engine backfires at varying speeds, especially when deceleration

"ironmonkeyto, Make sure carbs are adjusted correctly and fuel levels are good, one of the biggest reasons for this is if you have an exhaust leak. Check for leaks at all exhaust connections especially at exhaust port/engine connection as well as any cracks/breaks in exhaust system. Good luck.

I hope this helped you out, if so let me know by pressing the helpful button."

timing and valve adjustment can be the issue also