Zenith Idle Mix Screw Doesn’t Kill Engine

[Jim16A1]

Hello all,

I’ve been trying to tune a friend’s Zenith and eliminate the stalling while stopping feature. I found that the idle mix screw seems to have no effect on the idling of the engine. I can screw it in all the way, and it makes a really loud sucking sound, but the engine doesn’t die. I’ve had the carb apart several times and it looks immaculate inside.

Thanks in advance.

[johnbuckley]

"...idle mix screw seems to have no effect on the idling of the engine... " idle circuit should only come in to play at below about 500 rpm . If idle screw has insuffuicient effect on idle mixture suspect A) Poor seal on idle seat or worn idle screw? ( try a different idle screw). B) Idle jet orifice wrong calibre? ( have it flow test or get one from Renners). Stalling when suddden stopping can be multifactorial including float level or weak spark (don't ask!) ... it's a common problem on As.

[nkaminar]

The idle mixture screw on a Zenith works opposite of what you would expect. It controls the air feed into the idle circuit. So screwing in makes the mixture richer and screwing it out makes it leaner.

The loud sucking sound may indicate a vacuum leak somewhere. Spray carburetor cleaner around suspected areas to find the leak. If the engine rpm changes then that is where the leak is. Check for leaks around the carburetor throttle shaft.

[Jim16A1]

Thank you for the input so far. To add to this: the screw doesn’t really change anything until it is either tightened in the whole way, which makes the weird sucking noise, or it is backed out so far that it is removed from the carburetor, which will cause the engine to stop.

[BButturff]

As was mentioned above, the idle air screw only comes into play at, or below, 500 RPM. If your idle is too high, as most idle speeds are when people are trying to prevent a stall, the idle screw will have no impact when adjusted.

[Jim16A1]

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The idle speed is set very low. I’ve swapped this carb onto my roadster and it has the same issues. When set to the same RPM, the roadster’s idle mix functions normally, but my friend’s truck’s carb is still acting as described above.

[jeepguy1948]

The sucking noise is not a bad thing, that is telling you that when the throttle plates are fully closed and high vacuum exist’s

[P.S.]

Nobody has yet mentioned- When the air bleed screw is screwed in all the way (clockwise), and you hear the vacuum sound louder, that is normal operation!

Have you done all the usual checks yet? (Float level, leaks, etc.)

Edit: Just saw jeepguy1948's response

[Jim16A1]

Everything in the carb seems normal. I set it up the same as my roadster carb. One thing I did notice: when driving with the problematic carb, it needs a lot of GAV to keep it from backfiring and having no power. Like 1/2 to 3/4 of a turn open.

[P.S.]

Originally Posted by Jim16A1

Everything in the carb seems normal. I set it up the same as my roadster carb. One thing I did notice: when driving with the problematic carb, it needs a lot of GAV to keep it from backfiring and having no power. Like 1/2 to 3/4 of a turn open.

1/2 turn open seems pretty normal. My wife's roadster runs best at 1-2 to 3/4 open also. Car runs prefect!

[Jim16A1]

Originally Posted by P.S.

1/2 turn open seems pretty normal. My wife's roadster runs best at 1-2 to 3/4 open also. Car runs prefect!

Interesting! I was going by what the owner’s manual says about running it anywhere between 1/8 open or closing it entirely. My roadster always runs really well when using that setting, unless of course I have my friend’s carb bolted to it. Any idea what causes that much of a deviation in the GAV settings? I know you’re extremely knowledgeable when it comes to these cars.

Thanks!
-Jim

[P.S.]

That is kind of you to say, Jim.

Lots of things affect the GAV setting. The size of the main nets for one, atmospheric pressure also. Having the GAV 1/2 to 3/4 turn open is fine if that is where it runs best.

[Jim/GA]

Originally Posted by Jim16A1

Hello all,

I’ve been trying to tune a friend’s Zenith and eliminate the stalling while stopping feature. I found that the idle mix screw seems to have no effect on the idling of the engine. I can screw it in all the way, and it makes a really loud sucking sound, but the engine doesn’t die. I’ve had the carb apart several times and it looks immaculate inside.

Thanks in advance.

I am "late to this party", but I suggest you look at how the throttle plate is installed and its condition.

With the idle screw backed off entirely, close the throttle plate and look down through the carburetor throttle bore at some bright light. Can you see light at the edges of the throttle plate? If so, it is not installed properly.

You may also find that some guys in the past have filed a notch in the throttle plate -- this can sometimes result in what you found and may require you to replace the throttle plate.

To seat the throttle plate properly, loosen the 2 screws and work the plate around so that it is a tight, sealing fit inside the throttle bore all around. Hold it closed there while you tighten the 2 screws.

