Function of GAV

bobpo1

bobpo1

Senior Member
I've often wondered, is the GAV needle and seat supposed to shut the gas supply off. I know what the GAV does while the engine is running, but are you supposed to close it when parked.

Also, isn't there a needle that has a viton (sp) tip? :confused:

While I'm thinking of it, can a plugged Fuel Valve be cleaned to work like new again? My old one seems to be plugged so bad that I can't get it work with any cleaning agent I've come up with. Any Ideas appreciated.
 
Last edited:
To answer you first question, there is no need to close the GAV when parked. Fuel only flows through it when the running engine draws a vacuum through the carb. With the engine shut off fuel doesn't flow no matter what the needle setting is.
 
The GAV valve (gas adjust valve) is an adjustable valve between the fuel reservoir and the secondary fuel circuit which comprises the secondary well and the cap jet. Casting passages can be found drilled to either component. The end result is the same.

The comp jet is the small button like jet screwed into the fuel reservoir. It meters a fixed amount of fuel into the secondary well. The secondary well provides fuel for the idle jet and also provides a gulp of fuel to the cap jet when you accelerate.

When you are rolling down the road the cap jet and the main jet are providing the fuel to the engine. The secondary well is essentially empty. The fuel passing through the comp jet goes directly to the cap jet. The cap jet is limited by the small amount of fuel being emitted by the comp jet. This is where the GAV comes in. By opening the GAV you allow extra fuel to be ported around the comp jet to the cap jet. This allows you to set the proper fuel mixture when rolling down the road.

In most cases you should have the GAV cracked open from a quarter of a turn to three quarters of a turn if the jets are sized correctly. It all depends on the actual sizing of the jets. Many Zeniths have the jets sized incorrectly. If for instance the comp jet is sized larger than the cap jet the GAV has no function at all.

Once you find the optimum setting of the GAV you can generally leave it there. On a cold day you may have to open it slightly more until the car warms up. Also at high altitudes or climbing a steep grade you would want to richen the mixture by opening the GAV slightly more.

There is no reason to close the GAV when you shut the engine down. Most properly jetted Zeniths will not run properly with the GAV closed. For this reason there is no reason to be concerned about a GAV having the ability to completely close off. It is usually never fully closed.

The Viton tip valves on the market are the float valves. You want the float valve to have a positive shut off, otherwise you will have a leaking Zenith when parked.

Tom Endy
 
The fellas covered the GAV

The viton tipped needle you mentioned is the float needle.

The fuel valve should be able to be removed and cleaned. The fuel tank may have 'junk' [technical term] slopping around in the bottom which should probably be cleaned out and one of the aftermarket extensions installed so fuel isn't pulled directly from the tank bottom.
 
Tom Endy said:
The GAV valve (gas adjust valve) is an adjustable valve between the fuel reservoir and the secondary fuel circuit which comprises the secondary well and the cap jet. Casting passages can be found drilled to either component. The end result is the same.

The comp jet is the small button like jet screwed into the fuel reservoir. It meters a fixed amount of fuel into the secondary well. The secondary well provides fuel for the idle jet and also provides a gulp of fuel to the cap jet when you accelerate.

When you are rolling down the road the cap jet and the main jet are providing the fuel to the engine. The secondary well is essentially empty. The fuel passing through the comp jet goes directly to the cap jet. The cap jet is limited by the small amount of fuel being emitted by the comp jet. This is where the GAV comes in. By opening the GAV you allow extra fuel to be ported around the comp jet to the cap jet. This allows you to set the proper fuel mixture when rolling down the road.

In most cases you should have the GAV cracked open from a quarter of a turn to three quarters of a turn if the jets are sized correctly. It all depends on the actual sizing of the jets. Many Zeniths have the jets sized incorrectly. If for instance the comp jet is sized larger than the cap jet the GAV has no function at all.

Once you find the optimum setting of the GAV you can generally leave it there. On a cold day you may have to open it slightly more until the car warms up. Also at high altitudes or climbing a steep grade you would want to richen the mixture by opening the GAV slightly more.

There is no reason to close the GAV when you shut the engine down. Most properly jetted Zeniths will not run properly with the GAV closed. For this reason there is no reason to be concerned about a GAV having the ability to completely close off. It is usually never fully closed.

The Viton tip valves on the market are the float valves. You want the float valve to have a positive shut off, otherwise you will have a leaking Zenith when parked.

Tom Endy
Click to expand...

Tom, in your fourth paragraph, you mention that if two of the jets are improperly matched, then the GAV has no function. Well, I think that's where I'm at. It seems to have little to no effect when operating down the road. I am going to replace the needle however, just to be on the safe side. I used to be able to almost stall the engine by closing the needle. It doesn't do that anymore. I had changed the carb out within the last year.
 
