Could be a clutch issue, but I'm guessing it's the infamous fat-bottom tune problem.

On the box with engine warmed up, rev up to clear the motor out and then drop to idle. If the idle starts out high and then drops after a number of seconds, the low end is rich, top end is lean. Do the pinch test (at idle, pinch the fuel line closed), you should get 3-5 seconds before the engine dies with minor change in RPMs. If it's much longer, lean the bottom an hour or so at a time.

A lean top and rich bottom cause the run on problem. Running wide open throttle burns off all the fuel, when dropping to idle the mixture is initially very lean (so idle is very fast), but after a couple of seconds becomes very rich as fuel loads up. Fought this quite a while last weekend myself.

See the excellent tuning guide here:
http://go-racing.vpweb.co.nz/Tech-Tips.html

Grizz: question about the tuning guide, when doing pinch test for initial LSN setting, is that after idling for a bit or right after clearing out the engine and dropping to idle?