Before you try the bigger carb, you need to take the muffler apart and gut it. Take out any baffles, screens, etc., and leave only the bolt tubes or whatever is in there to keep the muffler from crushing when you tighten the bolts.

Then, you will need TWO outlets of 1/2" i.d. After trying it with the modified muffler, then try it with the larger carb too.

I recently put an aftermarket muffler on one of mine, and in place of my two normal 1/2" outlets, it had two slightly smaller exit tubes. The engine lost 200 rpms...

AV8TOR