Hey there,
I have a 72 Duster that sits for long periods of time. It is my Brother in law's car and he lives out of state. 360 with a Holley 600 4 barrel.
Here in CA, the blend is supposed to be "up to 10%" but testing has been reported to be around 6%. Regardless, the color of the fuel in the filter and the bowls is a pale yellow green. The last time I put more than 3 gallons of gas in it was several months ago, maybe last fall? Not sure.I did put about a gallon in it recently since I didn't know how much was in it.
My point....
The car idles fine. I did have to replace the needle & seat in both the primary and secondary bowls. Under a load, it sometimes backfires through the carburetor, It is not consistent like it would be with a bad exhaust valve or worn exhaust lobe, just occasionally. The ignition system is old. The engine bay wiring is old. The alternator is not charging even though I have tried three alternators and a different voltage regulator.
Yeah....The car needs work as many do,,,,But my question is: Can old ethanol laced fuel cause a backfire like this OR should I be looking at the ignition system?

Old bad gas with any ethanol can and will clog passages in a lot of carbs from corroding them Even Holley carbs . Unless they have the special coated bowls and metering plates for ethanol type fuel
Me thinks the best way to get around this is to run the tank dry when you know it will sit for more than 5 or 6 weeks and then add new fuel to run it on when you start it up the next time. Can you get any non Ethanol pump fuel there If so ,even if it is 87 octane regular, use a tiny bit of it (1/2 gallon) to run it on before letting it sit so it cleans out all the ethanol fuel in the system

Good idea. Thank you.

Never had a backfire from 3-month-old gas, even with ethanol.
But in anything that sits, I put Sta-Bil in the gas, and using an eye-dropper a few drops into the carb bowl vents.
You might have a bad accelerator pump diaphragm or seal.
I'd take a look at the cap & rotor for condensation/corrosion/carbon tracking.
Also make sure the mech & vac advance are operating.
Then I'd put a timing light on it to see where it's at & if it's moving around.
If the fuel filter looks like it served in WW2, I'd change it.

He should stick to stuff he knows for video.

Here's an example of umbrella check valve that got distorted. (first photo below)
Ethanol is just one of many components in automotive fuel that are not refined petroleum.
Any one of those components can damage rubber and plastics.
Find an 'old timer' who remembers the fuel reformulations in early 70s (see the end ofof post 9 here for example ).
There was another big change with introduction of reformulated gasoline (RFG). RFG first used MTBE, now pretty much phased out. Generally contains eth these days.

Ethanol street mixes do not evaporate quicker or cleaner.
Rather more of the light end evaporates at lower temperature, and complete evaporation (100% of liquid evoporated) occurs at higher temperatures.
Observe yourself. Take a couple ounces of your California summer RFG and a couple ounces of VP C9 or Sunoco GT or GTX and let them evaporate in something. That's not 100% fair test because race fuel is not the same as 1970 High Test. But the point is to see for yourself that today's pump gas doesn't do what Tony claims.

Here's a graph of fuel evaporation vs. temperature.
These are about the most recent distillation curves I've found for motor gas. But gives some idea of the effect of reformulation.
Read it like this.
On the left side look for a temperature. Then draw a straight line across and see how much volume turned to vapor at that temperature.

Re your charging issue. Does a 72 have the electronic regulator with the so called dual field alternator?

If so your problem may be under the dash. My dad had a 75 Gran Fury that randomly refused to charge and he blamed it on "that new fangled" electronic regulator. He changed it back to a mechanical regulator with a single field wire alternator and it never missed a lick.

Years later he was under the dash changing dash lights or something and noticed the ammeter connections were green as grass. Cleaned them up and all of a sudden the ammeter actually showed activity. Put the original alternator and regulator back on and it worked until he sold the car.

Apparently whatever tells the electronic regulator that the battery is full is sensed off the ammeter circuit and it was reading the resistance from the bad connection as a fully charged battery and not turning the ground on to the field.

Kevin

I watched the You-Tube video and the guy is spot on with daily drivers but anything that sits its a bad deal, had two trucks that it wrecked the pickup tubes & floats in the gas tanks. I use the premium in stuff that sits and has a carb with way better results.

Get a 5 gallon can of aviation fuel at your local airport. It will be leaded so do NOT use it in your daily driver as it will hose your catalytic converter. Run your fuel down and put a 1/2 gallon in at the tail end. It will last up to 3 years with zero additives. Ethanol cost me nearly $1000 to get a small boat running after sitting. Rebuilt carb and replaced the entire fuel system.

Thank you.
I did a search and found ONE gas station within 20 minutes that sells NON Ethanol gasoline.

I do the same thing for my Duster. I use 91 octane NON Ethanol "REC". gas in it at all times...from the local CO-OP Elevator, about 20 minutes from me as well. And in the fall before it goes into storage..I add Stabil to the tank for good measure.