Popping Sound

Franchise_24

Franchise_24

Well-known member
Was driving the Coupe home last night and going down a steep hill I let it coast not giving it any gas and it started popping a bit, like a mini backfire. What would cause that? Is it running too rich?

Neal
 
Could be, but it also may be that you have a leak in the exhaust manifold gasket. It may be letting in just enough air to ignite the left over charge.

Mike
 
Down hill, raw gas being pushed through the engine and into the hot exhaust system, even with the throttle plate closed, as the engine displaces, [keeps turning] air is evacuated from the cylinders and that air has to be made up and go's through the carburetor becoming an explosive mixture that does explode in the muffler. So it farts...some would say burbles.

skip.
 
skip said:
Down hill, raw gas being pushed through the engine and into the hot exhaust system, even with the throttle plate closed, as the engine displaces, [keeps turning] air is evacuated from the cylinders and that air has to be made up and go's through the carburetor becoming an explosive mixture that does explode in the muffler. So it farts...some would say burbles.

skip.
Click to expand...

I notice if I push the gas peddle going down the hill it doesn't happen, anything to really worry about if it does? I tried just going down a level road and let it coast and it does not do it.

Neal
 
What's happening is that the going down hill you have high rpms with the throttle closed. The engine is valiantly still trying to fire a plug on every compression stroke but because the only gas available is that from the idle circuit, and your rpms are way above idle speed, there's not enough gas to support combustion. So, the unburned gas is swept out in the exhaust and accumulates in the muffler or exhaust system somewhere, until eventually conditions are right for it to explode--"pop". Usually opening up the GAV--as much as one full turn--helps a lot. It's worth a bit of worrying about; I've blown a muffler apart with backfiring.

Backing down hill in 2nd gear is even worse. Anything that tends to lean out the carb mixture aggravates the situation, like a vacuum leak at throttle plate shaft, etc.

Steve
 
They are right. Enrich your mixture for long downhill coasts or put it into neutral. You will blow up a muffler if you continue with the enriched mixture building up in the muffler.

Dennis L Oberer
Green Bay WI
 
Hi Neal, Steve has it right, the popping sound is sign of a mixture too lean. If the hill(s) you are concerned about are descended with the foot completely off the accelerator pedal, look to an idle mixture that is too lean. It is possible that an adjustment as simple as turning in the idle mixture screw 1/4 to 1/2 turn will cure the problem. Check it out...
 
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"I notice if I push the gas peddle going down the hill it doesn't happen,"

When you open the throttle plate you let in more air to mix with the gas making a leaner more proper mixture that is not too rich and has less tendencey to burn and backfire in the can. Same as taking your foot off the gas on a level road and coasting. She'll only back fire if she has something along the lines of a too rich mixture to burn. [explode]

skip.
 
A bit lean...I thinks!

I agree with Chris in CT...you may be a bit lean on your mixture. Look for the obvious if the condition just started. Check for intake manifold leaks and then move on to check your main and compensator jets. Steve Pargeter the author of Zenith Model “A” Carburetor Restoration Guidelines describes the process in detail in this booklet. This booklet is available at most Model A parts sources. I have a brief description of Performance tuning of Zenith carburetors on my website that explains my method of checking the jets and adjusting them for the performance you desire.
You will find the article on my website: www.durableperformance.net Look on the Services page for the link.
Good day!
Dave in MN
 
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