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Old 02-17-2021, 11:15 AM   #1
Ed in Maine
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Default Engine Stalls at Stop Signs

I was recently helping a club member with a carb issue (dripping) and I ended up creating a new problem, "stalling on stops." I had removed the carb, cleaned it and installed clean, flow tested jets. This is the first time I have encountered a problem after installing flow tested jets and I found the cause.

There are several things that may contribute to stalling on stops. The carb bowl level should be set at 5/8 in. – 11/16 in. and there should be no vacuum leaks at the manifold or at the carb flange. The idle mixture and the idle speed should be set to specifications.

When the Model A is brought to a stop, the throttle plate suddenly closes. All gas flow from the Main Jet and the Cap Jet ceases. The only possible source of gas to the engine is from the Idle Jet. However, in the first few seconds after taking your foot of the accelerator, the Secondary Well is nearly empty because the Cap Jet has used all the gas from the well. The Secondary Well is the only source of gas for the Idle jet and a brief moment is required to get the level in the well higher. The length of time for this potential lack of fuel supply to the engine will be a function of the sizing of the Compensator jet, the Idle Jet and how far open the GAV is. The GAV will have been set for normal operation and most likely will not be set to eliminate the lean mixture on a stop.

The sizing of the Compensator and the Idle Jets becomes important in this situation. The published flow range for the Compensator is 138 - 142 ml/min. and the Idle Jet is 44 – 48 ml/min. I have found that if the Compensator Jet selected is at 138 ml/min. and the Idle Jet is at 44 ml/min. (both at their low end ranges), the length of time of the lean mixture on a stop may be excessive and the engine will stall. In a recent experience, the jets were changed as follows: Compensator at 141 ml/min and the Idle Jet at 47 ml/min. The nuisance stalling was eliminated. I strive to size jets at their mid flow ranges but in this case, the selected jets I put into this carb brought two jets together sized at their minimums.

In my experience, I strive to set the carb jet flows for a good balance of reliability and economy as follows:
Idle Jet: 46 – 47 ml/min.
Compensator Jet: 140 – 141 ml/min.
Main Jet: 155 – 156 ml/min.
Cap Jet: 160 - 185 ml/min.

I don’t like to see the Main Jet much higher because it reduces the need to add some fuel through the GAV for best performance. I don’t like to see the Idle Jet greater than 48 ml/min. because the higher idle flow forces you to open the idle adjusting screw too far for a good idle. The amount of flow through the Cap Jet is not very important just as long as it passes all the flow coming though the Compensator and some through the GAV. The combination of flow through the Compensator and the Main Jet should be about 295 ml/min.

I felt that if some you are experiencing stalling on stops you may not have considered looking for jet problems. Ed
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Old 02-17-2021, 11:18 AM   #2
alexiskai
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Default Re: Engine Stalls at Stop Signs

Great info here, Ed
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Old 02-17-2021, 11:59 AM   #3
Mike Peters
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Default Re: Engine Stalls at Stop Signs

I have flow tested some Zenith jets and found that size really does matter. The next Zenith i do will be with your values. Thanks for the great info, Ed!
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Old 02-17-2021, 12:10 PM   #4
Mike Peters
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Default Re: Engine Stalls at Stop Signs

Also, I have found that ball park flow values result in pretty decent operation, but does NOT solve stalling at stop signs, so your values are of special interest.
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Old 02-17-2021, 12:29 PM   #5
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Default Re: Engine Stalls at Stop Signs

Here are the ballpark values from fordgarage.com - the range of values is surprising.

Screen Shot 2021-02-17 at 12.27.23 PM.jpg

Last edited by alexiskai; 02-17-2021 at 12:35 PM.
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Old 02-17-2021, 02:18 PM   #6
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Default Re: Engine Stalls at Stop Signs

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Old 02-17-2021, 04:00 PM   #7
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Default Re: Engine Stalls at Stop Signs

I guess the other option is not to stop at stop signs, like my grandfather. He died peacefully in his sleep, not like the other screaming passengers in his car.
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Old 02-17-2021, 05:32 PM   #8
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Default Re: Engine Stalls at Stop Signs

I’d sure double check the flatness of the carb and manifold flange. Any leak at all here could cause this. Or some small leak in the manifold. Sometimes those are hard to find. Good luck and let us know how you fix it.
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Old 02-17-2021, 05:44 PM   #9
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Default Re: Engine Stalls at Stop Signs

