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Discussion Starter · #1 · (Edited)
I have a 29 model A coupe with 350.
Has been running great for awhile now.
Coming home the other day it just started coughing and backfiring through the carb.
Took it home and shut it down.
Next day it started fine, ran great until it warmed up then went back to the backfiring through the carb.

The fuel pump is a holley black, and it's getting good fuel pressure to the carb on the other side of the regulator 6.5psi.
The accelerator pump is putting out real good.
I drained all the gas, ran it out of gas and put new ethanol free gas in it, still happens.
The float bowl level was just a little low so I adjusted it a bit higher, just above center of the site glass.
I checked the timing its set at 12 BTC.
Also did a test to make sure power valve was not blown (turned in idle screw and engine died)

The carb is a quick fuel and is less than a year old.
The distributor is HEI, generic from speedway motors and is about 2 years old.

I'm going to check the compression tomorrow.
What are other things I can check?
Any thoughts on what things can cause it to suddenly start backfiring through the carb?
I need some sort of systematic approach.
Any help greatly appreciated.


Paul
 

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Is this a backfire "cough" in the intake manifold?
Camshaft gong flat will give that symptom if it is an exhaust lobe.

Dual plane intake?? A problem in one side of the idle circuit can give a pop only in every other cylinder occasionally in the firing order. Clean the air bleeds and idle passages with carb spray, including removal, spray out, and replace the idle mixture screws and see if it helps.

Carbon tracking inside the distributor cap, loose rotor, burnt rotor tip or burnt rotor carbon button are all possibilities creating misfire or fire at wrong cylinder timing

For sure not an afterfire pop in the exhaust?
 

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Discussion Starter · #3 · (Edited)
Is this a backfire "cough" in the intake manifold?
Camshaft gong flat will give that symptom if it is an exhaust lobe.

Dual plane intake?? A problem in one side of the idle circuit can give a pop only in every other cylinder occasionally in the firing order. Clean the air bleeds and idle passages with carb spray, including removal, spray out, and replace the idle mixture screws and see if it helps.

Carbon tracking inside the distributor cap, loose rotor, burnt rotor tip or burnt rotor carbon button are all possibilities creating misfire or fire at wrong cylinder timing

For sure not an afterfire pop in the exhaust?
Thanks for the input.
It is a dual plan e manifold.
It's definitely popping out through the carb.
Also I pulled some plugs, and the tips are grey, but otherwise they are black and sooty.
I'll try the suggestions.
 

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Black and sooty is running rich and or running cold. Inch and cold can come together if trips are too short and the engine doesn’t get hot enough to get off the choke nor warm the coolant.

This can also be the plugs are too cold a heat range, have been fouled previously and are unable to clean themselves which is common with modern peanut plugs, replacement is the only solution here. This is also a common problem to aluminum heads, they pull off heat much faster than iron so if you’re running aluminum heads going hotter on the plug heat range and the thermostat will help.

Sometimes these no name distributors are a bit light on secondary voltage so a little too much plug gap or once plugs foul you get misfires. I‘d put in fresh plugs not gapped wider than .035 inch to see if there is n improvement. In the vein of HEI distributors these do require full battery voltage all the time, so be sure the run wire from ignition switch to either the coil if independent of the cap or for an integrated HEI the correct B+ voltage is being supplied which would not be using a resistive wire nor external resistor in that circuit. If you use a CD box be sure the heads are grounded, the extra energy of things like 6AL will happily back feed ground through the electronics if the engine’s grounds are not good enough. Remember we’re talking potentials which doesn’t mean one side has voltage and the other side none; it means one side has more voltage that the other, the flow is from more to less not necessarily more to none.

Bogie
 

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Discussion Starter · #5 ·
Black and sooty is running rich and or running cold. Inch and cold can come together if trips are too short and the engine doesn’t get hot enough to get off the choke nor warm the coolant.

This can also be the plugs are too cold a heat range, have been fouled previously and are unable to clean themselves which is common with modern peanut plugs, replacement is the only solution here. This is also a common problem to aluminum heads, they pull off heat much faster than iron so if you’re running aluminum heads going hotter on the plug heat range and the thermostat will help.

