The output port in front of the muffler is partially clogged. Pull the muffler and take the spark plug out. Carefully scrap the hole clean while blowing air in the spark opening. Piston needs to be all the way down. If no air then just be as careful as possible. If you are unable to rotate the piston freely before you scape the debris come back and I will tell lyou what the problem is.
never had clogged muffler on about 10 of these and some with heavy use and 5 years old.