Air/Fuel mix, what are you running???

[Banky]

Hi all. Just wondering how you set your air/fuel mixture dial for driving? What seems to be the best setting? I am running at 1/4 turn open. Or is this one of those "ever A likes a different setting?". (Not talking about opening the choke)

Thoughts?

[Kirby1374]

I have been getting crummy mileage and a rich exhaust smell. I have been playing around to run leaner. I used to run about 1/4-1/2 open on the agv. With a few tuning changes I run with the agv barely open with better results. In my case the stubling I thought meant lean actually meant too rich. I am running a tillotson.

[Patrick L.]

Depends. Makes a difference what carburetor is used. Generally most richen/open the GAV a bit for starting and when cold. Most Zeniths use a closed GAV once warmed, but, some run open 1/4 turn or so. Tillotsons usually run at about 1/4-1/2 turn open once warm.
When driving along at a decent/normal cruise play with it and see what it likes.

[Mitch//pa]

every car has its own sweet spot depending on many variables

[marc hildebrant]

The owner's manual suggests 1/4 open...which I use.

Marc

[SDJason]

Sponsored Links (Register now to hide all advertisements)

Mine runs fine about 1/4 open...consistently getting 14 mpg, I tried running leaner - all I got for my troubles was an overheating engine that was still getting 14 mpg...

[Tom Endy]

The actual desired setting of the GAV control is dependent on the sizing of the comp jet and the cap jet. The two work in concert with each other. The comp jet should be much smaller than the cap jet. If they are correct and the GAV is completely closed the only fuel the cap jet will receive is what flows through the comp jet into the secondary well. Open the GAV slightly and it ports fuel around the comp jet into the secondary well and allows more fuel to be delivered to the cap jet. Open the GAV more that a turn or so and it is wide open and delivering as much fuel to the cap jet as it can handle. Opening it further is of no purpose.

If the comp jet is the same size or larger than the cap jet the GAV does not have any control as the comp jet is passing all the fuel that the cap jet can handle. That is why it is important to flow test the four jets that go in a Zenith. Repo jets are generally not sized correctly, if at all. Many original jets have been molested over the years by people pushing welding tip cleaners through them. Original and\or repo jets can be resized by soldering the ends shut and re-drilling to the correct size and flow testing them.

Flow testing and re-soldering is a tedious and time consuming task. But, it has to be done if you expect a properly functioning Zenith.

Tom Endy

[Peyton]

I'm running a balanced Zenth 2 which is trouble plagued, a Tillotson X which today performed very well at 1/4 open thru city traffic (temp 85 degrees), and a Marvel Schebler which seems to do real well but will not allow the car to get more than 14 - 15 mpg. I'm not sure which carb has the potential for better gas mileage.

I've sized the jets for the Zenith according to research that I have done on this website. My flow tester indicates the jet flows fall generally within the tolerances suggested by the research.

In the past I have gotten between 17 and 19 MPG on long trips. I'm perplexed about the car's overall drop in gas mileage. Running non ethanol seems to make no difference in MPG.

The Zenith 2 seems to shut down when the GAV is closed more than 1/2 turn open even when fully warmed up and at steady speed. I believe that I've got more work to do on the GAV needle seat.

[Bob-A]

The Zenith on my '29 roadster has been gone through by a much recommended rebuilder I met here on the ..'Barn. After his "magic"
the car ran a whole lot better. However, if I try and run with the gav
less then 1/2 open. The car will backfire every now and then when I let
off the gas like during shifting. No idea what my mpg is. I just add non-ethanol gas and some marvel mystery oil when needed. I've got a B-W
OD to, so I'm sure that it helps with the mileage.

Bob-A

[Bob-A]

Correction: if I try and run with the gav less then 1/2 "turn" open.

[Peyton]

I've heard that the Zenith 2 should be able to perform in a range of 0 - 1/4 GAV turn open when the car is running at a steady speed and the car is fully warmed up.

The main jet feeds off a separate passage and the cap jet continues to receive fuel via the comp jet/ secondary well even when the GAV is fully closed.

Admittedly, gas flow is reduced when the GAV is closed but should'nt the main jet plus the reduced flow from the comp jet/ secondary well/ cap jet provide enough fuel to propel the car when it is fully warmed up and at steady speed without any GAV assistance?

[Kurt in NJ]

I have run my car on a dynamometer while reading the exhaust with a 4 gas analyzer, I did it soon after a long trip that I was averaging 23 mpg at 55 to 65 mph --this was before ethonal was added to the gas and at that time I closed the GAV after warmup, more recently I checked the car again, I still get in the over 20 range of mpg, but have to open the GAV because of the ethonal ---there have been no changes in jets (or anything else) in the carb, t5he emissions are lower with the ethonal, and although I can still run with the GAV closed at lower speeds it is lean, to have reasonable power and driveability I now need about 1/4 at speeds in the 30-45 range, over 50 1/2 turn is better.

With the GAV closed the 3X plugs burn white, at 1/4 there is a slight tinge of light tan.

I have never flow tested the jets, they are original ones, I am happy with the way it runs , I can have it lean enough to run non ethonal gas, and make it rich enough to run the diluted ethonal stuff.

When I was running on the dyne I had to enter it as a modern car because model A wasn't in the database, I picked a 1995 Honda civic ---the A ran clean enough that it would pass NJ inspection---as the Honda

Run the GAV as closed as you can and still be able to drive the car ---same for the spark advance, run as much advance as you can, ---also using a thermostat to keep a steady operating temp can help you run less open GAV

[lindy williams]

Amen to everything Tom Endy said. After 80 years all of these carbs are different. You just can't go by the owner's manual or some other rule of thumb. Flow testing and correcting the flow of each of the four jets is critical to proper performance.

[Barney]

I run a tilly. My A runs fine about 1/4- 1/3 open. Fuel consumtion at 16mpg. But every car is something different.

[BILL WILLIAMSON]

I'll do this ONE more time: Chief taught me, with warm engine, run at a steady speed of 20 to 25 MPH in second gear, adjust GAV for best, smoothest running. Leave it there and if it does fine on a cold start & on through warm up, just leave the DANG thing alone & don't always be DIDDLING with it. It ain't supposed to be played with like a TOY!! Bill W.

[fordcoupe]

On my 31 coupe with a Zenith-1 1/4 turn runs good on my 28 pick up with a Zenith-1 1/4 turn runs good on the 31 power plant with a tilly 1 turn like Bill i don't have to touch them after i set them