Hi everyone. This is my second time posting a thread regarding my ride and my backfiring problem. I have a 90 integra with a b16a and 2 1/4" piping and whenever i shift from 1st to 2nd, to 3rd...sometimes to 4th gear there is a loud pop. I was told by some knowledgeable people on here that it could be my PCV valve, my CAT converter since its hollowed out, my ignition timing or my oxygen sensors. I was instructed to replace the PCV valve first since its the cheapest part ($10) , and if not replace my CAT. So i replaced the PCV valve a few days ago and it still backfires. I had a hollowed catalytic converter but I replaced it like 2 hours ago with a brand new direct fit CAT converter and....it still backfires. I was also told by someone on here that if i replace the cat converter and still have problems then its possible that my O2 sensors have gone bad. I've replaced plugs and wires in the past and the backfiring still persisted. I don't know what i should do but I don't wanna waste any more money. I just spent $240 bucks and the problem is still there. Can one of you guys please help me out and tell me what i should do now? Thanks.