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Old 12-06-2018, 12:24 AM   #1
SteveR.
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Default Engine runs rough unles GAV is open 3/4 turn

My engine idles good, but when accelerating and at higher speeds, the GAV must be open about 3/4 turn for the engine to be smooth and have good power. I remember reading that the GAV shouldn't be open more than 1/4 turn for normal driving after warm. I assume something on the carb needs to be adjusted, any tips?

Thank you!
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Old 12-06-2018, 01:07 AM   #2
Tom Endy
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Default Re: Engine runs rough unles GAV is open 3/4 turn

The sizing of both the comp jet and the cap jet determine where the GAV should be set. The comp jet is the little button looking jet in the reservoir and is much smaller than the cap jet. The fuel that feeds the cap jet has to come through the comp jet. The GAV allows fuel to be ported around the comp jet. If all the jets are sized correctly you should be able to run at high speed with the GAV set between a quarter and three quarters open. The jets in your carburetor may not be sized correctly, or you may have something plugging one of the jets. First take an air hose and blow each jet out. You also could have some debris plugging the internal passageways.


The jets in my carburetor have been re-sized and I leave the GAV set at three quarter open for most driving conditions. It doesn't sound like there is anything wrong with your carburetor, you just have the GAV closed too much.


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Old 12-06-2018, 03:27 AM   #3
johnbuckley
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Default Re: Engine runs rough unles GAV is open 3/4 turn

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Originally Posted by SteveR. View Post
My engine idles good, but when accelerating and at higher speeds, the GAV must be open about 3/4 turn for the engine to be smooth and have good power....
I find that that Tillotsons normally need to be run about 3/4 to 1 turn open .With Zenith carbs I reckon 0 to 3/4 turn is within normal range .
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Old 12-06-2018, 05:39 AM   #4
roy green coupe
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Default Re: Engine runs rough unles GAV is open 3/4 turn

If the fuel has up to 10% ethanol like here in nj, I would think that would be a reason to have to run the GAV richer (or more open). I am always kind of amazed that gasoline today will still run a vehicle designed ninety years ago.
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Old 12-06-2018, 08:23 AM   #5
Kurt in NJ
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Default Re: Engine runs rough unles GAV is open 3/4 turn

When NJ had regular gas I could close the GAV, I ran the car on the emissions dyno, 4%co, later with ethonal gas I ran it because the gav needed to be opened more, closed the co was 0.05 co, too lean to drive, the exhaust manifold would glow red, at 1/2 turn 3%up to 50 mph, to have 3% above 50 needed 3/4 turn, my jets have not been flow tested so I don't know how they compare to the"standards "
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Old 12-06-2018, 10:40 AM   #6
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Default Re: Engine runs rough unles GAV is open 3/4 turn

Had exactly the same problem on a couple of my A's - GAV needed open more and more to run well every year. Finally pulled the carbs and did a good cleaning with some carb cleaner. Problem solved and back to running the way they should. One had the jets so gummed up I had to really work to get them clean. Now I always let the carbs run dry when I'm shutting down for more than a few days.
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Old 12-06-2018, 01:47 PM   #7
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Default Re: Engine runs rough unles GAV is open 3/4 turn

I have the same problem...but I have to keep the GAV opened about 2 full turns open while cruising. (Oddly, if I turn the GAV back to 1/4 open the car idles higher and smoother.) Regardless...I too have self-diagnosed the problem as the cap jet and/or comp jet being gummed-up. That is on the list of to-do items this winter in the ol' shop.
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Old 12-06-2018, 01:57 PM   #8
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Default Re: Engine runs rough unles GAV is open 3/4 turn

A couple of years a wanna be expert tried to put me down on this barn because i had the same problem. I to had a problem running both of mine at 1/4 turn. 3/4 to 1 full turn is where they like to be. Glad to see others have the same scenario. Thanks to this expert, i don't post much any more.
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Old 12-06-2018, 02:40 PM   #9
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Default Re: Engine runs rough unles GAV is open 3/4 turn

