Carb. back fire on down hill drive

Bob from Northport

Bob from Northport

Well-known member
Can anyone advise what causes a Model A to back fire slightly when decelerating down a hill on a long drive.?? I assume that fuel is building up in the carb or intake and back fire occurrs. What is the cause, and is there a cure?? The little truck runs perfectly.

Thanks!!
 
This could indicate weak mixture .When next this occurs try opening the GAV half a turn more.I bet this will stop it. Also could be air leak in inlet manifold gasket etc etc.Also check timing is not a tad retarded.

John in lovely sunny there has to be a catch somewhere out in my flying 68B with the top down YIPPEEE !!! morning England.
 
Les Andrews Shop Manual States; [Does not say anything about vac leaks as the cause]

Engine backfires: Defective Distributor~Faulty Condenser, Ignition System.

Engine backfires down hill: Rich Carburetor Mixture, Fuel System.

skip. Lean and mean makes it go.

"With all due respect to Skip, I'd have to agree with John as it does sound like a lean misfire to me. I've found that I can stop carburetor backfire in my car when going down steep hills by pulling the fuel lever down slightly (thus opening the throttle just a tad), and slightly retarding the timing. I think this then allows the carburetor to run on the mid-speed circuit ."

The 'fuel lever' is the throttle lever on the column? And down is opening the throttle. Same as stepping on the gas. Dumping more air and fuel into the engine. A little retard timing for a longer burn time [sparking the plug before the pistons reaches TDC] and by doing this leaning out the mixture, ie, no more back fire. No more overly rich raw fuel in the exhaust system to burn and backfire. Me thinks. But I don't know.
 
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There are much better experts on this site than I, but it seems to me that going down hill with the throttle closed (foot off the gas) creates a very high vacuum condition, higher than at idle because of the much higher engine speed. However, with the throttle closed the carburetor should be operating on the idle circuit but cannot supply enough fuel to satisfy the engine, again because of the high engine rpm. With all due respect to Skip, I'd have to agree with John as it does sound like a lean misfire to me. I've found that I can stop carburetor backfire in my car when going down steep hills by pulling the fuel lever down slightly (thus opening the throttle just a tad), and slightly retarding the timing. I think this then allows the carburetor to run on the mid-speed circuit . But then, I could be wrong... $0.02.
 
Are you sure the popping is coming from the carburetor? If the car is coasting with the foot off the gas pedal then I would suspect a small exhaust leak in the exhaust clamp area. If it is not an exhaust leak it could be a valve problem. How does it idle?

Pete
 
I understand how too lean a mixture will result in a backfire (unburnt fuel as a result of incomplete combustion getting into the muffler) but how does an exhaust/manifold leak cause a backfire?
 
Let's assume it's really not backfiring out of the carb (which I believe would indicate a valve problem). What you're experiencing is a classic situation: too little fuel is getting into the combustion chambers to support combustion, so it's swept out on the exhaust strokes, accumulating in the muffler or thereabouts until conditions are right for it to explode there. The reason it happens going down hill is because the throtttle is closed then (foot off the gas pedal) so the only fuel is from the idle circuit. But, the engine is revving and still trying to fire on every combustion stroke. The idle circuit is hopelessly unable to provide enough fuel to support combustion. Backing down in a lower gear makes it even worse. Same thing can happen if you shift gears at high revs: backfiring will happen when you lift your foot off the gas to shift.

Anything that leans out the mixture will aggravate the problem. Opening the GAV will help.

You can distinguish this problem from electrical cause easily: ignition problems will show up more under load, the exact opposite of your situation.

Steve
 
Steve has the answer. Every model A that I have ever had or driven exhibits this phenominon....open the gav or dont coast downhill with the accellerator totally up....

Dennis L Oberer
Green Bay WI
 
I'll experience muffler explosions under similar conditions or just under a sudden deceleration and they continue for a 1/2 minute or so after stopping.. Closing the mixture helps me.. It seems to run best open 1/2 turn for the first mile or so then it wants to be closed to a 1/4 turn.. Its pretty fussy.. It also sounds as if I have a cold engine operation exhaust leak, but, can't locate it and can't seem to fix it no matter what I do[including regrinding the manifold taper]..
 
Let me expand on what Steve wrote.
When coasting down a hill or shifting you’re accelerator is closed off. The pistons are trying to draw air so the intake manifold pressure drops in the relation of the outside air pressure. That is why you vacuum wiper works really well in this situation. This also affects your exhaust manifold pressure. The exhaust manifold pressure also drops so, if you have any opening to the atmosphere the partial vacuum will draw in relatively cool air into this very hot environment (my guess it occurs between exhaust strokes). The cool air rapidly expands and the result is popped out the tail pipe.
I wonder if engine idle speeds we set are too low. I know they are as compared to modern standards. I have never seen a Model “A” Ford Bulletin that recommends a set engine idle speed. It was all done by ear I guess.

Pete
 
"but how does an exhaust/manifold leak cause a backfire? "

Unburned gases exiting the combustion chamber into the hot exhaust system are exposed to fresh oxygen via an exhaust leak and burned again. Result--> Backfire in the exhaust.

In modern cars an exhaust leak tells the oxygen sensor that the mixture leaving the engine is too lean and therefore the computer is commanded to richen the mixture fouling plugs and clogging the 'cat~a~lick' converter.

skip.
 
Exhaust Leak from manifold into exhaust pipe, happened to me. got short pipe from snyders that fits insde the too pipes, problem sloved....worked for me!
 
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