Could be timing, could be carburetor, could be ignition. Any one of these things could cause a backfire thru the carb. You could check the chain by removing the distributor cap and, if you can, grab the fan and turn it back and forth while watching the ignition rotor. There should be a little play, but not much. How much depends on how worn the chain is. If you have a clutch fan, then you'll have to put a socket on the balancer hub bolt and turn it that way. That's the way I do it. Another way is to just change it, it's probably time anyway.