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

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.



Lawrenceville, GA

Have you checked the timing with a light to see where it is set at?

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

55 Ford Crown Victoria Steel Top

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.


Lawrenceville, GA

Charlie,

Having the "when new" info. is great; I should, but forget! Very useful on most anything, though.

Regards, JLB

55 Ford Crown Victoria Steel Top

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