GAV during mixture set?

C

Commander Toasty

Member
Even in the original Ford operating book it does not state where to have the GAV when setting the air mix. And it does make a difference. I get a very different result in my mixture screw depending on where I have the GAV at the time. Even on Tom Endy's page it does not mention this. What are you guys doing in this regard?
 
Well, the GAV setting is much in direct corrolation to the condition of the carburetor. There really isn't anyway to give you any hard number. If I were to say I run mine at ¾ open, someone might say theirs runs better at ¼ open. The difference might be that their GAV is scored at the bottom and leaking. Also, many people who "rebuild" these carburetor do not open & clean out passages. Needless to say, that affects the vacuum signal & fuel flow internally too.

,
 
BRENT in 10-uh-C said:
Well, the GAV setting is much in direct corrolation to the condition of the carburetor. There really isn't anyway to give you any hard number. If I were to say I run mine at ¾ open, someone might say theirs runs better at ¼ open. The difference might be that their GAV is scored at the bottom and leaking. Also, many people who "rebuild" these carburetor do not open & clean out passages. Needless to say, that affects the vacuum signal & fuel flow internally too.

,
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Chief, (my dad), taught me this, with warm engine, run 25M.P.H. steady in second gear and slowly adjust GAV for best and smoothest running, simple!!!
THEN, JUST LEAVE IT THERE, DON'T DOODLE AROUND WITH IT.
 
It seems to me that the idle mixture should be set while the engine is warm and the GAV set where the engine runs best when warm, but, maybe I'm wrong.. Its happened before..
 
I 've always heard what Patrick L. suggested. Adjust the idle mixture once the engine is fully warmed.

My well-worn '28 seems to run best with the GAV open 1/4 to 1/2 turn from "lightly seated".
 
Looks like I wasn't clear in this question. I am not asking where everybody sets their GAV. This is not a question about how your car runs with different GAV settings.

I am asking about the procedure for setting the air mixture at the carb, which is done with one of the two screws on the carb body.

Ignored in most procedures is any reference to where - when setting this air mixture - the GAV should be set.

On the website model-a.org they do get specific and state that the GAV should be fully open during this procedure. But that is the only place I find such guidance, and there is no explanation as to why it should be so.
 
I turned my adjustment screw out 1 1/2 times from seated when I replaced with a newly rebuilt carburator. I found it ran better turned only 3/4 to 1 full turn out from the seat. Then I turn out the choke rod adjustment 3/4 from seat to start and turn it to 1/4 once warmed up. That's the way mine runs best. Hope that helps.
 
Cider Mill said:
I turned my adjustment screw out 1 1/2 times from seated when I replaced with a newly rebuilt carburator. I found it ran better turned only 3/4 to 1 full turn out from the seat. Then I turn out the choke rod adjustment 3/4 from seat to start and turn it to 1/4 once warmed up. That's the way mine runs best. Hope that helps.
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Great! you figured it out by yourself.As long as it runs good, just leave it be. Model A carbs are SIMPLE and very basic.
 
Commander Toasty said:
Looks like I wasn't clear in this question. I am not asking where everybody sets their GAV. This is not a question about how your car runs with different GAV settings.

I am asking about the procedure for setting the air mixture at the carb, which is done with one of the two screws on the carb body.

Ignored in most procedures is any reference to where - when setting this air mixture - the GAV should be set.

On the website model-a.org they do get specific and state that the GAV should be fully open during this procedure. But that is the only place I find such guidance, and there is no explanation as to why it should be so.
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Why would you set the idle mixture screw ( slotted screw on the front side of the carb) while the GAV is set at a position it will (probably) never be in when you're actually driving the car ? (IE: " all the way open " )

I've been wrenching on cars and farm machinery for nearly 40 years, and the proceedure has been the same for adjusting just about every carb: start the engine and run until warm, the proceed with adjustments.

On those models with an adjustable main jet ( GAV ), you adjust that first, to where the engine runs best, then idle down and adjust the idle mixture screw.

You may have to alternate between the idle mixture screw and the idle stop screw (which controls the opening of the throttle butterfly ) to get the ideal ideal mixture adjustment.

For my '28 (Zenith carb) , with the engine fully warmed-up, these settings are:

(First make sure points are clean and correctly gapped, and the engine is properly timed).

GAV 1/4 to 1/2 turn open from lightly-seated

Idle mixture screw - about 1-1/4 to 1-1/2 turns open from lightly seated.

Throttle and spark levers all the way up against their upper stops.


That said, apparently every A is slightly different... what I have listed for my car is a reasonable starting-point; tweak your car to where it runs best, and make a note of those settings.


I see no reason to adjust the idle mixture with the GAV fully open.
 
While the GAV setting may affect the idle mixture and speed setting procedure, it also affects how the engine runs at non-idle engine speeds. So would it make sense to set the GAV with engine warmed up and running at a comfortable rpm, then proceed to the idle settings? And is this what Frank is describing?
 
Pilotdave said:
While the GAV setting may affect the idle mixture and speed setting procedure, it also affects how the engine runs at non-idle engine speeds. So would it make sense to set the GAV with engine warmed up and running at a comfortable rpm, then proceed to the idle settings? And is this what Frank is describing?
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It sounds like what I was trying to describe... :o
 
This was a long trial and error setting the screw. Turned out after doing a vacume test that the manifold gasket was leaking and the carb needed a rebuild. Any air leakage renders the screw adjustment useless. Check the carb to the intake manifold as well.
 
Pilotdave said:
While the GAV setting may affect the idle mixture and speed setting procedure, it also affects how the engine runs at non-idle engine speeds. So would it make sense to set the GAV with engine warmed up and running at a comfortable rpm, then proceed to the idle settings? And is this what Frank is describing?
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And from this setting, the GAV can be used to adjust for non-warm, non-idle conditions.

On cold days at start up, open the GAV.

On hot (and humid) days at curb idle, "almost" close down the GAV (or wait til nightfall when the temperature drops-A's in heat are as bad as slopping roof tar on at Noon).

On long downhills in gear, but without much pressure on the loud pedal, open the GAV slightly to add more fuel to the mixture and avoid the tail pipe popping from a too lean setting.

It really is a marvelous device, wish I'd thought of it...
 
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