have a 1967 Cevelle SS 396. Has electronic ignition, new plugs, wires, rotor. Was driving it and engine started to backfire, making a "popping" noise from carb. Ran compresson test, and all cylinders indicated 120. Backfire not completely constant, but increases with RPM. Help!

Did this occur immediately after you gave it a tune up? Possibly a couple of plug wires not in the correct firing order?

Or did you tune it up thinking that the tune up would solve the problem?

thanks for replying. I had only replaced the wires. It was running well...no backfire or loss of performance. I was driving it at speeds of about 45-65 when it started to backfire. It was running good before then. I only replaced the plugs because I ran the compression test, and had broken one removing it. I replaced the rotor at that time because when I loosened the distributor cap for the compression test ( which I really didn't have to do), one of the wires got pulled into rotor and tore it up. Put everything back in place and it still backfires. I had thought it might be a valve problem but with the compression test and each cylinder at 120 lbs, I'm not sure what the problem is. Thoughts???
thanks for your help!

If it happened after changing the plugs then you may have a few routed incorrectly as mentioned above. It is backfiring through the carb because one or more of the plugs are firing when an intake valve is open. Also check your timing.

If all was well and it happened suddenly, a rocker may have broken
or came off some how. A compression test may not show the problem cylinder.
You can short out each cylinder untill the popping stops.
You'll have a miss, but it wont pop.
This will help you determine which cylinder is the culprit.
Remove the valve cover on that side and inspect the valve train.
Just a guess of coarse.

I had this problem once and it turned out that I left a few of the new plug wires too long and they burned on the headers. Caused some nice backfires. May want to check if the wires are not melted through somewhere.

FOUND THE PROBLEM WITH A STUCK LIFTER THAT WAS WORN THROUGH BOTTOM BY CAM hitting it but not lifting it...replaced lifter, but friend who was helping pulled distributor from intake before i had chance to align timing TDC with #cyl on rotor/distributor. Now I can't get the timing marks right to start the motor.help!

You assembly manual should explain how to time the engine withe the distributor position changed. Basicaly you can take on the drivers side valve cover. Rotat the engine to the number one position and reinstall distributor in the correct position. 120 compression seems a little low for a 67 BB.