If you have a stock exhaust, you must remove the plug from the bottom of the muffler (square drive pipe plug) and run the engine without the plug in. Rev up the engine a bit while blocking off the main exhaust outlet with a shop towel to help blow the crap out of the other hole (just blocked enough to clean it out- not enough to stall the engine) I suggest reistalling the pipe plug with a little anti-seize.
good luck