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, if your car backfires through the carb then the air/fuel mixture is too lean. Vacuum leak? Pete

Pete,
The backfire is in the exhaust, not the carb.
Thanks.
Jim Yergin

Jim,

The "exhaust" kind of "backfire" is usually somehow related to raw fuel getting into the exhaust system; then, getting ignited by a subsequent "fired" exhaust.  This could be related to cylinder misfires where the fuel is not burned.  At those RPM's I'd expect a possible continuing ignition issue.  Or, the carb is somehow on the verge of flooding-out [too much fuel].

For my part, I'd still look at the ignition first: Points and condenser; points could have too-soft spring tension, "Off-shore" condensers have a bad reputation.  Could also be the ignition lead from the coil rubbing-through it's insulation inside the distributor and "grounding" at certain spark advance, or where it's connection goes into the distributor wall through the insulated bushings [these wear/break]; and, could be an actual poor-ground [braided copper ground lead between distributor case and advance base plate].

Look very carefully at these "minuscule" details. Before retirement, I worked a lifetime of electrical/electronics; and, I can tell you for certain, that these little details can "bite" you severely.

Regards, JLB

55 Ford Crown Victoria Steel Top

Above Forum member suggestions are all GooD!  I recommend that you also take a hard look at the spark plug wire routing, i.e. what is next to what in the firing order and look closely for possible carbon tracking between the terminals inside the distributor cap.  I also would be highly suspicious of the condenser inside the distributor.

Hope this helps!  


NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

Check both ends of the ignition wire going from the coil to the distributor cap.  This caused me more than a few days grief some time back. Carbon core wires have a tendency the burn their core.

Thanks for all the suggestions. I cannot see any problems in the distributor wiring. I installed a new condenser. A little better but still occasional backfiring. I have a T5 five speed in my car. The backfiring is worse when in 5th gear and revs are under 2000 rpm. The car has dual exhaust and it sounds like the backfire is in the driver's side exhaust. Does that help at all as to where to look? How do I determine if the routing of the spark plug wires is a problem? I really appreciate the help.
Jim Yergin

bad plugs did it on my 56.in that case someone had gapped the plugs at.015.

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