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Old 09-21-2021, 08:44 AM   #1
CatMan1
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Default GAV Adjustment While Driving?

I got thinking about this. I tend to start the car and not do any adjusting of the GAV there after. Is that wrong? Should I be adjusting it while driving? I do have some backfiring between shifts. I tried to fix it by replacing the condenser, but it didn't help. The last time I was out with the car it wasn't running real well and may have been overheating. Going slow through a park in low gear it would 'buck' along.

This week we're going to try flushing the cooling system.
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Old 09-21-2021, 09:21 AM   #2
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Default Re: GAV Adjustment While Driving?

It's a little inconvenient in location of adjusting on the fly but I do it all the time. Turn for best performance (best running) and it will pull well. Turn it down a bit for best economy.
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Old 09-21-2021, 10:25 AM   #3
Patrick L.
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Default Re: GAV Adjustment While Driving?

Once you get it set where you like it, you shouldn't have to touch it. Except when the weather cools off, first start then it may help to open GAV a 1/2- 1 turn out for a few minutes.
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Old 09-21-2021, 11:57 AM   #4
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Default Re: GAV Adjustment While Driving?

I normally drive with mine 1⁄4 turn open but if I'm approaching a long hill I will open it 1⁄4 turn more
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Old 09-21-2021, 12:55 PM   #5
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Default Re: GAV Adjustment While Driving?

To be honest with you, I've never figured my GAV out. Once the engine is warm, the car runs fine with the GAV closed. At 1/4 turn it starts to become too rich so that at 1/2 turn the engine has lost power. I've compromised (since lots of folks on this forum always talk about 1/2 to 3/4 as best) and generally drive at 1/8 open. I check my plugs after each 50 mile trip and the tips stay light tan so I am happy. I suspect my GAV needle seat is worn open and imagine lots of folks have the same issue without knowing it.
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Old 09-21-2021, 12:56 PM   #6
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Default Re: GAV Adjustment While Driving?

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I forgot to mention I have a B carb and I suspect that makes the GAV a mute issue.
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Old 09-21-2021, 01:53 PM   #7
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Default Re: GAV Adjustment While Driving?

I like Patrick L's thoughts. I deal with GAV adjusting in the spring at the first start up, then I leave it alone. Choke on starting when cold. Nothing else. Usually get 17-18 mpg with spark 3 notches up from the bottom. Bill
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Old 09-21-2021, 01:58 PM   #8
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Default Re: GAV Adjustment While Driving?

Heard/read somewhere that the carb was designed to run a bit lean at sea level so it would be about right at higher altitude. I run mine just a touch open. I am at sea level.
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Old 09-21-2021, 03:18 PM   #9
Tom Endy
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Default Re: GAV Adjustment While Driving?

The setting of the GAV will be different for each car. The setting depends on the sizing of the comp jet and the cap jet. They are in the circuit with the GAV valve. On my Victoria I have it open about three quarters on a turn, and leave it there for normal driving around town. Out on the road when I encounter a long steep grade I open the GAV more and back off on the spark advance a little.

If the comp jet has been drilled out larger than the cap jet, the GAV will then have no function.

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Old 09-21-2021, 06:50 PM   #10
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Default Re: GAV Adjustment While Driving?

Quote:
Originally Posted by Tom Endy View Post
The setting of the GAV will be different for each car.

Tom Endy
Now there is a true statement if I ever heard one!! Set the GAV where your engine runs best. Never mind what everyone else sez.
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Last edited by Ruth; 09-21-2021 at 06:59 PM.
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Old 09-21-2021, 07:13 PM   #11
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Default Re: GAV Adjustment While Driving?

Bill Williamson once said to "drive at about 20 mph in second gear on flat ground, set the GAV where it runs best and then don't diddle with it"
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Old 09-22-2021, 05:23 AM   #12
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Default Re: GAV Adjustment While Driving?

Quote:
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Now there is a true statement if I ever heard one!! Set the GAV where your engine runs best. Never mind what everyone else sez.
X2 !
Also into the equation are different fuels , different weather/temp, and a host of other lesser factors including atmos pressure /hills /stop start traffic etc etc
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Old 09-22-2021, 06:36 AM   #13
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Default Re: GAV Adjustment While Driving?

Backfiring when shifting would indicate that the idle is too lean. A Zenith has an air bleed to adjust the idle mixture. That means that you turn the idle adjustment mixture screw in to richen the idle mixture. Turn it in 1/4 turn at a time and test. You may have to adjust the idle throttle setting after adjusting the idle mixture screw.
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Old 09-22-2021, 07:45 AM   #14
CatMan1
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Default Re: GAV Adjustment While Driving?

I often wonder if my choice of setting and around town driving has caused the fouled plugs? I pretty much have just set the GAV at what the Owner's Manual said to do. Maybe I should experiment?
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Old 09-22-2021, 08:48 AM   #15
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Default Re: GAV Adjustment While Driving?

Here is my practice:
Attached Images
File Type: jpg Driving With The GAV 237kb.jpg (77.7 KB, 66 views)
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Old 09-22-2021, 10:27 AM   #16
rotorwrench
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Default Re: GAV Adjustment While Driving?

The GAV is a mixture trim adjustment that works with the main jet to cover all possible fuel mixture needs. It will compensate for altitude and temperature changes by manual control. Auto engines will run from between a lean condition of 16:1 to a rich condition of 12:1. They pull good power at 13:1 but get better mileage at 14.7:1 so there isn't much variance there of air to fuel mix.

Manual control of fuel mixture takes some getting used to but it generally boils down to adjusting for best engine performance any time it feels necessary to the individual operator. A person can fine tune the system while running down the road or a person can get used to where it works best for their environment and adjust to their own desired settings at any time.

I've noticed that fuel mileage can vary depending on how you set it. A car engine will run just fine on a bit leaner mixture but it may not have best performance with that setting. It will still run and work and mileage will be better. If a person sets it too lean then they may notice a back fire hickup now and then when pulling a heavy load up a hill. Personally, I set it for best running and don't worry about the mileage. I just leave it where it is unless the temperature drops. If I store the car for longer periods I shut it off and pull the choke to keep stuff from getting into the carb (no air filter). I shut off the fuel valve and run the carb dry before messing with the GAV & choke. This modern fuel is not something I like setting around for too long in confined spaces that are vented to the atmosphere.

Last edited by rotorwrench; 09-22-2021 at 10:36 AM.
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Old 09-22-2021, 11:56 AM   #17
Patrick L.
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Default Re: GAV Adjustment While Driving?

Quote:
Originally Posted by CatMan1 View Post
I often wonder if my choice of setting and around town driving has caused the fouled plugs? I pretty much have just set the GAV at what the Owner's Manual said to do. Maybe I should experiment?



Like has been said [ as well as #3] set the GAV where your car seems to run the best, it takes a little playing around to get it where you like it. Just cruise on a level road and play with the funny little knob.

On a cool morning it may like the knob to be opened an extra 1/2-1 turn for the first few miles, then close to its normal position.
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