Poor fuel can cause this ensure fresh fuel mix is being used. Diagphrams in the carburetor can become hard and cause a similar problem. Another problem can be compression is low as well as a crank case airleak. Starting with the simple first check the piston for any damage when you check the spark arestor in the exhaust. Make sure there is no marks in the piston. If both are o.k check fuel line for cracks or restrictions. The next inspection I would suggest will be the carburetor. Normally I would pressure test and vaccuum test the engine but this would be too hard without special equipment.
