Re: Zenith Idle Mix Screw Doesn’t Kill Engine
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
)