I am having a backfiring problem with the 312 engine in my '56 Thunderbird. It occurs when accelerating under load between 1500-2000 rpm. Engine has been rebuilt, carb has been rebuilt, replaced the ignition condenser, rechecked point gap and timing, new spark plug wires and made sure they are properly seated. Still have the backfire. Any suggestions what I should be checking?
Jim Yergin

Jim;

Some stock 56 Intake Manifolds have the firing order cast into the top. If you have a shop manual it will be in there also. I think it is; 1-5-4-8-6-3-7-2, but double check that. Once you know the firing order, you can then simply trace the wires back from the spark plugs to the distributor. The rotor under the distributor cap rotates counter clockwise so trace the wires carefully in a counter clockwise direction around the cap. The 1st cylinder on the passenger side is #1, then #2, #3 and #4. Front drivers side cylinder is #5, then #6, #7 and #8.

  Ron Lane,  Meridian, ID

That Firing Order is correct. 1 5 4 8 6 3 7 2

  

check your valve adjustment&valve train

If you have checked all the ignition and carburetor isn’t to lean pull the valve covers and check valve train. Look for a broken valve spring, bent pushrod , or possibly worn cam lobe .

Not having much luck. I have replaced all of the spark plugs, the points and the condenser. I verified that the spark plug wires to #1 and #2 cylinders are not routed next to each other. I adjusted the valve lash. I inspected the valve springs and the push rods. They all appear OK. Still have the backfiring. Is it possible there is a problem in the spark plug wires? They are new but I have run out of things to check.
Frustrating.
Jim Yergin

Jim,

Measure each spark plug wire resistance from end to end (including the distributor-to-coil wire).  If "resistance" wire, they should be about 15,000 (15K) ohms or less (maybe down to about 5K ohms for the shortest); if solid wires, they should each measure very near zero ohms on a low-ohms scale (with GOOD / SOLID meter-lead connections).

Are you absolutely sure of no "carbon tracking" inside the distributor cap - those can be near impossible to see, especially on a black bakelite cap.

Good luck!  The only comment I now have is that you're probably going to find something "stupidly simple" as has been my experience on several previous occasions of my own - referring to myself: KISS Go back to simple basics and start over after letting the issue rest for a couple of days.

Wish I could be of more help; but, I'm out of ideas via this "remote" troubleshooting.

Regards, JLB

55 Ford Crown Victoria Steel Top

I drove around 500 miles this past weekend. My truck was backfiring occasionally under light load mostly on the drivers side. Mine is running too lean. My plugs are as white as the day I put them in, but they’re 2 or 3 years old.

  Dennis in Lititz PA