Motor runs eneven?

I have a lawn mower which suffered in the floods, totally submerged. I have drained the oil (several times until it stayed clean) cleaned the petrol tank and refilled with fresh petrol. Drained all water from within the engine. However the motor starts OK but once warm it starts to smoke (slightly) and revs up and down on its own very erratically. The throttle is auto set as it is a Briggs and Stratton classic.