Backfire after stopping

[rich40701]

Twice now I have gone to a local gas station and one way to get to it has a long down hill grade. Both times when I have stopped I get a loud backfire. Lol Yesterday it backfired and sounded like a gun going off and scared the bajesus out of some dude then about a minute later it did it again so loud I thought it would blow the manifold off. It has a Model B carburetor and is really loading up for some reason but not sure why also it seems to use a lot of fuel so for some reason it is not carbonating efficiently. Oh, it also came with electronic ignition and starts and runs great and I use Shell's highest octane. What do you think would be making it load up so much to explode twice?

[mcgarrett]

Not sure about the backfire issue, but I would offer some friendly advice about your fuel grade. It isn’t necessary to use the expensive high octane gasoline in a Model A. The gasoline they were designed to run on was very poor in the Model A era. High octane gas was formulated much later for engines with high compression ratios. Save yourself some $$$ by using regular unleaded.

[Marshall V. Daut]

Fully retard the spark lever and idle the engine for a couple of seconds before you shut off the engine. That will eliminate a backfire most of the time until you can trace the reason the engine is doing that. Backfiring is not a normal occurrence. Besides being loud and annoying, it could also blow out the muffler. Those puppies no longer cost $50, so to protect your $200+ Aires muffler (the only one we can buy these days), find the source of the backfiring post haste.
Marshall

[rich40701]

Thanks for the advice, I will start using cheap gas.

[Benson]

The down hill grade is most likely allowing un burned gasoline vapor to collect in muffler.

I would look for a leak in muffler somewhere.

Most likely it is in the muffler clamp area.

But rust holes in muffler (especially out towards the end of muffler) which cools off first is a "good" place for the air to enter muffler.

What happens is when you go to idle after coming down the hill the muffler starts to cool and then a small vacuum is created in muffler as it cools which sucks air into muffler. Then when air to fuel ratio becomes just right the mixture is ignited ... And BOOM.

I learned about this 53 years ago while working on Electronic Fuel Injected 1969 Volkswagen Type III engines in Fastback / Squareback models.

They had a circuit which turned off the injectors whenever there was deceleration condition to prevent backfiring.

I wondered why they did this and one of the German Factory Support guys explained it to me. FWIW

[john in illinois]

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I had this due to a leak at manifold clamp. Happened after I shut it off.



John

[Gene F]

High octane gas does not break down nearly as fast at that 87 stuff. That is why the Chevy Volt required high octane fuel - because it may set in the tank a good while.

Are far as the timing, there is little spring and plunger ahead of the camshaft. If either of them are broken in there the camshaft can walk out andall ow the gear (on the shaft) to change your timing until engine conditions change. Just something to think about.

[UPS Driver]

I actually use ethanol free gasoline available locally in my 29 Tudor and my antique tractors. It’s around.40 cents more a gallon and 90 octane rated. I quit having carburetor issues with the use of ethanol free

[OL JENNY]

I had the same problem and my cause was a small crack in my exhaust manifold that caused the fumes to ignite, sometimes up to 45 seconds after turning the engine off. Until I could replace the manifold I turned off my gas and starved the engine to shut it off. Prior to doing that I had upset people next to me at gas stations!

[Big hammer]

When I was a teenager my A would backfire sometimes, coasting down a hill with the key off and then turning it on. When I needed a new muffler dad dug out a discontinued heavy duty truck straight through muffler. I had it welded to the pipes and from then on no more backfiring ;-( I think that I learned about backfiring because the ignition switch was flaky and at times it would disconnect and re- connect with a backfire.

[29spcoupe]

unburned fuel due to too rich a mixture. lean it out a bit.

[nkaminar]

Add a spark plug at the end of the exhaust pipe, powered by a Model T trembler coil. Run the car down hill with the ignition off and the throttle partly open. Then turn on the trembler coil. What fun.

[rich40701]

Thanks for all the replies, I have been able to purchase ethanol free gas until recently and unfortunately so far it is looking like no one no longer carries it in my town. I was told a station sells ethanol free in the town where I work so I might have to start taking gas cans to work and making a gas run.
Thanks again, and to all the veterans out there; Thank You For Your Service.

[CT Jack]

Verify your timing is correct. Before turning off engine my sure it is idling. If your idle speed is too high you could be pulling excessive fuel in before the engine comes to rest. This fuel could explode without the ignition. Check the GAV to verify it works by opening it and closing it while running the engine. It should respond if it doesn't you could be running with a super rich fuel mixture. This would affect fuel consumption and cause backfiring.
A quick test would be to change carburetors without changing anything else. If the backfiring disappears the problem is within your carburetor. I have electronic ignition in my A and if I accidently over choke during startup it will sometimes backfire.