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Old 08-18-2019, 11:03 AM   #1
mike657894
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Default backfire out of exhaust after warmed up shut down.

Im getting a loud gunshot backfire out of my exhaust a second after the engine comes to a full stop. It doesnt do it on short drives. But if I go 10 miles shut down, and putt putt putt ...BANG! Wake the neighbors. The car is at another location so I have to plan to work on it. I have not looked at the plugs yet. I'm thinking tuning and it may be running rich. To save time though. I'm wondering if anyone else had this same issue and I could skip the trial and error. Stock enigne, head, B cam, and tilotson X carb. The last few times ive come off a country road after running a continuous 50mph. It has happened also on a hot day while doing city driving.
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Old 08-18-2019, 11:17 AM   #2
Jim Brierley
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Default Re: backfire out of exhaust after warmed up shut down.

The backfire comes from unburned fuel in the exhaust system. Start with a good tune-up. It could be a fouled plug, rich fuel mixture, etc., etc., or even a leak somewhere in the exhaust system.
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Old 08-18-2019, 11:49 AM   #3
Purdy Swoft
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Default Re: backfire out of exhaust after warmed up shut down.

Raw gas is collecting in the muffler .After the engine is shut off the temperature quickly increases and sets off the gas . Running too rich or the ignition is too cold or a combination of the two can cause this problem .This is fairly common with some model A's . Check carb adjustments . I usually after warm up adjust the GAV Check at one quarter turn off the seat for Zenith one or two carbs or closed completely if the model B carb is used . I adjust the air idle adjust screw at one and one half turns off the seat . This is a starting point for carb adjustment and may require a bit more or less turns open or closed depending on the carb and its condition . Check the gaps between the rotor tip and the contacts in the distributor cap body . I've read that Ford recommended a conservative gap of twenty five thousands . More gap to a certain extent will increase the spark at the cylinder . I run mine at thirty five thousands for hotter spark . Most model A guys have extra rotor caps and the gap can be adjusted by filing and checking the gaps as you go . The gaps need to be the same on all 4 cylinders . Sometimes the contacts in the reproduction distributor cap bodies can be all over the place and must be checked .
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Old 08-18-2019, 04:29 PM   #4
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Default Re: backfire out of exhaust after warmed up shut down.

I had it and it was an exhaust leak letting oxygen into exhaust after shut down.

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Old 08-18-2019, 10:15 PM   #5
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Default Re: backfire out of exhaust after warmed up shut down.

Interesting.
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Old 08-20-2019, 09:43 AM   #6
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Default Re: backfire out of exhaust after warmed up shut down.

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Originally Posted by Purdy Swoft View Post
Raw gas is collecting in the muffler .After the engine is shut off the temperature quickly increases and sets off the gas . Running too rich or the ignition is too cold or a combination of the two can cause this problem .This is fairly common with some model A's . Check carb adjustments . I usually after warm up adjust the GAV Check at one quarter turn off the seat for Zenith one or two carbs or closed completely if the model B carb is used . I adjust the air idle adjust screw at one and one half turns off the seat . This is a starting point for carb adjustment and may require a bit more or less turns open or closed depending on the carb and its condition . Check the gaps between the rotor tip and the contacts in the distributor cap body . I've read that Ford recommended a conservative gap of twenty five thousands . More gap to a certain extent will increase the spark at the cylinder . I run mine at thirty five thousands for hotter spark . Most model A guys have extra rotor caps and the gap can be adjusted by filing and checking the gaps as you go . The gaps need to be the same on all 4 cylinders . Sometimes the contacts in the reproduction distributor cap bodies can be all over the place and must be checked .


Had this issue when I received my A back from a Model A repair shop. Carb was not set correctly and the carb idle mixture was set way too rich.
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Old 08-20-2019, 10:37 AM   #7
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Default Re: backfire out of exhaust after warmed up shut down.

An exhaust leak will allow air to enter the muffler . Air is necessary for fire or an explosion from the muffler . If there is no raw gas in the muffler , there will be no backfire . The engine is running too rich .
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Old 08-20-2019, 03:11 PM   #8
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Default Re: backfire out of exhaust after warmed up shut down.

I got out and looked at the plugs. they were a good light chocolate brown. I started it up and smelled the exhaust. seemed rich so I worked the idle screw down to 3/4 and let it run for a few minutes. pulled a plug and it was wet and black. I didnt get a chance to drive it today. I have also been told to retard timing before shut down. I will have to wait on doing that until I see if the adjustment made a difference. I will have to check for leaks but I spent some time sealing the exhaust up so I would hope that held.
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Old 08-20-2019, 05:21 PM   #9
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Default Re: backfire out of exhaust after warmed up shut down.

