Small Engines (Lawn Mowers, etc.): drive belt, forward gears, forward gear

Question
i have replaced the back drive belt on my riding mower (I have no clue what kind it is cause it doesn't say) when it is in forward gear it runs fine. but when I put it in reverse the belt pops off the rear pully that is over the rear axle. I am getting tired of pulling the battery every time this happens to put it back on. I guesss my question is. is there anyway to adjust it so this will not happen again?

Answer
This usually indicates the transmission / axle mount has a broken mounting location.  The transmission can "jump" when put into reverse and this can cause the belt to become misaligned and jump off.  In forward gears, the transmission may move in such a way that it doesn't promote belt misalignment.  Let me know what you find.