[Justinh722]

It’s not so much an opened muffler as it’s still restricted by the inlet and outlet. I mentioned it’s more free flowing than the bad one simply because the other one had broken off inside and plugged it up. The issue is that it should be running with the muffler regardless. I can tune an engine if it turns out to be too lean but the pipe that’s more restrictive to the tune of the engine is the header pipe before the muffler. The warning should have been if I were to just drive the cart without a muffler at all and risk a melted piston. I’m wondering why it’s still not running correctly with the same back-pressure the stock muffler would be giving it. It’s being starved for air somewhere. You would have made a valid point had I said that the new muffler was causing the engine to over rev or run hot. When opening a muffler you are introducing more air. More air would mean it needs more fuel. It will only run well with the muffler removed. The carb is sucking plenty of air, but not enough for it to run correctly.