Don't change the muffler, it's fine as is. Plugging off the muffler and using pumps are a less than ideal way to address the real issue, which is an easy fix anyway. If you look carefully at the fuel nipple on super tigre's carbs, it's VERY tiny. Measure it with drill bits and you'll find the fuel inlet area is less than half that on any other engine brand out there, even 40 in sized engines. Compare it to the volume in the needle valve assembly. It's simply too small to consistently supply the engine as the throttle setting changes without unnecessarily high pressures and too much back pressure can cause overheating. The needle should control the mixture, not the fuel nipple. Either drill the stock fuel nipple out to 1/16" or better yet, I just replace them with pressure nipples of large ID off ill fated mufflers. You should be using large fuel tubing to ease the fuel draw, especially if the tank is large and has 12+ inches from carb to clunk. Be sure to use a large ID clunk and brass tubing as well. Sullivan's "HD Klunk" works great. Do remember, if this is your first big tigre, don't expect it to be broken in until you have nearly 2 gallons or more through it. Bench run it and get it broken in and set and youll be very happy with the engine. Gobs of thrust and great throttle response. BTW, I have 5 big tigres and 4 ST90's. I find the 90's are also much easier to adjust, have excellent midrange now, and make more power by simply drilling the fuel nipple out to 1/16" and removing the muffler baffle. Best wishes.