Originally Posted by Frankee
sorry I missed this its probably a clogged exhaust port.
remove the muffler and very carefully with the piston all the way down scrape out the carbon in the cylinders exhaust port. try and not scratch the piston or cylinder wall while doing this. they usually recommend a wooden or plastic toll but they don't work very well. use a small screw driver and be careful. once your done pull the plug out (actually pull the plug before you start) and blow the cylinder out before trying to start the engine.
You can do this, but you also want to check the exhaust screen for clogging with carbon. There is probably a screen in the exhaust that is held on by a metal plate secured with screws. Take off the screws, remove the plate, pull out the screen and check to see if it is clogged with black carbon residue. If it is, hold it with a pair of pliers and burn the carbon off with a torch or lighter, then re-install it. Then you can check the exhaust port as described above.
Also, check to see if the air filter is dirty and inspect the spark plug.
Definitely sounds like an intake or exhaust restriction and the plug is probably now fouled because of the other problem.