stay open about 1mm. if it is closing tighter than that, then this is your problem. while keeping the brakes applied, adjust the ISS until the carb remains open at braking.