Start at the beginning of things.
Pull your plugs and not the colors. Note which plug goes to which cylinder. Get a spark plug color chart out and match them up the best you can with the pictures. Are they all too dark? than its too rich. Are they all light colored then it's too lean. Is there black sludge on one side of the plug? Most likely oil leaking past a valve. etc. etc. Is one plug mcuh different in color thatn the others? That cylinder is most likely the problem.
Do a compression test. Are all the cylinders pretty close in compression, or is one way off?
Do a leak down to determine if there is a leak through a valve. etc. then check the valve adjustment on the one in question to see if that solves the problem. You may get the valve adjusted just right and make a bad adjustment on another creating the same (or different) problem elswhere.
Don't jump around guessing. Check the easy things first and draw a conclusion from your findings.
When you say "backfire" is a loud bang like a firecracker, or a rumbling popping? If it is the rumbling popping when you let off the gas, my next question is "Do you have glass packs or a real muffler?"
