The only things I can think of are...plugged air filter, plugged fuel filter/line, mixture settings or messed up carb, clutch bearings(are rear wheels on the ground?).
Oh yeah, and toasted piston[:@].

When it starts up, how does it sound? Bubbly and soggy, or crisp and lean?
Do you have temp gun?

First reaction...plugged fuel filter, or a piece of flashing from mfg. on the fuel inlet in the tank acting like a one-way valve.
just a shot in the dark