You idle screw may be too far in or your throttle cable is too tight. Your engine does not spark when your foot is off the throttle and if your throttle is slightly open when coasting it will pump fresh gas through your engine and gather in the exhaust pipe. When you touch the throttle at the bottom of the hill you ignite the fumes in the exhaust pipe then "BAM", you have a backfire.
Yamaha Golf Cart Dealer
Repaired over 3,000 carts last year