Lawnmower starts easily but has no real power?

I have a Champion 35 with a Briggs and Stratton engine. It did get a bit wet in the floods some time ago. Changed the fuel in case of water contamination. However since, it starts first time, but doesn't run smoothly, nearly stalling when cutting and the engine rpm keeps changing when idling. Even if you hold the throttle open it still fluctuates. Afterwards if I take the plug out it is well carbonated and a little wet. Carbon wipes off easily.