Small Engines (Lawn Mowers, etc.): RE Ride on problem, fuel tank cap, water in the fuel

Question
Machine starts ok but after a few minutes starts to ref up and down with carb level moving forward and back. Eventuallly it cuts out. If it leave it a while it starts again and same happens after a few minutes. Checked usual things and chenaged to new spart plug but no joy.
ANy ideas

Answer
Hi Joe,

It can be either of two things:  A weak carb governer spring or a dirty needle and seat in the float bowl.  I suspect the latter.  

Let me know how you make out.
Michael

Afterthoughts:  Check for water in the fuel or a clogged fuel tank cap vent.