Now screw the idle speed adjustment screw in until it touches the stop and go 1.5 turns more for initial setting. Bottom out the idle mixture adjustment screw (gently) and back it out 1.5 turns for initial setting.

Adjust both screws from there on a running warmed up engine. Don't waste your time adjusting them on an engine that you just started. Use the hand throttle and open the GAV to get a cold engine to run better while it warms up.

Hope this helps.

[RENNERS CORNER]

A few notes on the subject of stalling while stopping with the Model A Zenith carburetor.

There is a lot of useless folklore, wives tales and YouTube nonsense out there concerning this topic !!


Some examples of this nonsense:

- Fuel slosh causes the fuel in the bowl to flood the carburetor out or starve it out.

Not possible... how would the car go up or down hills or bumpy roads ???
It is impossible for fuel to slosh in the bowl enough to uncover any jet, the jets are in the very bottom of the bowl and even at a 45° angle, they cannot be uncovered.
The fuel inlet valve needle and seat keep the bowl almost full even under hard acceleration.


- Some folks think that the 1932 model B Zenith carburetors had the float turned 90° from the direction of travel so that the slosh front to back would not affect the float and inlet valve.

If that were true cars with model B Zenith carburetors would stall when you turned hard right or left and they don't. Model B Zenith carburetors with issues will stall when stopping just like A carburetors with issues.


- Adding an extra gasket under the fuel inlet float valve will fix it the stall.

This only changes the bowl level by .030" this is not going to matter on a bowl that will supply fuel for over 2-5 minutes of run time at idle.
Fact is the level of fuel in the bowl does not determine how the engine runs, that job is done by the jets and the adjusting needles.
The adding of a gasket makes no logical sense.
If the carburetor is starving for fuel when it comes to a stop, why would lowering / lessening the fuel in the bowl make this starvation condition better?!?


-Throttle plates are not made correctly and are not sealing up the throttle bore.

Fact is no engine will ever run with the throttle plate completely closing off the throat, a small amount of air needs to pass the plate to be mixed with the idle circuit fuel.
Of course, proper centering of the throttle plate in the bore is required, but the angle of the plate bevel is irrelevant.


-Sloppy throttle shafts.

Although they cause a vacuum leak, this is usually not a problem when coming to a stop. This is typically only an issue when you're trying to set a low idle speed.


5 million Model A's equipped with A Zenith carburetors did not stall when coming to a stop when they left the factory.
There is no way Ford would've kept producing the model A Zenith carburetor for 4 years if there was a problem with it.
Ford would have released Service Bulletins on how to solve this and it wouldn't be an issue today.

So what's happening????

When you come to a stop your carburetor transitions to the idle circuit from the high speed circuit.

The throttle closes requiring engine vacuum to pull air past the mostly closed throttle plate, along with fuel out the idle aperture hole in the throat of the carburetor.
When returning to idle the air to fuel ratio is very sensitive because the volume of fuel being used is very little.
If you have the slightest bit too much air due to a vacuum leak, the air to fuel mixture will not be in ratio and will stall.

If the idle circuit fuel supply is restricted or does not have enough engine vacuum to pull fuel through it, it will stall.

So people will notice when it's going to stall if they quickly pull the choke or increase the engine rpm up this stalling doesn't happen.

This is because you have temporarily fixed the air to fuel ratio and or not allowed the carburetor to drop onto the idle circuit where it is more sensitive.



There are many reasons why engines stall when coming to a stop.

#1 reason

Vacuum leaks that create weak vacuum on the idle circuit.
There are many areas for vacuum leaks.

Undersized or plugged idle jet.

Undersized compensator jet starves the idle well.

Secondary well issues, wrong out dated style, the version sold by vendors today is the correct style.
The brass secondary well not seated in the bore and compensator to secondary well interference.

Engine idle speeds less than 400 RPM or grater than 600 RPM.


A few things to make sure are 100% correct:

The fuel level in the bowl must be 5/8" to 11/16" below the gasket line, set with an external sight tube threaded into the carburetor drain plug hole.
No other method is acceptable !
Even though the level in this bowl is not directly connected with stalling at stops, it is something that must be verified so that we're not troubleshooting multiple issues.
The level of fuel in the float bowl has nothing to do with how the engine runs. This fuel is just a reserve to make sure the jets don't starve when going up or down hills or under hard acceleration.
The jets and the adjusting needles control how the engine runs not the fuel level in this bowl.
It is next to impossible to set the level in this bowl so low that it will starve the jets
If the level is too high, it will overflow the main and cap jet, even when the engine is not running.


