Question about backfires?

[Chaotic_Warlord]

Ok here's my question what causes a car to backfire? I'm not referring to a modded engine with a turbo or anything like that, just your standard run of the mill stock block. THe reason I ask is I took my car in for inspection last week and well after my 2 attempts to hold it at 2500 RPMs for like 2 minutes, and the inspection guys 2 attempts I think my car is running a little to rich now. When I get my car (95 S14 stock KA with 235K on the clock) up to around 55-65 (normal highway speeds) the old girl makes this popping noise when I am on and off the the throttle. the sound comes from the back so I know its backfiring. What would cause this just so I have so starting points. I know I need to do a tune up badly and I will after xmas (as long as the weather is reasonable ). Could my stock fuel map be thrown off orcould it be my fuel filter or is it that one of my spark plugs has decided it doesn't only wants to fire every other cycle (very good possibility). I'm just looking for some possible reasons for it to be backfiring so I have a couple starting points. But like I said after the holidays I do fully intend to replace the plugs and wires since I have a new valve cover gasket in my room collecting dust. Should i just fork out the extra cash and replace the fuel filter (and maybe the PCV valve)? I just had the EGR solenoid replaced last week too so could they have messed up something in the process of replacing it?

[s13sr20chris]

2 kinds of backfire... intake

exhaust

backfires through the intake are dangerous. they are caused by too much fuel, too little fuel, or advanced timing. i dont know how many stock airboxes i have blown up fixing cars with this problem.

backfires through the exhaust are usually cars running rich. in fact, all cars are popping and gurgling all the time but you dont hear it through most exhaust systems. when they get real loud you have probs. catalyst problems can cause these too. im sure retarded timing would agrivate the problem.

[Chaotic_Warlord]

Thanks, I kinda have a feeling its my cat going bad, while I did pass the emissions for my state I failed the CO tests and the exhaust has a weird smell to it kind of like an acidic smell not your normal exhaust smell, definately smell it when I start the car up for teh first time in the morning to let it warm up. Was looking to replace the cat anyways, it's just kinda weird to me rolling down the road and be popping when I get on and let off the gas pedal. Actually its cool when I let off it but not when I'm on it. So are there any other signs of retarded or advanced timing, my gas mileage is starting get real ****ty can't enough go 300 miles on a full tank anymore. oh well I don't think it's on the intake side cause I hear it popping from the muffler not the engine.