Ok you will need to take the muffler off and check the exhaust port for carbon build up. If it has any blockage clean it out(make sure that the piston is covering the hole so no carbon falls into the cylinder).
Once you have done these things let me know if you still have a problem and we will go to the next stage.
Regards,
Bruce
Please do not PM me asking for support. Post on the forums as it helps all members not just the individual.