I believe an oscilloscope is the test equipment to which you refer. Without one of those I would connect your timing tight to each plug wire and accelerate the engine watching for a miss in the light flash and also watching for the timing mark to be "jumping around" indicating a problem in the distributor.
A backfire through the carb is usually caused by a "too lean" fuel mixture. Have you watched your AFR gauge when it happens? When you goose the throttle and it backfires you should see a full lean (20:1) condition on your gauge. Have you confirmed that your accelerator pump is functioning correctly?
 

It's an Edelbrock. I put the next size bigger accelerator nozzle in it and the next size bigger float nozzles. It was running out of gas in 2nd or 3rd gear when I had it WOT. The AFR stays where it is when it starts to miss. When I hold the throttle open for a length of time or move it slowly to WOT it moves to indicate rich. That's the way I have it set up. No matter what speed I'm going if I WOT it pops. I When it backfires it starts missing and continues till I back off. The accelerator pump squirts a good strong shot.

Does this occur when engine is cold or up to operating temperature?

What ignition system?

Does this happen at different initial advance settings?

Last edited by Nos681 (3/21/2021 7:32 AM)

I have two fuel pumps. The engine driven pump (brand unknown) and an Edelbrock electric that is switched. It makes no difference on or off. That led me to think the secondary's were only partially working causing it to run lean and backfire. I eliminated that theory because I can spray carb cleaner through the jets and it comes out all the holes and the AFR indicates that it is going rich. I'm running Autolite plugs that are not very old via a DuraSpark ignition system with a new Spectrum distributor. I had to bend the mechanical stop way back to adjust it so it wouldn't go past 35° BTDC. Ford distributor parts don't interchange with Spectrum distributor parts. I had Accell ignition wire wires but changed them to Taylor carbon wires after the last bash because of radio and AFR interference. (And MS doesn't like yellow wires) It's probably the wires. They came too long so they are wrapped allover the place. No matter the speed or RPM when just barely stepping on the gas it will back fire and continue to run rough till I let off the gas.     

HudginJ3 wrote:

I have two fuel pumps. The engine driven pump (brand unknown) and an Edelbrock electric that is switched. It makes no difference on or off. That led me to think the secondary's were only partially working causing it to run lean and backfire. I eliminated that theory because I can spray carb cleaner through the jets and it comes out all the holes and the AFR indicates that it is going rich. I'm running Autolite plugs that are not very old via a DuraSpark ignition system with a new Spectrum distributor. I had to bend the mechanical stop way back to adjust it so it wouldn't go past 35° BTDC. Ford distributor parts don't interchange with Spectrum distributor parts. I had Accell ignition wire wires but changed them to Taylor carbon wires after the last bash because of radio and AFR interference. (And MS doesn't like yellow wires) It's probably the wires. They came too long so they are wrapped allover the place. No matter the speed or RPM when just barely stepping on the gas it will back fire and continue to run rough till I let off the gas.     

Are those spark wires touching any metal?  They could be arcing and making the ignition short out.
 

How old is the gas tank and THE SOCK/float assembly in the tank?
6s6

HudginJ3 wrote:

It's all OEM. ???   I know that stuff get's old and deteriorates. I've had the car since 78 and don't really suspect the sock filter but I get the idea that you do. I just changed one in a 68 Mustang that has been sitting for the last 16 plus years but it had old gas in it. The sock was shot and the gage didn't work. I would suspect that because of the old gas. The gas gage in my car still works and has always had fresh gas in it. So I guess I'm asking if you think it's deteriorated causing a restriction and not an ignition issue. I hate the idea of changing it with an inferior new part. Especially when I just filled it up. 

I'm with you on that.  If the gauge works I would probably investigate this last.  The OEM sender is going to be MUCH better than the new stuff.

Bearing Bob wrote:

HudginJ3 wrote:

I hate the idea of changing it with an inferior new part.

I think people are saying to inspect the sock to see if it's clogged or damaged, and not just replace the float unit. Since your gauge still works, worst case scenario would be to replace only the sock.

What HE^^^^^^^^^^^^^^said!!
This junk they call "gas"... turns to goo sometimes if it sits in a tank for any length of time. (stinks too!)
Might be a 'last-thing-to-check' item.
I hunted and hunted a strange miss while at Cruis'in-the-Coast a few Bashes back. Never did figger it out until I got home and decided to change the tiny-lil glass filter going into the Eddy carb.
BINGO.....prob solved!.....Crazy
6s6
 

John Ha wrote:

HudginJ3 wrote:

After a while longer it would not start because it was backfiring as it cranked. 

I've always thought that backfiring suggests a timing issue.  Have you put a timing light on it (you can check when someone is cranking it if it won't start) to see whether you're on the mark?  It sounds suspiciously like the timing chain has gone off.

That was something I initially considered as well, but was hesitant to go there because he stated the engine was rebuilt.  He did say it was awhile ago, but given how little mileage most of these cars see I assumed (possibly incorrectly) that there's no way the timing chain could have that much slack in it.  If all the easy, normal stuff checks out though this is where I'd go next.  Good call John. 

John Ha wrote:

I've always thought that backfiring suggests a timing issue.  Have you put a timing light on it (you can check when someone is cranking it if it won't start) to see whether you're on the mark?  It sounds suspiciously like the timing chain has gone off.

Something else just popped into my mind while I was reading this over again  Long ago I'd bought a chrome distributor hold down clamp and installed it.  One day as I was driving the car started running worse and worse - had to keep my foot on the throttle when I stopped or it would stall.  I managed to get it home and found that the clamp had bent and wasn't holding the distributor securely.  The distributor had rotated way out of time.  So I put the original clamp back on it, retimed it and we all lived happily ever after (at least as far as the distributor clamp was concerned).

So maybe that's another place to look.

Last edited by John Ha (3/24/2021 5:56 AM)

Glad you found the issue Doug. 

Hmm... It appears the hot tip was from post 5 back in March.

TKOPerformance wrote:

This could certainly be fueling related, BUT its worth looking at the ignition side, even if you don't end up replacing anything. 

I would start by pulling the plugs and looking at them.  If they look worn I would replace them with a set of good copper plugs (Autolite is my personal go to).  If the wires are more than a couple years old I'd replace them too.  Wires tend to fail due to age more than use.  When routing the wires make sure that wires which fire cylinders which are next to each other in the firing order don't run next to each other as much as possible to prevent an inductive misfire.  I'd also check inside the distributor cap for any carbon tracking, etc., and replace the cap and rotor if they look worn.