Did you take the muffler apart and remove any and all baffles, etc. from inside, or just drill holes in it? You need to remove all the power robbing guts of the muffler, leaving only the support that prevents the mounting screws from collapsing the muffler.

If the engine was new, it will need some time to seat the rings. If it was used, it likely needs a new ring. I recommend rings from Frank Bowman.

AV8TOR