I think some engine react very different when run without a muffler. I took the muffler of my MB290 leaf blower to use on another MB290 on my test stand. Putting a muffler on the engine on test stand helped the idle on that one a lot. But my leaf blower now will not shut off and I need to choke it to kill it. Before it would idle lower than you could imagine. Bottom line...I think different engines can react very strange when removing the muffler!! Capt,n