OLD beetle Backfires ??
 

I got this engine from a friend and he said he couldn't get it started. I found that one of the push rods was bent and the valve was rusted tight, I loosened the valve and bought a new pushrod. Now back in the car I have adjusted the tappet’s ,the timing, new coil, and a new rotor (the old one was missing). Eventually I got the engine running but it backfires a lot. You can hear it only backfires either on piston 3 or piston 4.

How can I fix this problem?

Thanks in advance