Zenith and the Gav adjustment On a 31

[old31]

Have a zenith on my 31 that I just can’t figure out the problem.

The carb has been soaked, all passages blown out, all new renner parts, and wire pass through all the holes.

Above 35 the car runs best with the gav turned 2 1/2 to 3 turns. at half a turn at highway speed’s it just doesn’t run right. With the gav turned up that much does not make any difference on idle or low speed.

With my other car that has a zenith it runs best either closed or 1/8 of a turn on the highway. And on start up I usually do a quarter turn.

Any ideas?

[johnbuckley]

Airleak at carb/manifold or manifold/head? Have you tried swapping carbs on the vehicles to confirm it carb or car? half an hour switching carbs may avoid a few hours of head scratching.

[redmodelt]

Sounds like a vacuum leak. Have you done any checking for vacuum leaks? Check and make sure the gasket between intake and carb is in place.

[Brentwood Bob]

Rereading your post I notice you took care to clear all circuits, that looks correct.But, You may try a more thorough cleaning, using air to check each circuit.
John Buckley has a good check list for trouble shooting the zenith. If you would do a search of recent posts on this issue you will locate his ideas.
The surface of the two halves needs to be flush, and true.
The surface of the top of the carb, and the mating surface of the intake need to also be flat, and true.
The manifold gasket likes to fail, or disintegrate, and a new gasket for the carb to intake is required. There are other details covered under vacuum leaks. Try these suggestions first.
I like buckleys approach.
Include an inspection of the throttle shaft. A quick method I have used is removing the shaft, and adding some petroleum resistant grease to each end of the shaft then reinstall and test.
Doing one thing at a time will eliminate guesswork, and locate the reason, that is why it takes as long as it does to really locate the true cause.

[Chuck Sea/Tac]

Good point about the throttle shaft, leaking because of wear. I also use petroleum resistant grease on the shaft.

[bobbader]

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You say that your other car runs fine with what would be considered normal adjustment parameters for the GAV. Why not try swapping carbs from one car to the other. This could help determine if the problem is this particular carb or an issue with a different component (ie: vacuum leak from cracked intake manifold, etc)

Good luck,

Bob Bader

[rocket1]

just a thought,if the gav has a jet might be the wrong one.

[Purdy Swoft]

I have built up worn throttle shafts with solder and filed to fit . This has worked for me on carburetors that new throttle shafts were not available for .

[History]

Make sure the plugs are clean and the gav passage is actually clean. I thought my gav passage was clean but I hadn't even put the clip through it. I finally got it clean and the car kept running poorly, more than before I cleaned the gav passage. I cleaned the plugs and it ran like new.

Your plugs are probably ok but just in case.

[Ed in Maine]

Hello Old 31, your statement that the GAV must be open 2 1/2 to 3 turns would indicate that the Main Jet is undersized or plugged. The Main Jet is fed directly from the bowl. At higher speeds, the GAV feeds gas into the CAP jet as a supplement to the Main Jet. At higher speeds the Secondary Well becomes empty and this turns off the Idle circuit. You should confirm that the (4) jets have been flow tested; Main Jet = 150-160 ml/min., Cap Jet = 150-180 ml/min., Idle Jet = 44-48 ml/min. and Compensator Jet = 138-142 ml/min. When a Zenith has its flow passages checked by wire, soaked and flushed out and rebuilt with flow tested jets, you can't miss having a properly running Model A. One parting thought, make sure the vent to the carb bowl is clear. It is easy to miss. Ed

[johnbuckley]

Good point Ed... we've all assumed the jets are OK as they came from Renners. But even experts get things wrong sometimes !