I just had that problem with an old Quantum, it partly rolled off the shed shelf - ended up with the rear wheels on the shelf, and the front wheels off the shelf and the body resting on the shelf.
Smoked really badly and just wouldn't clear up. stopped it and immediately restarted - barely a trace
MF - probably throttle cable adjustment or someone has screwed the idle screw in a fair way - mine idles down to a nice near silent tickover
