It could be a few things. possibly the timing is off or the carbeurator isn't adjusted properly
probably 180 deg out on timing
probly needs a valve job
timing chain or distributer --or gypsies, gremlins etc.......
Sounds as if you have some wires crossed.
may have plug wire crossed
Possibly, especially if it is contaminated with water
Having no catalytic converter installed is illegal and can cause a myriad of problems. Probably not a backfire though.
it's lean
timing or a plug not sparking
backfire timing off
A sparkplug misfiring.
from unburned fuel in the cylinder