If the speed control has no effect, the govenor is faulty. Remove the air filter assembly, and observe the long lightweight spring. It should be connected at the front to the speed control lever that moves when you move the handlebar lever, and at the rear, to a link attached to the carb throttle plate and the govenor vane (out of site under the cowling. Verify that this is all present, and FREE MOVING (engine OFF).
If that's all ok remove the cowling (3 bolts).
Check that the cooling fins are not blocked. If they are, this will reduce the airflow past the govenor vane, allowing the engine to overspeed.
If all that checks out, it's likely that the plastic vane that controls the throttle is broken.
phil_saunders(a)bigfoot.com
Phil Saunders, April 2010