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Originally Posted by Tom Endy View Post
The sizing of both the comp jet and the cap jet determine where the GAV should be set. The comp jet is the little button looking jet in the reservoir and is much smaller than the cap jet. The fuel that feeds the cap jet has to come through the comp jet. The GAV allows fuel to be ported around the comp jet. If all the jets are sized correctly you should be able to run at high speed with the GAV set between a quarter and three quarters open. The jets in your carburetor may not be sized correctly, or you may have something plugging one of the jets. First take an air hose and blow each jet out. You also could have some debris plugging the internal passageways.


The jets in my carburetor have been re-sized and I leave the GAV set at three quarter open for most driving conditions. It doesn't sound like there is anything wrong with your carburetor, you just have the GAV closed too much.


Tom Endy
Terrific, thank you Tom! I must be remembering something incorrectly about driving with 1/4 turn max on the GAV.
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Old 12-06-2018, 04:37 PM   #10
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Default Re: Engine runs rough unles GAV is open 3/4 turn

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A couple of years a wanna be expert tried to put me down on this barn because i had the same problem. I to had a problem running both of mine at 1/4 turn. 3/4 to 1 full turn is where they like to be. Glad to see others have the same scenario. Thanks to this expert, i don't post much any more.
I've had trouble here too with a guy (now banned) who thought he knew the lot but didn't even read my posts properly. Don't worry about him. As they say, an EX is a has been and a SPERT is a drip under pressure!
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Old 12-06-2018, 09:36 PM   #11
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Default Re: Engine runs rough unles GAV is open 3/4 turn

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Originally Posted by rosenkranswa View Post
Had exactly the same problem on a couple of my A's - GAV needed open more and more to run well every year. Finally pulled the carbs and did a good cleaning with some carb cleaner. Problem solved and back to running the way they should. One had the jets so gummed up I had to really work to get them clean. Now I always let the carbs run dry when I'm shutting down for more than a few days.
This is the experience I've had.

First you need to get a handle on what's "normal" for you car. For example, my Dad's car likes to run with the GAV closed. My cars like to run with the GAV open 1/4 turn open (B carbs). Then, after that, if it seems like you need to open the GAV more, you probably need to clean it up inside.

I will mention one thing, though: a richer mixture will compensate for ignition being weak. So make sure ignition wire connections are all clean and tight, make sure ignition points are shiny and gapped correctly, make sure the coil polarity is correct. Then set the GAV to what it needs to be to run well.
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Old 12-10-2018, 09:33 PM   #12
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Default Re: Engine runs rough unles GAV is open 3/4 turn

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Originally Posted by SteveR. View Post
My engine idles good, but when accelerating and at higher speeds, the GAV must be open about 3/4 turn for the engine to be smooth and have good power. I remember reading that the GAV shouldn't be open more than 1/4 turn for normal driving after warm. I assume something on the carb needs to be adjusted, any tips?

Thank you!
I am not sure you have a problem, if you can run the GAV 3/4 turn open. (and the car idles right).

Just to point something out, the GAV is adjustable to allow adjustment to get the right amount of fuel mix in the carb to run the car.

I run mine at between 1/2 and 3/4 and have never been concerned about adjusting the GAV in this range to have the car run good.

The way I look at it, the GAV is able to be adjusted for a reason. I also understand that altitude affects how it should be adjusted. Where I am located, I am 751 Ft above sea level. Just for grins, Check your altitude above sea level?
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Old 12-11-2018, 04:14 PM   #13
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Default Re: Engine runs rough unles GAV is open 3/4 turn

The town we live in is at about sea level. Our car runs great at 1/4 to 1/2 turn open.

We use to live in up-state New York, at about 1,200 feet elevation. GAV needed to be at 3/4 turn open.

New Zenith carburetor.

As stated in other comments, the GAV is adjustable for a reason. Enjoy.
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