If you have an older machine you may have an idle adjust screw and a power adjust screw for the fuel. The power adjust may be located right next to the drain valve on the carb bowl.
Turn the screw completely in carefully counting the # of turns. (Probably 1&1/2 to 2 turns.) Turn the screw back out to the same setting it was at.
This procedure may clear any varnish buidup in the jet and allow the engine to start.