Stalling at Stop Signs

E

Ed in Maine

Well-known member
As I rebuild a carburetors, I occasionally create a "staller"! What is happening is the car starts to run lean when you take your foot off the gas and step on the brake. It is a nuisance because you end up increasing the idle speed, wasting gas and making it difficullt to get into first gear without grinding first. As you approach the stop sign, the throttle goes closed and stepping on the brake makes the gas level shift forward in the carburetor bowl. At this moment, the Cap Jet has nearly emptied the Secondary Well and the Main jet is closed off because the throttle is closed. The only jet that will keep the car running is the Idle Jet and its source of gas is dwindling. It is a race to see if the Secondary Well will fill in time before the car stalls. The question is why on some cars and not others?

Today, I think I found the answer. I was getting sloppy with my restoration procedures. If your car is stalling, check the length of the Idle Jet. It should be 3 in. overall. The Idle Jet has a beveled end at the end that is down in the Secondary Well. My jet was 1/16 in. short. When the gas level was low in the Secondary Well, the idle jet was pulling air instead of gas. If enough time went by during the stop, the Secondary Well would fill enough to allow the car to recover. If necessary, use one or two small ring gaskets to extend the Idle Jet down into the Secondary Well. Better yet, when assembling the carburetor, carefully fit the idle jet to the bottom of the Secondary Well.

If you have a stalling problem, let us know if you were able to stop the problem with the above fix. Good luck with it, Ed
 
Ed in Maine said:
As I rebuild a carburetors, I occasionally create a "staller"! What is happening is the car starts to run lean when you take your foot off the gas and step on the brake. It is a nuisance because you end up increasing the idle speed, wasting gas and making it difficullt to get into first gear without grinding first. As you approach the stop sign, the throttle goes closed and stepping on the brake makes the gas level shift forward in the carburetor bowl. At this moment, the Cap Jet has nearly emptied the Secondary Well and the Main jet is closed off because the throttle is closed. The only jet that will keep the car running is the Idle Jet and its source of gas is dwindling. It is a race to see if the Secondary Well will fill in time before the car stalls. The question is why on some cars and not others?

Today, I think I found the answer. I was getting sloppy with my restoration procedures. If your car is stalling, check the length of the Idle Jet. It should be 3 in. overall. The Idle Jet has a beveled end at the end that is down in the Secondary Well. My jet was 1/16 in. short. When the gas level was low in the Secondary Well, the idle jet was pulling air instead of gas. If enough time went by during the stop, the Secondary Well would fill enough to allow the car to recover. If necessary, use one or two small ring gaskets to extend the Idle Jet down into the Secondary Well. Better yet, when assembling the carburetor, carefully fit the idle jet to the bottom of the Secondary Well.

If you have a stalling problem, let us know if you were able to stop the problem with the above fix. Good luck with it, Ed
Click to expand...
The idle jet has NO gasket!!
 
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Thanks for the info. When I work on a carburettor, it is hit and miss (mainly miss) as to whether I improve things. I'll remember this for next time.
 
Ed ............. just curious to understand what you mean by "fitting the idle jet to the bottom of the secondary well". How do you propose doing this???
 
Might also have the wrong secondary brass well, or not screwed in to the casting far enough. There were three different ones I believe.
 
Thanks, Ed. Like Bob, I'm wondering if the secondary well is in all the way also.
 
This slide may help you.
 

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Thank you for your comments, answering your replys:
Gary - yes, the Idle Jet does not normally get a gasket but if you want to extend the Idle Jet down into the Secondary Well, adding a gasket or two will get you closer to the bottom of the well without having to modify the jet.
Bobbader - I should have been more specific about fitting the Idle Jet to the Secondary Well. When you are putting the two halves together, I usually don't use the gasket at first. If there is interference you will see it at the seam of the top and bottom castings. The two halves will rock on the tip of the Idle Jet. Then add the gasket to see if the two halfs come together. If the two halves come flush together, then it is possible that the Idle Jet is not at the bottom of the Secondary Well. Then, shim the Idle Jet with a gasket. By doing this procedure, you know you have the Idle Jet tip very close to the bottom of the Secondary Well.
Brentwood - I think you are referring to differences in the Secondary Well for dual venturi and single venturi carburetors. I don't think there is any difference in the length of the Secondary Well used with single venturi carburetors.
Model A Fords - This back firing is the accumulation of gas fumes in the exhaust system after the ignition is turned off and the engine is coasting to a stop. Try to have the idle speed at its lowest when turning the key off. This can be from an accumulation of carbon in the engine. Look at the spark plugs, are they black or light grey. You also might look at the sizing of the Main Jet. For cleaner burning, size the Main Jet for 152 - 153 ml/min. Strive to turn the Gas Adjusting Valve to a minimum while still having good operation. ED
 
I believe I read that there were up to 5 different shapes for the secondary well (brass insert). It may have been read from Tom Endy's article at santaanitaas.org


I don't think that the early dual venturi carbs had a brass insert at all??? Please correct me if I'm wrong.
 
Guy's, don't forget that loose manifold nuts will cause the same issue, dyeing at a stop.

Regards
Bill
 
Bob Bidonde said:
This slide may help you.
Click to expand...

I respectfully disagree with all of these statements in this posted diagram.

1 - gasoline slashes forward in the float bowl.... why would this be a problem as fuel moving forward is no different than going downhill... Do model A's stall going downhill?

2 - car noses downward... same thing as driving down a hill.

3 - main jet and cap jet ingest air ... These high speed circuit jets are not in operation when the throttle plate is closed and the carburetor is on the idle circuit while stopping. Fuel moving forward in the bowl can only go so far, it will not move enough to uncover the bottom ends of the main or cap jet.

4 - inrush of fuel as the float moves downward... just the opposite happens as you tip the carburetor fuel moving forward and up is going to push the float up, closing the fuel inlet valve.

5 - car recovers and too much gasoline in the float bowl... where did this extra fuel come from and what difference would it make? When transitioning back to the idle circuit the engine is stalling because it's lean not because it's too rich.

The fuel inlet valve float valve and float keep the level high enough even while stopping and some amount of fuel moving towards the front of the car the jets are always covered and supplied with fuel, especially when transitioning back to idle as very little fuel is being consumed.

This diagram is very misleading. Try holding a glass of water and stopping in Model A, You will not see fuel move this drastically, they would've put baffles in the bowl if it was possible to uncover the jets while stopping.
The fuel being only 5/8" below the gasket line and the volume of the float bowl will not allow the Jets to become uncovered.

The arrow showing fuel or air moving backwards through the idle jet is impossible carburetors work on vacuum and when you're on the idle circuit, engine vacuum on the jet would be pulling fuel into the throat of the carburetor. Fuel moving forward isn't going to pull a stronger vacuum on the Jets.

The stalling issue can also happen with model B carburetors with both styles of float which are turned 90° from the Model A design, further proving that fuel moving in the bowl is not affecting the Jets, fuel inlet valve or float.

Why was none of this happening back when the cars were built?
They never would've sold a single car if this was an issue, there would've been Ford released service bulletins addressing the problem.

At any rate, I'm not trying to stir up trouble or cause problems but I think more thought needs to go into this before it's published.
 
BillCNC said:
Guy's, don't forget that loose manifold nuts will cause the same issue, dyeing at a stop.

Regards
Bill
Click to expand...

This is correct.

Manifold leaks create a lean condition.
Engine vacuum pulls air from the leak rather than fuel through the idle jet causing the engine to stall.
 
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