GAV Question

[Model A Ron]

So how far open do you run your GAV? Will a lean or rich setting cause a car to run hot?

Thanks
Ron

[Tom Endy]

The GAV adjustment is affected by the size of the cap jet and the comp jet. When the GAV is closed fuel that is emitted by the cap jet is determined by the size of the comp jet. When you crack the GAV open you are porting fuel around the comp jet so that the cap jet will emit more fuel. If the comp jet has been drilled out to the same size as the cap jet the GAV does not have any control.

Each car is going to be different depending on the sizing of the two jets. If the two jets are within spec (most are not) the GAV adjustment should be between 1\4 and 1\2 a turn cracked open for normal driving. The jets in my Vic have been properly re-sized and I run with the GAV cracked open 1\2 turn for normal driving.

When climbing a steep mountain grade I open the GAV more and back off on the spark advance a little.

Tom Endy

[Gene F]

Explained well Tom. FYI; I don't know anyone that drills their jets.

[Kurt in NJ]

I did testing on emission machine under load , on 'regular" gas I got 3-4% CO with GAV closed, ran fine, gas mileage over 20mpg
Then NJ got 10% ethonal, this made the car too lean with the GAV closed, 0,5% CO (car would pass emissions as a 90s car), good running took 1/4 open up to 60, 1/2 above 60.
I found that 3-4 % the plugs would burn light tan, at 0,5%-2.0% they burn pure white
When I used gasoline with over 25% diesel it would take 1-2 turns to even run

[nkaminar]

Normally a richer mixture will make an engine run cooler because of the latent heat of evaporation of the gasoline (and ethanol). There are limits and this pertains to the normal range of air/fuel ratio.

[Bob Bidonde]

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Heed Tom's advice. Always seek the sweet spot for the GAV adjustment, and it will vary from car to car. Here's my practice driving the GAV:

[Chuck Sea/Tac]

Originally Posted by Gene F

Explained well Tom. FYI; I don't know anyone that drills their jets.

I drill, if needed, and flow test. I believe Renners jets are all flow tested, so I assume they may drill if not enough flow?

[jayvee34]

I am somewhat confused. My car starts @ 1/4 turn open, idles perfect. Will not
run past 10 MPH. Have to open gav to 1 1/2 turns, car runs up to 35 MPH,
open gav to 2 1/2 turns, car runs perfect up to 55 MPH, runs cool (160 degrees). I can leave gav @ 2 1/2 turns will slowing down or even stop and
start again. Turn ignition off. Turn ignition back on touch starter, instantly starts.
When on line and found this:ZENITH CARBURETOR ADJUSTMENT
Turn idle adjusting needle in lightly against its seat, then back out needle l 1/2 turns
as a preliminary adjustment.
With throttle held in closed position, turn throttle stop screw in until stop screw just
contacts throttle stop and then turn stop screw in 1 1/2 additional turns as a
preliminary idle speed setting
Set main jet adjusting needle at 4 1/2 full turns open.
Start engine and let it run a few minutes until warm enough for final adjustments. Set
the throttle stop screw for the desired idling speed.

Set the idle adjusting needle for smooth idling. Turning the needle in (clockwise)
reduces air and makes the mixture richer, while turning it out (counter clockwise) admits
more air into the mixture making it leaner (avoid using a mixture that is too lean).
To set the main jet adjustment, hold the throttle open so that the engine is running at
about 1/2 its normal operating speed. Turn the adjusting needle toward its seat until it
causes the engine to give evidence of a lean mixture by running irregularly. Then turn
the needle away from its seat, carefully counting the turns made until the engine shows a
trace of rich mixture. Set the adjustment. Then at just about halfway between the lean
point and the rich point, hold the adjustment at this point and tighten the packing nut
so there is no leakage of fuel around the adjusting needle.
Note: Check the fuel line connections so there is no leakage.
Look at magneto points and make sure they are not pitted and are adjusted to the proper
gap (.020" if Fairbanks Morse - .014" to .016" if Edison Splitdorf). Check spark plug to
make sure it is clean and properly gapped at .027".

[Patrick L.]

