neither of my 72's have this problem... I have seen post's on this very problem though. It's the plastic back plate flexing that allows the intake pipe to backout, I guess some flex more than other's? anyway IIRC there is not a metal back plate that fit's the 72, the only solution I remember reading about was placing a zip tie around the head and intake pipe, lock's that intake pipe in place so it can't work it's way out. The other thing you may look for is vibration, it your getting to much vibration I'm sure it's not helping matters.

John