I have a Kohler 26 HP that runs fair but it will change sound and speed. It will go from sounding like the muffler is gutted (this is when speed increases) then it goes to a muffled sound (this is when engine speed slows down). My though is a baffel plate inside the muffler moves around and causes this. I will take muffler off if any one thinks this might be the problem. My problem with muffler bolts is that they tend to break off leaving broken stud inside of block. A pain to drill and tap out. I will put penetrating oil on bolts over nite. Some times this helps. Engine has new carb., plugs, air filter and and is in good condition. Any help is welcome--Cambre