Originally Posted by jayvee34

I am somewhat confused. My car starts @ 1/4 turn open, idles perfect. Will not
run past 10 MPH. Have to open gav to 1 1/2 turns, car runs up to 35 MPH,
open gav to 2 1/2 turns, car runs perfect up to 55 MPH, runs cool (160 degrees). I can leave gav @ 2 1/2 turns will slowing down or even stop and
start again. Turn ignition off. Turn ignition back on touch starter, instantly starts.
When on line and found this:ZENITH CARBURETOR ADJUSTMENT
Turn idle adjusting needle in lightly against its seat, then back out needle l 1/2 turns
as a preliminary adjustment.
With throttle held in closed position, turn throttle stop screw in until stop screw just
contacts throttle stop and then turn stop screw in 1 1/2 additional turns as a
preliminary idle speed setting
Set main jet adjusting needle at 4 1/2 full turns open.
Start engine and let it run a few minutes until warm enough for final adjustments. Set
the throttle stop screw for the desired idling speed.

Set the idle adjusting needle for smooth idling. Turning the needle in (clockwise)
reduces air and makes the mixture richer, while turning it out (counter clockwise) admits
more air into the mixture making it leaner (avoid using a mixture that is too lean).
To set the main jet adjustment, hold the throttle open so that the engine is running at
about 1/2 its normal operating speed. Turn the adjusting needle toward its seat until it
causes the engine to give evidence of a lean mixture by running irregularly. Then turn
the needle away from its seat, carefully counting the turns made until the engine shows a
trace of rich mixture. Set the adjustment. Then at just about halfway between the lean
point and the rich point, hold the adjustment at this point and tighten the packing nut
so there is no leakage of fuel around the adjusting needle.
Note: Check the fuel line connections so there is no leakage.
Look at magneto points and make sure they are not pitted and are adjusted to the proper
gap (.020" if Fairbanks Morse - .014" to .016" if Edison Splitdorf). Check spark plug to
make sure it is clean and properly gapped at .027".

Not sure what those adjustments are for, but, they aren't for our stuff. Especially the last paragraph.

[jayvee34]

Originally Posted by Patrick L.

Not sure what those adjustments are for, but, they aren't for our stuff. Especially the last paragraph.

Not sure either, but that is actually how my car runs. That is why I am some
what confused.

[alexiskai]

It's for a tractor.

[jayvee34]

Originally Posted by alexiskai

It's for a tractor.

WOW!! Wonder why my car won't run unless I use use those settings as I
described?

I would appreciate very much if someone would have an idea as to how I can fix this.
Thanks.

[alexiskai]

Originally Posted by jayvee34

WOW!! Wonder why my car won't run unless I use use those settings as I described?

Maybe your comp jet is damaged. The GAV controls additional fuel supply to the cap jet, but this jet is supposed to normally be fed by the comp jet from the fuel bowl. If you're needing to open the GAV 2-3 full turns, that could indicate that the cap jet isn't being fed anything via the comp jet.

I'm not sure what rpm the main jet kicks in at, but you might could test this hypothesis by throttling up above where the main jet kicks in and then closing the GAV. In theory the engine would continue running off the main jet, and then when you lowered the rpm, the main jet would shut off and the engine would die immediately.

[alexiskai]

You can see in this image, the cap jet is fed both from the blue pipe and the green pipe. Green pipe is the GAV, blue pipe is the comp jet. Sounds like you're getting nothing via the blue pipe, so there could be a problem with the comp jet or the secondary well.

[nkaminar]

John, There is a really good site for all things Zenith. For your situation see: http://model-a.org/mixture_lean_heat.html. For a good explanation of the different parts see: http://model-a.org/components_full.html.

The Zenith carburetor seems simple but the devil is in the details.

[jayvee34]

alexiskai, Thank you so very much. I will go to work on it. I hope you don't mind that
I copied the attached picture. Being a Zenith Carb dummy I will need to refer to it.
Thanks again.

[jayvee34]

Originally Posted by nkaminar

John, There is a really good site for all things Zenith. For your situation see: http://model-a.org/mixture_lean_heat.html. For a good explanation of the different parts see: http://model-a.org/components_full.html.

The Zenith carburetor seems simple but the devil is in the details.

Thank you very much also, I will certainly check it out.

[alexiskai]

Originally Posted by jayvee34

I hope you don't mind that
I copied the attached picture.

Go right ahead, I copied it from modelabasics.com.