If you can't afford an expansion chamber then try the "dual tailpipe" mod. Drill a 10mm hole beside the existing tailpipe, then insert a 3/4 brass male coupling. You have to rebore/redrill the male coupling to have a 8mm inner diameter. The maximum tailpipe diameter should be 16mm based on the exhaust port dimensions. Existing tailpipe is only 8mm, but you can measure it just to be sure as long as the total is not greater than 16mm. Set the male coupling with some muffler glue if you don't have a welder. However, this causes your engine to run lean so you have to adjust your mixture and/or your carb to run richer. I think this mod only works on mufflers that has a bolt in the middle - the one that has no cat. (meowwww !!!)
Check your plug afterwards and engine temp.