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By Jim Yergin - 5 Years Ago
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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
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By Pete 55Tbird - 5 Years Ago
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Jim, if your car backfires through the carb then the air/fuel mixture is too lean. Vacuum leak? Pete
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By Jim Yergin - 5 Years Ago
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Pete,
The backfire is in the exhaust, not the carb.
Thanks.
Jim Yergin
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By bergmanj - 5 Years Ago
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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
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By NoShortcuts - 5 Years Ago
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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! 
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By Florida_Phil - 5 Years Ago
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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.
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By Jim Yergin - 5 Years Ago
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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
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By darrell - 5 Years Ago
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bad plugs did it on my 56.in that case someone had gapped the plugs at.015.
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By Rono - 5 Years Ago
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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.
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By oldcarmark - 5 Years Ago
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Rono (5/23/2019)
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.
That Firing Order is correct. 1 5 4 8 6 3 7 2
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By bergmanj - 5 Years Ago
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Jim,
I just recalled that there was a technical service bulletin on switching [#6 & #8 - I think] spark plug wires in the square rubber loom at the back of the engine under the exhaust manifold [driver's side] to prevent "cross-firing" between those two cylinders. I do not remember the specifics, nor do I have a copy to include here. Someone else please post if you have the drawing.
Regards, JLB
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By Jim Yergin - 5 Years Ago
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Thanks JLB. That's what I meant about the routing of the spark plug wires. I have the correct firing order. Sorry for any confusion in my question.
Jim Yergin
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By Jim Yergin - 5 Years Ago
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Found this:
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By NoShortcuts - 5 Years Ago
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Jim. Earlier in this thread you wrote:
'I have a T5 five speed in my car. The backfiring is worse when in 5th gear and revs are under 2000 rpm.'
I hope the spark plug wire routing of number 6 and 8 is responsible for your problem. Finding that service bulletin was GreaT. Another possibility to
consider . . . I had a late model Ford that had a miss only in overdrive. The car had quality spark plugs which had less than half their 100,000 mile rating of use, BUT after checking out numerous other possibilities, the spark plugs were determined to be responsible!
Let us know what you find to be the problem.
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By bergmanj - 5 Years Ago
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Jim,
THAT's the one [the TSB I was thinking about]!!!
Mentioning the O. D: There's a "spark-kill" circuit involved with O. D. "kick-down" & Solenoid I wonder if this may be involved due to O. D. relay points, solenoid points, other connections, chaffed primary spark wiring, etc. Might be worth a look, too!
Hope it helps you and others.
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By Gene Purser - 5 Years Ago
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I'm glad to hear about the induction misfiring bulletin. I never would have figured that one out.
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By Jim Yergin - 5 Years Ago
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JLB, can’t thank you enough for the tip. I repositioned the spark plug wires and there was a big improvement. However there is still occasional backfire so there must be another problem. There is no solenoid/spark cut out in the T5. Over drive is just a fifth gear.
Charlie, what brand of spark plugs solved your problem?
Jim Yergin
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By NoShortcuts - 5 Years Ago
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Jim, a faulty spark plug could occur with any brand name. Mine happened to be an original from the factory installed MotorCraft plug. The miss only occurred in the overdrive gear, and only when under load and pulling at lower rpms. Finally found the culprit with an oscilloscope mounted in the car out on the road duplicating the engine load condition.
John's suggestion is equally as possible as the one you found in the Service bulletin. It's all about the parallel AND proximity routing of wires to plugs that fire in sequence. You have to know how the cylinders are numbered on the particular make and model of engine you are working on, though.
Hope this helps!
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By emtdude56 - 5 Years Ago
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check your valve adjustment&valve train
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By Andy - 5 Years Ago
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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 .
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By Jim Yergin - 5 Years Ago
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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
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By bergmanj - 5 Years Ago
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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
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By pintoplumber - 5 Years Ago
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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.
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By Sandbird - 5 Years Ago
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Have you checked the timing with a light to see where it is set at?
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By bergmanj - 5 Years Ago
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Charlie,
In my experience over many decades, resistance wires are not all that "accurate" as to resistance per foot; they just don't seem to have had "precision" manufacturing. As a matter of fact, precision is not really important for this application. That's why I earlier suggested "around" 15K or somewhat less.
You are absolutely correct about one being "way-out" (much higher resistance) as compared to the rest as probably being bad.
The way most are manufactured is by taking raw core material (cotton, rayon, etc., chord/string) and impregnating it with, essentially, carbon granules (sometimes graphite), then molding that core into the outer insulation. Be aware that this core can "burn-out" if it has a "weak" spot (low quantity of carbon) - I've actually had this happen once years ago on my Crown Y-block with standard points ignition; it's probably worse with HEI.
I retired out of 45+ years of working in electrical & precision electronics (power plant instrumentation, and precision military electronics manufacturing and test); and, have never seen "precision" resistance wires or, for that matter, resistance plugs for regular "consumer" use (for the military, "yes" - so, it is "possible"). Their manufacturing processes for consumer use vary all over the place: I never take their "specs." for granted.
Regards, JLB
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By bergmanj - 5 Years Ago
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Charlie,
Having the "when new" info. is great; I should, but forget! Very useful on most anything, though.
Regards, JLB
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By Jim Yergin - 5 Years Ago
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Good news/Bad news.
The good news is that I have solved my backfiring issue. The bad news is that in doing so I have confirmed I am an idiot.
JLB nailed it when he suggested to "Keep It Simple Stupid" (in my case the emphasis is on "stupid"). I went back to basics and found that the spark plug leads for cylinders #3 and #6 were switched on the distributor cap. Corrected that and no more backfire and the car is a true pleasure to drive.
Thank you all for your advice. Please forgive my incompetence.
Jim Yergin
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By MoonShadow - 5 Years Ago
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Glad you found your problem but you are not an idiot. Many of us have crossed some plug wires along the way leading to mysterious problems. Often time a return to the basics will fix it. Sometimes its best to back away from the problem and come back at it with a fresh mind. After checking the same thing more than once we tend not to check them as closely as needed. Great news!
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By bergmanj - 5 Years Ago
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Jim,
Good for you! We've ALL done many "stupid" things (LOL - on ME).
Regards, JLB
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By Hoosier Hurricane - 5 Years Ago
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Jim, if crossfiring was happening between 6 and 8, it could also happen between 1 and 2, one follows 2 in the firing order.
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By charliemccraney - 5 Years Ago
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You need to know the manufacturers specification for the resistance of the wires. If you don't know the manufacturers specification for the plug wires, then you cannot know what the resistance should be. What you can do then, is measure all of the wires and calculate the per foot resistance and make sure it's all very close. If you find one that is significantly different, then it could be a problem.
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By charliemccraney - 5 Years Ago
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Sure, don't take it for granted but the manufacturer's specification is the best thing we have to work with. It may not be the most accurate process but it should have a spec and possibly, tolerance. If they say their wire should have a specified resistance, then it should have that specified resistance. If it doesn't there's probably a problem. If we can't rely on a manufacturer knowing their product, what can we rely on?
What I've been doing is measuring the resistance of wires when I get them. brand new. If there is a spec, I make sure it matches. If there isn't, I make sure all of the wires match per foot. If I find anything anomalous, I'll get another set. When I have the set I will install, I record the resistance so that way I have the new measurements to compare against in the future if a problem should arise.
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