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Old 12-17-2018, 07:27 PM   #1
Russ/40
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Default Carb expert needed

I'm having trouble with the GAV in my 29. I have been plagued with a hard to turn GAV since day one. I think I now have it where I want it. Not too loose or to tight. Problem is, the GAV doesn't seem to make any difference any more. Does not seem to change anything no matter where I put it, totally closed or two turns out. All jets and passages have been checked, and are clear.
The only thing I could find when checking it out was the Comp Jet was loose. I took care of that as well as cleaned everthing but now.....no adjustment.

The reason I pulled the carb was the lack of adjustment.
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Old 12-17-2018, 07:39 PM   #2
Brian T
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Default Re: Carb expert needed

Hello Russ,
As long as the idle is below 6-700 Rpm then you will not notice any change with the GAV at idle, however driving along you will notice a change in the engine performance, try it and let us know.
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Last edited by Brian T; 12-26-2018 at 02:01 PM. Reason: added text
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Old 12-17-2018, 08:47 PM   #3
Wick
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Default Re: Carb expert needed

Your gav needle at the carbutor is bent or the tube is bent. Replace it.
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Old 12-17-2018, 11:18 PM   #4
Mike V. Florida
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Default Re: Carb expert needed

http://modelabasics.com/carb%20basics%202.htm
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Old 12-17-2018, 11:23 PM   #5
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Default Re: Carb expert needed

Part of the Zenith design is the Gas Adjusting Valve (GAV), which provides a manual fuel adjustment for extra fuel from the main float bowl through the Secondary Well to the Cap Jet when needed. The carburetor was designed to be slightly lean to allow for high altitude driving and the GAV provided additional fuel. This extra fuel is needed at sea level, and even more so when the engine is cold.
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Old 12-21-2018, 01:13 PM   #6
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Default Re: Carb expert needed

My GAV on one of my cars sometimes seems to be "insensitive" and has little to no effect under certain conditions. But when I adjust it when the engine is cold, or I'm climbing a fairly steep hill, I will definitely notice its impact. Maybe you could find a good hill that you need to climb in 2nd gear around 15-20 mph, and then close it off. You should really start to lose power. If not, as others have said, you need to replace the needle or seat. Good luck
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Old 12-21-2018, 01:33 PM   #7
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Default Re: Carb expert needed

Wick is most likely correct. I am having the same problem and after doing some work I found the GAV needle bent making it almost impossible to turn.
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Old 12-26-2018, 12:16 PM   #8
redmodelt
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Default Re: Carb expert needed

Could it be that someone in the past rebuilt the carb and left the seat out? Some have the brass seat some do not for the GVA.
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Old 12-26-2018, 12:31 PM   #9
Al 29Tudor
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Default Re: Carb expert needed

If your carb doesn't have the replaceable brass seat, how can you know the condition of the cast iron seat?
Is there a tool that can re-surface that seat?
I'd sure like to know.
Thanks,
Al
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Old 12-26-2018, 01:46 PM   #10
larrys40
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Default Re: Carb expert needed

Quote:
Originally Posted by Al 29Tudor View Post
If your carb doesn't have the replaceable brass seat, how can you know the condition of the cast iron seat?
Is there a tool that can re-surface that seat?
I'd sure like to know.
Thanks,
Al

Al,
You can take a black permanent marker and darken up the needle tip and by hand turn it against the seat. If it has a decent seat you will see it on the needle .

sometimes lightly spreading the GAV needle hold ring will help a little. Yes, making sure the needle isn't bent is first but I have had the grip ring too tight on a few.

Also, is someone has a large vacuum leak that can affect how the GAV reacts, as it is all mixture related. Check you wiper tubes and connections, carb tight, manifold gaskets, wiper lines as manifold and firewall, inside and outside.


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Old 12-26-2018, 01:58 PM   #11
Benson
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Default Re: Carb expert needed

The most common way the needle gets bent, that I have seen, is that someone installs the GAV housing with the needle screwed into housing, if they tighten housing too much, the needle can be bent.


Solution: Remove needle from housing before installing housing.



FYI for those that may not know it:

The housing is a tapered pipe thread so do not over tighten it or use a standard tap to clean out the threads in the casting.


I use small amount of "Seal All" sealer in yellow tube on threads ... it's fuel proof.
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Old 12-26-2018, 02:08 PM   #12
larrys40
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Default Re: Carb expert needed

Agree with Benson, I usually don't install the needle at all until the housing is installed and then there is not that problem . Needles get bent as sometimes when carbs are off or have been they get banged up on the gave housing... etc.



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Old 12-26-2018, 10:39 PM   #13
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Default Re: Carb expert needed

I like Al 29Tudor would like to know if the brass seat is not installed is there a tool to resurface the cast part of the carb to make the GAV seat?

Any one?
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Old 12-26-2018, 11:04 PM   #14
larrys40
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Default Re: Carb expert needed

Typically it’s not a problem unless the carb is severely rusted and then that wouldn’t be a favorable core anyway. I’ve not had one that I’ve ever had to resurface the seat because it was that poor. Just my experience. If I have a carb bottom that bad I’d use another one.
I haven’t done it but it may be possible to drill and tap for a replaceable seat . You would have to do some carefull drill work and use a special seat cutting tool to get a good flat tight seat.
Easier to use another carb bottom.
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Old 12-27-2018, 12:08 AM   #15
Al 29Tudor
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Default Re: Carb expert needed

Thanks guys.
I'll try the black marker.
I would still like to re-surface that cast in seat to save another bottom casting if I can..
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Old 12-27-2018, 10:30 AM   #16
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Default Re: Carb expert needed

This is strange but I was going to ask about the replaceable GAV seat in my '29 carb, too. I've inspected that there is a seat down in the bottom and have tried to remove. Is there a procedure for removing this and retapping the threads? I know I will have to drill out the seat if need be but what size tap should I use?
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Old 12-27-2018, 12:40 PM   #17
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Default Re: Carb expert needed

I had the same problem with a hard to turn GAV, turned out that the rear motor mount was cracked and sagging, changed them out and all is well.
Can't hurt to check it out.
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