VERY LOUD Backfire After turning off car

[Theo]

Hey guys. Ive had this problem ever since I first got my car. I dont know why it does it. Every time I turn off the car, about 5 seconds after I get an EXTREMELY LOUD backfire (almost like a gunshot). This only happens after running the car for an extended period of time, at different speeds. Say if I let the car Idle for 10 minutes, and shut it off, it wont do it. But, if I drive the car for 5 to 10 minutes, or longer, then shut it off it does it.

This did it with the original engine and fuel system (zenith carb), and now its doing it with the new engine, with electronic ignition, fuel pump, and weber 2 barrel carb.

Does anyone know what could be causing this? I know its nothing with the timing, so maybe a fuel issue or exhaust problem?

[Steve Wastler]

Most all backfires after engine is turned off is due to unburned fuel vapors in exhaust, my guess is running too rich...

[Brentwood Bob]

check for leaks in the exhaust/intake manifold. WD40 sprayed around the gasket to the block as the car idles. either your manifolds are not flat, or the nuts holding the carb, or manifold on are not tight enough.

[2manycars]

High idle with rich mixture will cause shutdown backfire. Now that you have added a carburetor that was not designed for the model A, it is a guess as to wether or not it is correct. Is your idle slow when you shut down?

[Rainmaker Ron]

Originally Posted by Theo

Hey guys. Ive had this problem ever since I first got my car. I dont know why it does it. Every time I turn off the car, about 5 seconds after I get an EXTREMELY LOUD backfire (almost like a gunshot). This only happens after running the car for an extended period of time, at different speeds. Say if I let the car Idle for 10 minutes, and shut it off, it wont do it. But, if I drive the car for 5 to 10 minutes, or longer, then shut it off it does it.

This did it with the original engine and fuel system (zenith carb), and now its doing it with the new engine, with electronic ignition, fuel pump, and weber 2 barrel carb.

Does anyone know what could be causing this? I know its nothing with the timing, so maybe a fuel issue or exhaust problem?

Theo, Some time ago I turned some common Model A problems into stories based on things that might have happened around the corner filling station of my youth.
Rainmaker Ron

Curb Service.

Problems that can be solved curbside using few tools and lots of logic.

THE SURPRISING “REPORT”

Ernie was tinkering with a carburetor in the back room of the station when the driveway bell announced the arrival of a customer. Ernie was reaching for a shop towel to wipe his hands when the sound of the bell was followed by a loud backfire. Ernie nearly tripped over the hoist in his haste to get outside to see what was going on.

At the pumps was Bob Brickey’s Model A Tudor. Bob was already out of the car and standing there with a puzzled look.

“It’s been doing that once in awhile lately”, Bob said, as Ernie walked up. “I don’t think it is an ignition problem because it happens after I turn the car off, most of the time I am already out of the car and walking away”.

“ Have you changed anything lately, like the GAV setting”? Ernie asked.

“Nothing has been touched”, Bob said, “the only thing different lately is a slight exhaust leak but I am thinking that was caused by a backfire”.

“Maybe not”, Ernie said, “It would seem that the chicken came before the egg, but in this case it could be they arrived together. Let me explain. We are taught that to have a fire or explosion we must have fuel, oxygen and ignition. So where did the fuel in the muffler come from? Your car is facing north so I will assume you were traveling north on the street out front which is downhill. As you decelerated, the engine was turning at above idle speed with the throttle closed. The resulting fuel mixture at closed throttle was too lean to burn in the cylinder and was pumped on into the muffler.”

“So where did the spark come from”? Bob said.

“The ignition source was always there in the muffler in the form of red hot metal parts and glowing carbon bits, but remember, we also must have oxygen to complete the fire triangle”. Explained Ernie.

“Let me guess”, Bob said, “you are going to tell me the oxygen came in through the exhaust leak, but I still don’t get why it only backfires after I shut the car off and walk away”.

“Yep”, said Ernie, “when the engine is running, exhaust gases escape through the leak due to pressure in the system. When the engine is shut down the muffler immediately begins to cool, creating a partial vacuum and drawing in oxygen-laden air through the leak. Bingo! Our triangle is complete and our explosive combination is announced by what else but an explosion!”

“So actually, the leak was first and not caused by the backfire,” Bob said, “Looks like I better tighten up that exhaust clamp before I blow the muffler off the car or get a ticket for disturbing the peace!”

[steve s]

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I agree with Steve W that there must be unburned fuel in the exhaust system, but agree with Ron's parable that it's more likely due to running too lean rather than too rich. Fresh air being drawn in as the exhaust system cools down after shut down is the only explanation I've ever heard that makes any sense for the delayed backfire, however, I'm amazed that there can be a sufficient partial vacuum in there with the tail pipe open to the air.

Two things to be alert for: Be careful tightening the manifold bolts; it's not hard to bust the ears off. If you can't get a good seal at the muffler clamp, you might consider getting a good Aries muffler; the lip on it is made to original specs and gives a superior seal compared to cheap repro mufflers. Others may disagree, but I vote against using a sleeve insert to help the seal; there's an important reason that the inside diameter of the exhaust pipe is what it is.

Steve

[Tom Wesenberg]

Ron, you are the prefect replacement for Gus in the Popular Mechanics magazine.

[Ron Lachniet]

I had the same problem last smmer and found that my idle was too fast. I slowed it down and the backfire went away.

[DaveB]

When I was in East Bakersfield High School I drove a nice 1930 coupe everywhere. It was my first car and I bought it for $1 in 1959. Friday or Saturday nights were big events in those days since EVERYBODY drove their cars from drive-in to drive-in hopeing the car would help attract a pretty girl. My coupe was special !! It helped me get lots of attention with the "backfire" trick. I would pull into any space where we would wait for a car-hop to take our order. This Model A never failed me. As soon as I stoped and turned the key off, I would retard the spark, step out of the car, turn to face the front of the Model A and point my finger at the radiator and it would respond with a VERY loud backfire. I still don't know why or how it did that and didn't want to change anything. That little Model A coupe and I had lots of fun and it was just full of tricks to make my life full of wonderful memories in Bakersfield in the late 1950's and early 1960's

(A true story Rainmaker!!)

[roccaas]

Condenser could be shot. Cheap fix.

[Kohnke Rebabbitting]

You are running to LEAN! When you run lean, the parts of the upper cylinder run will glow inside, like the exhaust valve, but mostly just a small amount of carbon, will glow Red, and when the motor has been shut off, and loads up on gas, and the carbon is still red, the gasses that form in the cylinder, will Ignite off the Glowing carbon, acting like a Glow Plug. Air cooled lawn mowers are good for that. So when you run the engine a little before you turn it off, you cool it down, and it won't set off the chamber of gas!

thanks Herm.

[Mike V. Florida]

Mine was timing, went throught it again and the back fire stopped. Mine was so bad and so delayed I could stop the car get out and when it would backfire I would duck. Scared the hell out of people, they though a gun went off.

[Jerry Parr WI]

I agree with Ron. I have seen it a couple times both caused by cracked exhaust manifolds. Theo, you don't say but I bet your using the same exhaust manifold on the new engine.

[Theo]

Thanks for all the responses guys! I am going to try adjusting the carb this weekend. I checked the exhaust clamp and manifold, and no leaks. The manifold is not the original one from my other engine, it came on this engine when I bought it. I checked it out and nothing was wrong with it. Im rulling out timing since I have electronic ignition and the timing is set perfect. I also had the timing set dead on on the original engine.