sticky/frayed throttle cable,
disconnet each end and make sure it runs smoothly.
Failing that it has a monster air leak from some where, let it idle, then with small sprays of wd40 spray around the inlet side of things, in order, if the revs change doing this, the air leak is somewhere in that area...
Oh and don't spray too much on the exhaust/dizzy cap/plugs etc, engine fires suck...
Cheers
kraM