Well crud

As I have posted in other post I have been mounting a used 8HP. Briggs on my 2550 after the old 5HP. Tecumseh gave up the ghost. Well I had several bugs to work out with the new engine but yesterday I finally got it running. I cut the whole yard and it ran great. The added power was a big plus over the old Tecumseh. When I was finished I stopped the mower, disengaged the blade, and put my hand on the key to shut it off when suddenly POW! The entire mower shook. I shut it off as I was already in the motion of doing. I looked it over and noticed that the muffler was almost blown apart. I pulled the rope and notice right away that there was no compression. The crankshaft spins freely and there is no air puffing out of the exhaust. I just walked off and left it sitting there. :banghead3