GAV Valve function?

Z

zemana

New member
I am not sure what the GAV does. Don't all the gas ports have jets to control the gas flow? How can opening the GAV give more fuel?
 
The GAV is the main or high speed jet. You are regulating the gas flow into the venturi at all speeds above idle.
 
zemana said:
I am not sure what the GAV does. Don't all the gas ports have jets to control the gas flow? How can opening the GAV give more fuel?
Click to expand...

Good question.

The carb basically has two "main jets" that put gas out into the air stream and they are in parallel to each other (they act independently of each other). One main jet is of fixed size and puts the same amount of gas out all the time. The other main jet is fed by the variable jet that you call the GAV, so the more you open the GAV, the more gas you put into the air stream from that second main jet..

So imagine you are filling a large tub outside, first with just one hose running. That is the fixed main jet. But if you then start a second hose to run into the tub at the same time, that is your second main jet. And you can control how much water runs though that second hose by only opening the faucet a small amount. That is your GAV.
 
Jim/GA said:
Good question.

The carb basically has two "main jets" that put gas out into the air stream and they are in parallel to each other (they act independently of each other). One main jet is of fixed size and puts the same amount of gas out all the time. The other main jet is fed by the variable jet that you call the GAV, so the more you open the GAV, the more gas you put into the air stream from that second main jet..

So imagine you are filling a large tub outside, first with just one hose running. That is the fixed main jet. But if you then start a second hose to run into the tub at the same time, that is your second main jet. And you can control how much water runs though that second hose by only opening the faucet a small amount. That is your GAV.
Click to expand...
Well stated. Even a shade tree like me can understand.
ThaNK YOU.
 
On a modern car the air/fuel ratio is controlled by the engine computer. On a Model A the air/fuel ratio is controlled by the driver via the GAV. More fuel is needed when starting a cold engine and when more power is needed, such as when going up a long grade.

It is also handy to adjust the air/fuel ratio because of different ambient conditions and different gasolines. For instance, alcohol in the gasoline requires a slightly richer fuel mixture.

There is a jet for the GAV but it is large enough so that the fuel flow can be controlled by the position of it's needle valve.
 
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The gas adjustment valve allows for mixture adjustment on the fly. Following the owner's instruction manual for operation of the engine gives a good overall instruction for adjustment. Ford also wanted the owners to be able to adjust out altitude related metering concerns. The air is thinner at higher altitudes so the mixture needs to be leaned for best operation as the altitude of the car increases and then enrich the mixture as as it decreases.

For average driving, it only needs one adjustment for the area of operation. A fuel miser will adjust it slightly lean and a lead foot will adjust it to the rich side. It does affect fuel mileage over time and distance. Ford's instruction manual stated to open it one full turn for cold starting and use of the choke. After the engine warms then it recommended closing it down to only 1/4 turn open. This is fine and good but in reality, folks adjust it all over the place for smoothest operation or for best fuel mileage.
 
Zemana: For what's it worth...I have driven my car since 2014. I opened the GAV about a 1/4 turn, maybe a touch more. Never adjusted it again. Get in, gas on, key on, timing lever up, gas lever slightly down about a 1/4 from top...hit the starter button real hard...pull the choke (GAV) and as it fires immediately let go of the choke and it's running.
Don't agonize over it...
 
Tom Endy gives us the simple and accurate explanation of how the GAV varies the Air fuel mixture using the Cap Jet.
 
I often hear fellow club members saying that they don't adjust the GAV. Thats sad, it is part of the Model A experience and an experience that the modern driver never gets. Others have already explained how the gas gets into the engine. In the literature, it is mentioned that Model A dealers often got complaints that the Model A used too much gas. It is because, after a cold start, the GAV was not closed down for optimum performance. You should not be afraid to practice with the adjustment and you should feel the increase in power when the air/fuel ratio becomes correct after your warm up when the GAV is closed slightly. My 1932 Coupe carburetor is different. This carburetor has you closing the GAV after warmup because the Main Jet and the Power Jet (which is new for 1932) is all the gas you need. You don't need the GAV for driving, only for cold startup.

For those of you that are rebuilding your Model A carburetor, I would recommend that your Main Jet be sized for a maximum flow of 153 ml/min. so that there is a slight need for gas provided by the GAV (about half a turn open) and to obtain a light grey tail pipe indicating that you are not driving too rich. ED
 
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