Sometimes these no name distributors are a bit light on secondary voltage so a little too much plug gap or once plugs foul you get misfires. I‘d put in fresh plugs not gapped wider than .035 inch to see if there is n improvement. In the vein of HEI distributors these do require full battery voltage all the time, so be sure the run wire from ignition switch to either the coil if independent of the cap or for an integrated HEI the correct B+ voltage is being supplied which would not be using a resistive wire nor external resistor in that circuit. If you use a CD box be sure the heads are grounded, the extra energy of things like 6AL will happily back feed ground through the electronics if the engine’s grounds are not good enough. Remember we’re talking potentials which doesn’t mean one side has voltage and the other side none; it means one side has more voltage that the other, the flow is from more to less not necessarily more to none.

Bogie

Thanks for the input
During this covid thing, I have just been driving it a few miles once a week to keep it running. I do drive it until the temp gauge reaches 180, but it doesn't stay there long before the trip is done.
I'll try some new plugs and go up a heat range and see if that helps.

What is odd to me is how suddenly this problem came on, and how it runs fine when it is cold.
I did notice that I did not do a good job keeping the plug wires separated when I put them on, the set was just a bit short for the wire holder setup I used, and they are touching each other near the distributor. It did run fine for a couple of years though.
I'm really just a car guy novice, no expert here. This forum has gotten me out of a few head scratchers with input form you and others on this forum. Thanks.



Paul
 

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JMHO, sounds like it's time for a tune up to me. Resistance goes up with heat, chance of cross wire goes up with temp increase. SBC are susceptible to crossfire between #5 & #7 as they are next in firing order and adjacent each other, keep em separated from their cap to the plugs.

For street with carburetor, grey/lt brown tip is not unusual with oxygenated fuels and usually good, black sooty thread ring indicates you're idle, I like a full thin black ring at the face of the ring on the plug, going down the outside of the threads tells the heat (as will the strap), two to three rings is good to me (with color change midway on strap). I ain't talking reading racing plugs and I know I'll get whipped.

If it has been running good and suddenly started these symptoms, I would do as suggested by others, tune it up, new cap, rotor, wires & plugs. And as stated, plug gap 35-40 thousands on our old school street motors (anymore gap is just asking for the spark to jump through average wires).
 

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Discussion Starter · #7 · (Edited)
Thanks for everyone's input.

I replaced the spark plugs, with some AC delcos, and went up a heat range.
Now it runs like it used to again. No backfiring.

So I really want to understand how cruded up spark plugs can make it backfire.
Do the carbon deposits in the black soot retain heat, and then cause it to fire off as the intake valve is trying to let in gas/air?
A couple of them were also a little oily.
 

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Thanks for everyone's input.

I replaced the spark plugs, with some AC delcos, and went up a heat range.
Now it runs like it used to again. No backfiring.

So I really want to understand how cruded up spark plugs can make it backfire.
Do the carbon deposits in the black soot retain heat, and then cause it to fire off as the intake valve is trying to let in gas/air?
A couple of them were also a little oily.
Usually, the way this works in this situation, is the cylinder misfires. Then on the exhaust stroke the unburned mixture is pushed out into the exhaust where it is lit off by hot exhaust gases from another cylinder, and then burns back into the cylinder through the open exhaust valve. This late burn then also catches the next fresh intake charge at the point in the overlap period in cam timing where both valves are slightly open, intake opening and exhaust closing as the TDC piston pushing exhaust out and getting ready to draw new mixture in.
Root cause was the fouled plug failing to jump a spark across the gap sufficient to light the mixture at the correct time. Cold mixtures light easier, as they are richer...warm mixtures are leaner and harder to light.
 

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Discussion Starter · #9 ·
Usually, the way this works in this situation, is the cylinder misfires. Then on the exhaust stroke the unburned mixture is pushed out into the exhaust where it is lit off by hot exhaust gases from another cylinder, and then burns back into the cylinder through the open exhaust valve. This late burn then also catches the next fresh intake charge at the point in the overlap period in cam timing where both valves are slightly open, intake opening and exhaust closing as the TDC piston pushing exhaust out and getting ready to draw new mixture in.
Root cause was the fouled plug failing to jump a spark across the gap sufficient to light the mixture at the correct time. Cold mixtures light easier, as they are richer...warm mixtures are leaner and harder to light.
Thanks. I can visualize that.
 
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