In regards to keeping the muffler in place its actually really simple if it has exhaust studs, all you need to do is go to a hardware store and get a couple extra M6x1.00 nuts and double nut each stud after muffler is installed.
Get the first nut good and snug with even pressure on each side of muffler flange, then put an additional nut onto each stud tighten until it is touching other nut, then with a 10mm open end wrench on each nut, while making sure the first nut you put on doesnt move tighten the second nut down good and tight against the nut that is against muffler flange.
This simple technique works good for preventing nuts from backing off.
If. Your engine has socket head screws instead of the studs, I suggest a dab of either blue or red loctite thread locker, I've used both with great results on exhaust studs that were loosening from vibrations.
I may be way off base here but for aomereason Im thinking to tell you this, if engine revs up when you give it more throttle while clutch is released, your clutch is slipping.... if engine over revs when you disengage clutch and doesnt want to idle down, suggest as others have said, you have an air leak at a gasket more than likely.