Patrick L. said:
The fellas covered the GAV

The viton tipped needle you mentioned is the float needle.

The fuel valve should be able to be removed and cleaned. The fuel tank may have 'junk' [technical term] slopping around in the bottom which should probably be cleaned out and one of the aftermarket extensions installed so fuel isn't pulled directly from the tank bottom.
Click to expand...

What can I use to get all those years of rust out of the old valve?
 
bobpo1 said:
Tom, in your fourth paragraph, you mention that if two of the jets are improperly matched, then the GAV has no function. Well, I think that's where I'm at. It seems to have little to no effect when operating down the road. I am going to replace the needle however, just to be on the safe side. I used to be able to almost stall the engine by closing the needle. It doesn't do that anymore. I had changed the carb out within the last year.
Click to expand...

That is why you need flow tested jets for your carburetor. Otherwise it will be by accident that it runs correctly.
 
You might take a look at page 206 and 207 of the Service Bulletins, it provides a good description of the various jets also covers the air adjustment needle and the GAV.

Note: it also states the GAV can be closed when driving for Maximum Economy.

I size my jets so the GAV can be closed after the engine warms up. When climbing hills or around town when making stops having the GAV 1/4 turn open will help keep the engine running smoothly and not heat up for lack of fuel.

Just my opinion.

Ron
 
Some of you may remember a few weeks ago I was having trouble with flooding. I had a carb rebuild kit on hand I had bought 4 years ago from I can't remember who I bought it from. I didn't have my reading glasses on and done a rebuild on my carb. The comp jet in the rebuild kit was three times the opening it should have been for a comp jet. Needless to say I had big time flooding till I got that problem located. All this time I was leaning toward the float valve not holding. And to further complicate matters I was also trying to make another carb work that was also flooding. After I finely discovered the oversize comp jet in the one carb I tour down the second carb and found that the cap jet had the orifice gone out of the tip so that was the problem with the second carb. I'm still learning. Learning to wear glasses too.
 
Simply put, the gas adjusting jet is just an EXTRA jet, to supplement the MAIN jet, when it's too lean & causing coughing back or surging.
Chief taught me, with a warm engine, run at a steady 25 MPH in second gear & adjust the GAV for smoothest running. Most carbs have their own personality & require a DIFFERENT setting. IF, at that setting, it starts OK when cold & during warm up while driving, just LEAVE it ALONE.
On Minerva, she ran well with the GAV CLOSED, except on really cold days.
IF your GAV affects the idle, then it's idling TOO FAST!
MODELABASICS.COM has some great carb info.
Bill W.
 
Tom Endy said:
The GAV valve (gas adjust valve) is an adjustable valve between the fuel reservoir and the secondary fuel circuit which comprises the secondary well and the cap jet. Casting passages can be found drilled to either component. The end result is the same.

The comp jet is the small button like jet screwed into the fuel reservoir. It meters a fixed amount of fuel into the secondary well. The secondary well provides fuel for the idle jet and also provides a gulp of fuel to the cap jet when you accelerate.

When you are rolling down the road the cap jet and the main jet are providing the fuel to the engine. The secondary well is essentially empty. The fuel passing through the comp jet goes directly to the cap jet. The cap jet is limited by the small amount of fuel being emitted by the comp jet. This is where the GAV comes in. By opening the GAV you allow extra fuel to be ported around the comp jet to the cap jet. This allows you to set the proper fuel mixture when rolling down the road.

In most cases you should have the GAV cracked open from a quarter of a turn to three quarters of a turn if the jets are sized correctly. It all depends on the actual sizing of the jets. Many Zeniths have the jets sized incorrectly. If for instance the comp jet is sized larger than the cap jet the GAV has no function at all.

Once you find the optimum setting of the GAV you can generally leave it there. On a cold day you may have to open it slightly more until the car warms up. Also at high altitudes or climbing a steep grade you would want to richen the mixture by opening the GAV slightly more.

There is no reason to close the GAV when you shut the engine down. Most properly jetted Zeniths will not run properly with the GAV closed. For this reason there is no reason to be concerned about a GAV having the ability to completely close off. It is usually never fully closed.

The Viton tip valves on the market are the float valves. You want the float valve to have a positive shut off, otherwise you will have a leaking Zenith when parked.

Tom Endy
Click to expand...

Tom, Thanks for that very easy to comprehend explanation of the GAV :)
 
bobpo1 said:
While I'm thinking of it, can a plugged Fuel Valve be cleaned to work like new again? My old one seems to be plugged so bad that I can't get it work with any cleaning agent I've come up with. Any Ideas appreciated.
Click to expand...

The valve on the tank can be taken apart, cleaned and rebuilt.
 
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