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I got out and looked at the plugs. they were a good light chocolate brown. I started it up and smelled the exhaust. seemed rich so I worked the idle screw down to 3/4 and let it run for a few minutes. pulled a plug and it was wet and black. I didnt get a chance to drive it today. I have also been told to retard timing before shut down. I will have to wait on doing that until I see if the adjustment made a difference. I will have to check for leaks but I spent some time sealing the exhaust up so I would hope that held.


Screwing in the idle mixture screw makes it richer - so being wet, sooty/black makes sense when you screwed in the mixture screw. If you wanted leaner, should turn the screw out (counter clockwise). To test/adjust teh mixture screw the motor must be at a very low idle. less than 500-550 Rpm, with spark lever 1/4 down . You will know if testing/adjusting idle is at low enough rpms by turning the GAV from closed (fully clockwise) to 1/1/2 - 2 turns clockwise and the idle speed does not change. If running too fast RPMs adjusting the GAV affects the idle.


Above low Idle other Jets take over the mixture. Adjusting for higher than low idle is adjusted by replacement/changing the size of the Jets.
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Old 08-21-2019, 03:30 AM   #10
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Default Re: backfire out of exhaust after warmed up shut down.

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Originally Posted by mike657894 View Post
Im getting a loud gunshot backfire out of my exhaust a second after the engine comes to a full stop..... But if I go 10 miles shut down, ...BANG! ... I'm wondering if anyone else had this same issue ....
Yes had identical symptoms for a while... backfired about 10 seconds after I turned the ignition off after hot fast runs. I overcame the issue by ensuring I idled it slow for about 10 seconds on full retard instead of just reaching out and switching off the ignition whilst the engine was still doing about 1000 rpm. It doesn't do it now. Possible causes I reckoned were weak spark, incorrect fuel mix ( either too rich OR too weak- either will be more difficult for a spark plug to trigger ignition), brand/quality of fuel ( yes that does make a difference). Whatever ----if you switch off with the engine still running fast then unburnt fuel is going to enter the exhaust system and maybe find a hot spot in there which ignites it .
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Old 08-21-2019, 08:35 PM   #11
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Default Re: backfire out of exhaust after warmed up shut down.

I guess I was thinking backwards on the idle screw. ill have to do some more adjustments and figure that out.
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Old 08-22-2019, 01:29 PM   #12
Purdy Swoft
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Default Re: backfire out of exhaust after warmed up shut down.

Try the idle AIR adjust screw on the upper top of the carb housing, not the idle adjust screw on the throttle shaft lever . Just a thought , you probably already know this .
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Old 08-22-2019, 01:35 PM   #13
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Default Re: backfire out of exhaust after warmed up shut down.

Don't forget to retard the spark all the way...that helps alot.
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Old 08-22-2019, 06:48 PM   #14
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Default Re: backfire out of exhaust after warmed up shut down.

Quote:
Originally Posted by Jim Brierley View Post
The backfire comes from unburned fuel in the exhaust system. Start with a good tune-up. It could be a fouled plug, rich fuel mixture, etc., etc., or even a leak somewhere in the exhaust system.
Quote:
Originally Posted by john in illinois View Post
I had it and it was an exhaust leak letting oxygen into exhaust after shut down.
No doubt, unburned fuel exploding is a backfire. Unburned fuel can come from either engine running too rich OR too lean (because of ignition failure causing unburned fuel/air mixture to pass on thru to the muffler).

Two things are required for explosion: a proper mix of fuel and air, AND high enough temperature. SO, why would it explode only after shutdown?

Although thermal seep from the combustion chamber after shutdown causes the coolant temperature to go up a few degrees for a few minutes, I don't think this causes backfire; the exhaust valves are never as hot as when the motor is running, and the exhaust manifold only gets cooler after shutdown.

The only thing that makes sense for backfiring only AFTER shutdown is that a leak at the muffler clamp or the exhaust manifold gasket lets air be drawn INTO the muffler as it cools down after shutdown. As the magic fuel/air mix is reached while the system is still hot enough, BAM!

Anything that results in unburned fuel in the exhaust system and overheated exhaust system will aggravate the situation: improper tuning, improper carb setting, and retarded timing makes engine run hot. BUT without an air leak on cool down, your car may struggle but it won't backfire.
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