Gas adjustment valve question

Billmc52

Billmc52

Member
I have a 1928 Phaeton I got from my father in Aug. and put a rebuilt Zenith carb on in Sept. I've driven it for maybe 150 miles since then and the car wuold run best with the GAV open a full turn or more. I've been reading that the GAV should be open about a half turn and then closed down to a quarter or less as the car is driven. At a quarter open the car will hardly run , at half open still runs badly (backfiring, no acceleration, etc) but it idles fine. At 1.5 to 2 turns open it runs much better, accelerates to about 45 but gas mileage is less than 10mpg, maybe around 8mpg. Is there an adjusment to the GAV at the carb to correct this, or do I need another rebuilt carb. I don't have the know how or experience to attempt to rebuld a carb myself. Need help and advice. Thanks,
Bill Mc
 
You probably just have a vacuum leak, maybe between the manifold and engine block. A leak makes it run lean therefore you have to enrich the mixture by opening the GAV.
 
8 to 10 MPG is so bad that I wonder if you have other problems, or a gas leak? Do you use the spark control and have the timing correct? How is the compression?
 
Tom Wesenberg said:
8 to 10 MPG is so bad that I wonder if you have other problems, or a gas leak? Do you use the spark control and have the timing correct? How is the compression?
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Spark control 3/4 to full advanced when running. Timing was checked by local car restorer here last Oct. and said it was fine. Have not checked compression. I'm estimating mpg. Last weekend I put about 45 miles on it and used a little less than half a tank of gas.
 
try contacting your local model A club, i am sure that someone would be willing to help you out and check all the basics plus you'll meet some great people.
 
dave in australia said:
Check the float level, if it's set too high it can cause a constant rich running condition.
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True, but he has a "too lean" condition, that's why he has to open the GAV. Vacuum leak is a common source.

Sometimes when a car just needs a tune up really bad, you can compensate by making the mixture richer and it will run better. So check the plugs, check the condition of the points. Look inside the distributor cap and on the rotor for signs of carbon tracks, cross-firing, that sort of thing.
 
Jim/TX said:
True, but he has a "too lean" condition, that's why he has to open the GAV. Vacuum leak is a common source.

Sometimes when a car just needs a tune up really bad, you can compensate by making the mixture richer and it will run better. So check the plugs, check the condition of the points. Look inside the distributor cap and on the rotor for signs of carbon tracks, cross-firing, that sort of thing.
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Thanks, I'll see if I can get aomeone with the local club to help me go through those things. Need to learn how to do this stuff on my own.
 
Had the same problem with my Murray. Tune up was right and could not find a vacume leak. Main jet in my Zenith 1 turned out to be .025. Way too small. I changed it to .037 and now its OK. To start, I have to open the adjusting needle one quarter turn and quick choke. Then its closed (gently) for great performance and decent mileage. I have the cap jet and the comp jet at .038.
 
I read in one of the many articles or books on carbs that there is a jet that if too lean you need to open the GAV more than normal. It has to be the jet that feeds gas to GAV.
 
Bill Goddard said:
I read in one of the many articles or books on carbs that there is a jet that if too lean you need to open the GAV more than normal. It has to be the jet that feeds gas to GAV.
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Smokey Pickett said:
Had the same problem with my Murray. Tune up was right and could not find a vacume leak. Main jet in my Zenith 1 turned out to be .025. Way too small. I changed it to .037 and now its OK. To start, I have to open the adjusting needle one quarter turn and quick choke. Then its closed (gently) for great performance and decent mileage. I have the cap jet and the comp jet at .038.
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Thanks for both of these suggestions. I have good fuel flow to the carb and thhis may be the problem. This is a locally rebuilt carb and maybe there is a problem with the jet sizes.
 
Billmc52 said:
Thanks for both of these suggestions. I have good fuel flow to the carb and this may be the problem. This is a locally rebuilt carb and maybe there is a problem with the jet sizes.
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It may not be the rebuilder's fault either. A small bit of dirt or crud can find it's way into one of the jets and screw up your running mixture. That happened to my Dad not too long ago. We took the carb bottom off of the top, sprayed some carb cleaner in the jets, shook it out and put it back on. Ran like new again. The carb rebuilder should do this for you. Look for simple things before looking for harder ones.
 
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