it is a tuned pipe, he must be a noob and calls it a muffler...

There should be a metal wire that is attached to the end of the pipe and is also attached to the chassis. Make sure that this is on and tight. Also make sure that it is on straight. Then test it by pulling on the pipe away from the manifold... It shouldn't pop out.... If you lost the wire I' talking about, either use hangar wire, or scarp it and use hose clamps on the silicone coupler.

The reason why it runns really well when the pipe pops off is because the tank is not getting any pressure any more, making your engine run EXTREMELY LEAN! shut it off immeadiately when this happens...