Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Welcome to JUST ANSWER small engines, let me try to help you !! I don't know it all, but maybe together we can get you a satisfactory answer!
Running with only gas and no oil mix, you may have ruined the piston rings and sleeve. Remove the muffler and look into the cylinder for damage. Or run a compression test, and needs to be over 125 psi. If not, cylinder and rings are bad!
Remove the # 40 cover, exposing the two muffler bolts,remove them and muffler assembly will come off.
I see question is still open, did you find problem?