The secondary well must be of the latest style and positively threaded to the bottom of its bore.
If the bottom of that idle well bore is not clean and the brass secondary well is not fully seated, pathways will not lineup and fuel flow can be obstructed.


The compensator jet in the float bowl must not be blocked by the secondary well.
Some versions of the compensator jet and its threaded hole can allow the jet to thread too far and the opening of the jet will contact the brass secondary well restricting flow through the compensator jet.
This can sometimes be hard to verify, usually I take a sharp scribe and mark the secondary well through the compensator jet threaded hole. Next remove the secondary well looking to see if the groove in that area on the secondary well is wide enough. If the wider part of the groove lines up with the end of the jet, I often turn the groove back on my lathe so that it doesn't restrict the end of the compensator jet.
You can also add another gasket under the compensator jet to help with this, although I don't really like to do that because people in the future will not understand or miss why there are two gaskets under this jet.


All of the passages in the carburetor must be clean and unrestricted like when they were new.
Thorough cleaning and checking passages with a rubber tipped air blow gun are a must.
It is almost never necessary to drill brass plugs. Drilling plugs is an absolute last resort and a good way to ruin a carburetor.


All of the jets must be flow tested and sized for todays fuels.
Jets that are undersized, even slightly can result in fuel supply issues.


There can be ZERO vacuum leaks, the only way to check for manifold to engine block leaks is to take the manifolds off and inspect and replace the gasket
with the 1931 style copper gaskets.
Don't waste your time checking for vacuum leaks using starting fluid or propane.
All those spray can methods are folklore nonsense.
You have to disassemble things that actually look for compromised gaskets and imperfect mating surfaces.
At this time you should also inspect your intake manifold and carburetor flanges for cracks that could leak air and create a vacuum leak.
If you have a vacuum operated windshield wiper, it should be plugged off at the intake manifold until the stalling issue is resolved.
A lot of the model A's out there are tired with worn engines and don't create great vacuum.
So any vacuum leak between the combustion chamber and the carburetor is going to be a problem for the idle circuit.


The engine idle rpm should be between 450 and 500 rpm, no higher than 600 and no lower than 400.
You cannot guess at this, just because the engine sounds like it's idling low doesn't mean anything!
I never work on a carburetor without good tachometer.
If you have your idling rpm too low, the engine dips down suddenly when the throttle is closed and cannot recover.
If the engine idling rpm is too high you won't utilize the idle circuit.

Verify that the idle air fuel screw is set correctly, if it is out too far it will lean out the idle circuit.
Typically, with my carburetor jets, I start with the idle air fuel screw at 3/4 turn open from seated.
The GAV (gas adjusting valve) needle is a high speed adjustment adjusted on the dash and should have no effect on the idle circuit
If your GAV adjustment is affecting how it's idling then you're idling faster than 600rpm it's higher than the operating range of the idle circuit.
The fuel supplied by the GAV enters carburetor throat through the cap jet, which is only active when the throttle plate is open.


Our fuels today, even the non-ethanol types are full of additives, reducing the actual amount of gasoline in every gallon of fuel.
So jetting and needle adjustments need to be current to accommodate these ever-changing fuels.



When you figure out which of these OR a combination of these is the issue, you will stop the engine from stalling when stopping.

If you'd like to discuss this in more detail , please feel free to contact me.

David

RENNER'S CORNER
EARLY FORD PARTS
Ph 734-428-8424

[P.S.]

The authority has spoken! I agree with every word.

[nkaminar]

Here is a Zenith on a 1924 Lancia.

[Big hammer]

Thank you David Renner ! Thank you for sharing your years of experience !

[bobv]

[QUOTE=RENNERS CORNER;2450780]A few notes on the subject of stalling while stopping with the Model A Zenith carburetor.

Some examples of this nonsense:

- Fuel slosh causes the fuel in the bowl to flood the carburetor out or starve it out.

Not possible... how would the car go up or down hills or bumpy roads ???
It is impossible for fuel to slosh in the bowl enough to uncover any jet, the jets are in the very bottom of the bowl and even at a 45° angle, they cannot be uncovered.
The fuel inlet valve needle and seat keep the bowl almost full even under hard acceleration.


The level of fuel in the float bowl has nothing to do with how the engine runs. This fuel is just a reserve to make sure the jets don't starve when going up or down hills or under hard acceleration.
The jets and the adjusting needles control how the engine runs not the fuel level in this bowl.
It is next to impossible to set the level in this bowl so low that it will starve the jets
If the level is too high, it will overflow the main and cap jet, even when the engine is not running.



That seems like a contradiction ??

[old31]

Contradiction, what do you mean?

[johnbuckley]

Thank you David Renner; best expalantion I have ever read on the subject (and cofirms all my beliefs so it must be correct )