I would be inclined to look at the exhaust
system, take the muffler off and see if it
is clogged or if it has the spark arrestor
screen and it is clogged, if it is ok, look
at the exhaust ports and see if they may be
partially clogged.
If all these look ok, then you have a partially clogged carb, look at the inlet
screen and at the fuel filter in the tank.
Bill