I've had the same set-up for years with few changes: Indy SR-headed 440, with Mallory/Hyfire 6AL ignition box (mounted under the hood), MSD coil, Mallory distributor. The only recent change was a couple of months back when I swapped out my 0.031" Holley squirter for a 0.035" squirter to help with a slight off-idle bog (I don't believe that this is the culprit).
The car has been having some kind of strange starting issues lately. The battery will seem near-dead after sitting for a few days, and will sometimes die before I can get the engine running. The battery's only 4 years old, and didn't seem to have any issues when I recently put it in my truck.
Out of the blue, the engine started backfiring through the exhaust yesterday. At first, I thought that it was loose plug wires, but they checked out. I also pulled all of the plugs and the distributor cap, but everything looks good. I tested the coil's resistances and they were 0.8 ohms across the primary and 7500 ohms across the secondary (higher than the advertised 4500 ohms).
I could be wrong, but I'm convinced that it's an ignition problem. Other than the backfires, the engine seems strong while running with the car in park. The backfires aren't consistent. If I bring the engine up to about 2500 rpm I'll get a loud backfire about every 30 seconds.
Any idea what I should check next? ...pull the valve cover? ...check compression?
Thanks in advance for any help?

fouled spark plug, that is what I would check 1st.

My 511 is doing the same thing. I think its a bad plug. Gonna change plugs tomorrow and see if it goes away. Mine pops out exhaust while at idle in gear only

#1 on the hit parade should be to get the battery problem fixed. Start with the battery - charge it and load test it. Then check to see that the charging system is working - 13.8 to 14.2 with the battery fully charged. Next check for an excessive amp draw with the key off. If that all checks out, your back fire problem is not related to the above ( based on what you have already checked). First thing to check is for exhaust leaks at the header flange.

I agree to make sure its a good strong battery. Also when does it backfire like when stepping on the gas or when letting off the gas ? Ron

Exhaust leak?

Had the same problem after chasing it through the charging system found the header gasket blow drivers side. Replaced and all was well... good luck

Had a bud with intermittent miss and it was his plug wires gone bad. We opened the hood at night, put the car in drive and brake stalled it up against the converter to load the engine. You could see the spark jump to ground and it would start missing.

After dark, with the car running in the garage; turn all lights off and watch all spark plug wires and the ign ystem. If they are leaking you´ll see it. Been there done that.

Ragnar in Iceland.

The plugs looked good, but I can go ahead and swap them out and see if anything changes. I'll also swap my truck battery in and see if that makes a difference.
I can't tell if it is backfiring through the same side of the engine each time or not. I'll also check and see if I've got an exhaust leak near the heads.
Thanks for the input.

Parts store batteries typically only last 4-5 years. 5 if it's a good one. So get a new battery first and make sure your charging system is working.

If the plugs don't fix it and your wires check good, it's possible you damaged the MSD box w/ the low voltage. MSD boxes don't deal too well w/ low voltage when cranking and trying to run. I've had this happen a couple of times.

I have found that on MSD distributor that the "pic-up Coil " in the distributor is the problem! Had that problem twic this year on two different engines! Good Luck

I agree low voltage and 6A boxes don't like each other. Also, where is the 6A located? Mine have always been pretty heat sensitive and act up with they get hot or have been heat cycled a bunch.

Header loose or bad header gasket

Changed plugs today. Exhaust pop gone. Runs smoother to. All were dark.

Well I have the same problem. Battery is good, engine starts right up . It has a miss . Pops through the exhaust going down the road . Not a gasket leak , put in new plugs, no change, started in the garage at night with the lights off , reved engine with no spark tracing anywhere. Used those wire testers with the lights in them to check to see if the wires were firing at the plug , all good, Inspected the cap, checked the point gap,and timing . Finally did a compression check. All cylinders were 150 except 2 which were 145. Now what did I miss?

I was swapping out spark plugs, just to make sure that one of the plugs hadn't gone bad, and decided to check the plug wires while I was at it. The multimeter showed that two of the eight had no continuity.
...not 100% sure that that's what is causing the backfires (could still arc across whatever gap exists), but they definitely need to be replaced.

Had the same problem yrs ago. I grounded the block directly to the battery. I put a terminal strip under the dash, grounded everything to it i.e.,tach, ing box & gauges & ran it directly to the battery. No more miss.

I would check the float bowls and float needles to see if there is any debris in either one. Or possibly a leak near one on the collectors. Something has to be putting enough extra fuel or oxygen into the exhaust system to produce the backfire. Maybe a rear float set too high and you are getting a splash of fuel into the intake on bumps or decel? A bad power valve will do this but usually that will show up as a super rich mixture and foul plugs.

I did notice the small ground strap was broken from the engine to frame. Do you think that could be the problem?

X2 then (charged up (from a charger not the alt) take it out & if still bad change the coil, put in any one from your stash just for a test (with a ballast if need be). EDIT vac leak is another potential & you did change plugs correct? plug wires would be further down on the list

after running my new plugs a while the pop out the exhaust came back. I discovered that I had the magnetic plug hooked wrong. ya see with MSD the orange wire is the positive and on an Ice ignition box the purple is positive. Reversed the plug, reset timing and so far no pop out exhaust.

Mine is a stock 1964 points ignition. Coil, condenser what's left ? I guess I'll just start throwing money at it till it stops .

Well, first I would check all your grounds because its most likely electrical issue causing that. Next I would take a hard look at your distributor. The reason mine was popping was because with the polarity of the pickup coil reversed it put the rotor out of phase. Might want to check that. Maybe you just need a new cap and rotor. I'm sure its something in your electrical system between dist. and spark plugs to ignition box imho.

Do any of you guys have power brakes?

I do not have power anything.

I had asked when it did it because if it backfires when letting off the gas then I would look for an exh leak at a header gasket as it will suck in air on decell and can cause it. If it does it when stepping on the gas and feels like it misses at the same time then I would be looking at the ign system real close. Or does it do is just sitting there idling ? Ron

I swapped out the battery and the plugs and the backfire went away. I was surprised, considering that I didn't replace the two plug wires that lacked continuity. I guess that the spark is able to arc across whatever gap is in the wires with a good battery. Of course, I'll be replacing the wires soon.
On a side note: my engine seemed to like the NGK V-Power plugs that I used to have in there a lot more than the Autolite Racing plugs that I swapped in.

When motors talk to me I listen, especially on spark plugs
Most racing plugs are COLD, some are to cold for pump gas on the street