Yes tiger you have found the problem that I hate as well. It's not just the powertorque engines that this problem comes to,but also the full crank box mufflers as well. Unforunatly there is no easy solution,but as said before the angle and the bottom first is the best way. Then finding something or even yourself push like all hell to get that top clip on. All the time making sure not to let the bottom clip come off. If all else fails I have found putting the muffler on off the base is some what easier. As you can lay the engine on the carby side (carby off of course) and have no problems pushing down on muffler to get it. So I guess removing engine to do it that way is always an option. As said there really isn't an easy way as they have to be tight so they don't leak. Best of luck just keep trying you'll get it. You will find a way as we all find our own way to make it work.
