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Old 11-08-2019, 09:10 PM   #1
JP31
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Default Backfire or popping when going down hill

I am having an issue with what sounds like popping/backfire in the exhaust when traveling down the road and letting off the gas when going down hill. I have just set the points and confirmed the timing is correct. Any thoughts on what else to check. I have only about 250 miles since engine rebuild. I have a standard head and did have the top of the block milled flat. I do not know how many times this has been done on this block but wondering if this could be the cause.

Thanks in andante for your input.
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Old 11-08-2019, 10:54 PM   #2
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Default Re: Backfire or popping when going down hill

The back-fire is the exhaust gas that has not 'fully' combusted/burned and is then ignited by the heat of the exhaust system. Generally, this takes place when an engine has been driven fast and the exhaust gases have not had time to 'push' completely through the muffler system. Let off and slow down well before you are coming to a stop and you will find this to be less of a problem. Correct value adjustment will also help with this issue.
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Old 11-09-2019, 01:23 AM   #3
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Default Re: Backfire or popping when going down hill

Quote:
Originally Posted by JP31 View Post
I am having an issue with what sounds like popping/backfire in the exhaust when traveling down the road and letting off the gas when going down hill. I have just set the points and confirmed the timing is correct. Any thoughts on what else to check. I have only about 250 miles since engine rebuild. I have a standard head and did have the top of the block milled flat. I do not know how many times this has been done on this block but wondering if this could be the cause.

Thanks in andante for your input.
"confirmed the timing is correct." Since the timing is set by the "spark" (or timing) lever, how do you know when the timing is correct? Is it with the lever at 9 o'clock? 8 o'clock? somewhere else? How many degrees BTDC is the timing with the lever at 9 o'clock, for example? Very, very few drivers know what the timing actually is with the lever at their favorite position.
When going down hill, do you retard the timing (lever up)? This will make it pop! When I was a teenager, this was a favorite trick going downhill. Throw the spark lever up, close the GAV, and see how many pops it would make. A classmate bragged that his Friend's A "would pop 40 times going down a hill!".
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Old 11-09-2019, 04:27 AM   #4
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Default Re: Backfire or popping when going down hill

Some times this can be caused by a weak mixture . Open up the GAV a turn or two and see if this "cures" it .

John in Bath city in Somerset County England .
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Old 11-09-2019, 05:46 AM   #5
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Default Re: Backfire or popping when going down hill

Intake gasket or carburetor gasket sucking air. First just take and make sure manifold bolts are tight.
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Old 11-09-2019, 07:09 AM   #6
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Default Re: Backfire or popping when going down hill

A lot of guys will have a lot of complicated answers for you, but if you richen your idle mixture screw the popping will stop.



I learned this on a Briggs and Stratton powered minibike when I was in my early teens and used it successfully on any carbureted vehicle I've ever had, from flathead V-8's to 600 horsepower dirt track modifieds. If it pops when decelerating with the throttle closed, enrich the idle mixture.
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Old 11-09-2019, 08:43 AM   #7
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Default Re: Backfire or popping when going down hill

For what's it's worth, I dealt with the popping/backfire issue for years on a 28 CCPU, adjusting the GAV, checking for vacuum leaks, etc., etc. The issue was finally resolved by replacing the tailpipe/muffler that apparently did not lineup properly with the exhaust manifold (tried copper gaskets & sealants to no avail) with a new Aries muffler. No more backfire.
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Old 11-09-2019, 09:16 AM   #8
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Default Re: Backfire or popping when going down hill

My A makes a little what I call a "coughing/quiet pop" sound when shifting and foot off the gas slowing down from higher rpms. It is not a true firecracker/shotgun sound that would wake the neighbors, which is an exhaust backfire. Instead of running at 1/8 open on the GAV, run the GAV about 1/3 open and issue is resolved.
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Old 11-09-2019, 11:57 AM   #9
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Default Re: Backfire or popping when going down hill

As others have said, open the GAV a bit. I lean it out a 1/4 turn when cruising down the highway, but before I slow or come into town I open up the GAV the 1/4 turn again. That's half the fun of driving a model A, lots to do, knobs and levers to adjust, pedals to push, engine sounds to pay attention to, etc etc!
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Old 11-09-2019, 10:37 PM   #10
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Default Re: Backfire or popping when going down hill

Thanks for all the input. I opened the GAV about half a turn and the issue is resolved. Thanks again for the help. My issue was more like "coughing/quiet pop " described above and opening the GAV solved it.

Thanks all
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Old 11-10-2019, 10:34 AM   #11
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Default Re: Backfire or popping when going down hill

Popping in the exhaust takes both unburned fuel and fresh air in order to happen. Most of the time it's a leaky exhaust that allows fresh air to leak in as was already mentioned. changing the mixture will affect the combustion process more that what happens in the exhaust. When you let off the throttle the fuel mixture drops of with the closing of the throttle plate but the idle mix portion is still there. A leaky exhaust valve or valves can do the same thing by allowing fresh air into the exhaust.

Harley Davidson motorcycles had problems with this but it was aided by the way the ignition system functioned. They used a dual coil that fired both plugs every time the breaker points opened. Folks also used short open exhaust systems a lot so that didn't help either.
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