Re: operation question
Model A losing idle upon Deceleration :
Over the years, I have had trouble with various A engines dying out during a fast stop. I would question the float or something else in the carb. And I was never sure what cured it, if anything did. Many times, people would just increase the idle speed and keep driving it.
I think I have figured out at least one thing that can cause this problem.
If the carb. flange, or the intake manifold flange is warped, a leak can occur between the two flanges. This air leak is insignificant at road load, but at idle, it can be a large percentage of the total air going into the engine. I always thought that an air leak here would cause a squeal. Sometimes it does, but my last two occasions of this problem gave no sound at all.
To fix it, simply take a large file and clean up the two surfaces until they are shiny over the entire flange surface, then put on a new gasket and install the carb. I have done this twice recently and the problem went away. Try it if you have this problem.
You don’t have to take the intake manifold off to do this. Use a mirror and a light to see the manifold flange and file it until it is flat. The heat is very hard on the gasket and the flatness of the two flanges. They don’t stay flat over time.
Other things that can cause a tiny air leak down stream of the throttle blade are a cracked intake manifold or a break in the manifold gasket. These, both can be very hard to find.
While at a constant road speed, the float level in the carb stays fairly level with the fuel right up near the tips of the jets. As you decelerate, the fuel in the carb. all moves forward which means that the level of the fuel in the jets is a little lower. This should cause a little leanness, so a slight air leak at the flange, or anywhere, hurts even more.
If your car was running good last year, or recently, and this problem occurs, don’t start taking your carb. apart and changing things. The float level did not magically change, and I doubt that your carb. suddenly got plugged up.
Look for an air leak. Jack Bahm 2019
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