Earl the have a gold muffler for high rpm as I stated. the blue evo muffler is about 500 rpm less than the red mufflers. qwk measure the hole in the crank as that can limit the size of carb. so if the crank hole is .400 then a carb bigger than doesnt help. all of the Jett mufflers will load up on the midrange on most engines i have seen. just something we have to live with. Jett mufflers are for boring holes lol I have used them on some fun fly planes and work well just have to run to full power for a couple of seconds to clear engine. Something else on engines with enough clearance I like to run the four stroke plugs as it has helped on some engines.