no! do not solder the pipes together.
dave I seriously doubt the back pressure is blowing your muffler off, its probably not tight enough.
there should be a locking nut at each end of the pipe that connects the actual muffler to the engine they need to be pretty tight,not so tight that your breaking any thing but pretty snug.
after a few runs any fuel residue that leaks out through the gaps will burn and seal any leaks that are there.
you need to screw the pipes together as much as possable then tighten the lock nuts with a wrench on both parts,the muffler,and the nut.the photo of the engine clearly shows a space for a wrench on the muffler and the lock nut.
like any 4st engine there is some vibration so nuts,and bolts need to be tight,along with engine mounts etc...