What make a 1997 Yamaha royal star backfire?

Backfiring means that unburned fuel-air mixture is getting out of the cylinder. Likely cause is a weak spark -- are the spark plugs fouled or just old? Weak coils can also reduce spark. A less likely cause (but more serious) is a sticking or mis-timed exhaust valve.

what ever. the biggest reason for back fire with the royalstar is exhaust leaks at the mufflers and some times the head pipe. I own the first year(1996) Royalstar and have 60,000 or so miles on it. change to aftermarket exhaust sys. when i had my first problem with backfire. Which most if not all mistake for a REAL back fire.

This is type of leak is really popping on decelaration which is usually called a back fire. NOT.