I had the same problem with mine. I went to a hotter plug. Then I pulled off the exhuast manifold and cleaned it. Then I pulled the card and the crankcase manifold and cleaned them both. You wouldn't believe the carbon build up inside. Anyway, motor runs well now. Good luck. Don't run any leaner than 50:1 in the two strokes unless the owners manual will allow deviation. Use premium oil if possible.