DO NOT TAKE MY WORD AS GOSPEL!!!!
Now when mine was doing that I adjusted the idle mixture until it was at its highest rpm, then turned the idle screw back until the wheel only
just began to move when it was on the stand, and before the engine started to 'stutter', if you will.
So Idle mixture screw: where carby joins onto inlet manifold, there is a single flat head screw that if you turn it anti-clockwise
will adjust the idlemixture. If the revs go up, good, if they go down , bad.
The side of the carb where the accelerator cable is, the idle screw.
AGAIN: WAIT UNTIL SOMEONE AGRESS WITH ME OR RIPS ME TO PIECES BEFORE YOU DO OR DON'T DO THIS!!!