Ed, what are you using for tools to change and test flow rates?
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Old 02-17-2021, 05:50 PM   #10
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Default Re: Engine Stalls at Stop Signs

Another issue can be wear of the side of the butterfly valve along with throttle shaft wear, the throttle valve has to be closed more to have reasonable idle speed, this causes less airflow over the idle mixture port--- back the idle speed screw out and look for gaps around the edges, shine a light in and look for light coming through, sometimes the butterfly just needs to be centered
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Old 02-17-2021, 09:18 PM   #11
Chuck Sea/Tac
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Default Re: Engine Stalls at Stop Signs

Something I never hear mentioned, is if your engine is modified, bored out, bigger cam ect, those jet values wound be off.
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Old 02-17-2021, 09:47 PM   #12
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Default Re: Engine Stalls at Stop Signs

Nice research Ed.

Could you explain how you 'set the idle mixture and idle speed to specifications?" You passed over that point quickly,

Thanks to an early mentor, Marshall Dault, with the correct idle setting procedure, no more stalling.
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Old 02-18-2021, 12:09 AM   #13
Jack Shaft
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Default Re: Engine Stalls at Stop Signs

Multiple jet carburetors faded away not soon after the model A in part due to the difficulties tuning it,in part to the emulsion tube design,which basically incorporated all jet functions with one tube drilled with a "ladder" of delivery holes. The Allstate/ Marvel Schebler is an emulsion tube carburetor..probably the easiest to service with superior drivability..the stromberg 97 is an emulsion tube carburetor as well.
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Old 02-18-2021, 10:09 AM   #14
Ed in Maine
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Default Re: Engine Stalls at Stop Signs

Please note that I did not develop the flow data ranges for Zenith carbs. I highly recommend that you get a copy of Steve Pargeter's "Zenith Model A Carburetor Restoration Guidelines." I have heard a rumor that this booklet was re-published but when the current stock is gone, they are not planning a reprint. I built the plastic pipe water test setup shown in the guidelines. For years I drove a Model A with a black sooty tail pipe because a I installed newly purchased jets that had not been flow tested. I would further state that soldering and drilling the jets using the recommended drill sizes is not accurate enough to get to the correct flow for a jet. How the drill leaves the exit hole in the jet affects how much gas will pass through the jet. You have to flow test and then using the micro sized drills as a file, file the hole to the final flow capacity. You cannot use the micro drills in a hand drill because they will brake immediately. They are too fragile. Before I solder up a jet, I insert a 24 ga SST wire (Idle Jet) or a 20 ga SST wire (Main, Cap or Compensator Jet) and solder the hole. For the Main and Cap Jets, I use a 1/16 in. drill to make the dimple at the tip to minimize gas weeping after soldering. Then flow test, and slide the appropriate size micro drill in and out until the jet is exactly at the flow you want. I use electrical shrink tubing to fasten the jet to the test rig. You have to be patient, this is a trial and error process that can take up to an hour just for one jet. I often tell members of my club that just cleaning a carb and re-assembling is a waste of time if you do not test jet flow rates. Good luck with your rebuilding project, Ed
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Old 02-18-2021, 10:15 AM   #15
KenBolton
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Default Re: Engine Stalls at Stop Signs

When everything else has been checked and is good and it still stalls, clean the idle jet.
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Old 02-18-2021, 10:29 AM   #16
Jerry in Shasta
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Default Re: Engine Stalls at Stop Signs

Try adjusting the FUEL level up or down a 1/8", cured my problem.
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Old 02-18-2021, 05:29 PM   #17
Gene F
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Default Re: Engine Stalls at Stop Signs

I have heard over the years that an idle that is too high can cause this. I have no idea if this is true. Always sounded odd to me, but I don't know everything. It sure would not cost you anything to try it. You'd be no worse off, that's for sure.
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Old 02-18-2021, 05:52 PM   #18
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Default Re: Engine Stalls at Stop Signs

Gene,
If you have a 'high' idle speed more likely you have an air leak some where .
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Old 02-18-2021, 07:05 PM   #19
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Default Re: Engine Stalls at Stop Signs

Open the GAV a bit..............
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Old 02-18-2021, 07:25 PM   #20
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Default Re: Engine Stalls at Stop Signs

That is what I do
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