Lean fuel mixture.

It probably has a restriction on fuel delivery in the carburetor. If its a Tecumseh engine, stand the mower up on its wheels so the carb is on the high side. Remove the brass bolt that holds the fuel bowl on. Clean this bolt which is also a metering device, particularly the tiny hole on a slant near the end of the threads. Strip the paper off a wire tie and use the soft steel wire to rod it out. Replace the metering bolt and be careful with the torque. Sometimes it will leak when the fuel reenters the bowl - tighten 1/8 turn at a time until it stops. If this doesn't fix it, get back to us.