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Old 03-30-2016, 04:58 PM   #1
woodrow
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Default Poor gas mileage

Recently I posted a question about our '30 coupe and the poor gas mileage. I showed a picture of the Zenith carb, which had an air filter. A suggestion was made that I remove the air filter, which I did, however seems like I'm still getting less than 7 or 8 MPG. Then I realized I was running with the choke rod all the way in, turned clockwise and started running with it about 1 1/2 turn counter clockwise, but the mileage is still very low---about the same as before. Should I turn the choke rod more to counter clockwise or have the carb rebuilt? I'm open to suggestions. The car really runs great and I don't smell gasoline but sure would like to get the mileage to a more reasonable level. Thanks
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Old 03-30-2016, 05:13 PM   #2
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Default Re: Poor gas mileage

How about timing?
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Old 03-30-2016, 05:23 PM   #3
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Default Re: Poor gas mileage

Turning the choke/mixture rod counterclockwise will richen the mixture and reduce the mileage more. Turn the mixture rod in until it seats. Then turn out 1/4 turn for running.
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Old 03-30-2016, 05:24 PM   #4
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Default Re: Poor gas mileage

1 1/2 turn to start. Retighten 1/4 turn to run.
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Old 03-30-2016, 05:44 PM   #5
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Default Re: Poor gas mileage

How long are the trips and how many stops along the way?
Stop lights and stop signs are mileage killers.
How do you check fuel economy?
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Old 03-30-2016, 06:26 PM   #6
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Default Re: Poor gas mileage

7 or 8 miles to the gallon is not normal even with some stop and go driving.
Pull a spark plug and check to see if it's color is black and looks sooty.
If so recalibrate your carb.

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Old 03-30-2016, 06:59 PM   #7
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Default Re: Poor gas mileage

All of the above advice is good, especially timing.

You should also check the compression of each cylinder with a compression tester gauge. I've owned some A's with low compression (due to worn rings or bad valves -- or both) that got poor gas mileage. No amount of fiddling with the carburetor would fix it. You may just have a tired engine.
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Old 03-30-2016, 07:50 PM   #8
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Default Re: Poor gas mileage

Have the four jets in your Zenith been flow tested? If not, that could be your problem. Over the years people have been poking welding tip cleaners through the originals to clean them, and most of the reproductions are not much better. Most are way too big allowing too much fuel to flow. If you have a proper set of jets that are within the flow window, you should set your GAV at somewhere between a quarter and three quarters open.

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Old 03-30-2016, 08:25 PM   #9
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Default Re: Poor gas mileage

Quote:
Originally Posted by 29spcoupe View Post
Turning the choke/mixture rod counterclockwise will richen the mixture and reduce the mileage more. Turn the mixture rod in until it seats. Then turn out 1/4 turn for running.

Well hell...now I know why my gas mileage is crap....I've been turning the GAV in the wrong direction!!!!!!!!
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Old 03-31-2016, 08:26 AM   #10
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Default Re: Poor gas mileage

Thanks for all the good replies to my question. Now I have some checking to do. Thanks again.
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Old 03-31-2016, 09:11 AM   #11
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Default Re: Poor gas mileage

That is half the mileage you should be getting.
At this rate, you should have black smoke pouring out the pipe in a huge cloud, and your plugs should look like blobs of fluffy charcoal.
Something else is wrong besides carb/mixture.
I hafta guess there is a leak somewhere

Also, please describe exactly what math you are using to determine mileage.

Start by pulling a plug, get a pic, and post it. We can go from there
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Old 04-03-2016, 04:19 AM   #12
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Default Re: Poor gas mileage

Few month ago, my car got exactly the same gas mileage but smells gas. I cleaned carb and filter, changed the float value shim and now : #14mpg !

Last edited by Gabalus; 04-03-2016 at 04:30 AM.
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Old 04-03-2016, 06:55 AM   #13
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Default Re: Poor gas mileage

Tune your car with the GAV 1/8 to 1/4 turn open. Also take advantage of modern gasoline by setting the timing more advanced than at TDC where the mark in the camshaft gear will put it. Run with the GAV no more than 1/4 turn open, and the spark lever all the way down.
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Old 04-03-2016, 08:04 AM   #14
Tom Wesenberg
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Default Re: Poor gas mileage

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Originally Posted by Bob Bidonde View Post
Tune your car with the GAV 1/8 to 1/4 turn open. Also take advantage of modern gasoline by setting the timing more advanced than at TDC where the mark in the camshaft gear will put it. Run with the GAV no more than 1/4 turn open, and the spark lever all the way down.
I would never do either. 28* advance is the most I'd use for the stock head, and even less for the higher compression head, like about 23*. I want my babbit to last.
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Old 04-03-2016, 08:13 AM   #15
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Default Re: Poor gas mileage

Quote:
Originally Posted by Tom Wesenberg View Post
I would never do either. 28* advance is the most I'd use for the stock head, and even less for the higher compression head, like about 23*. I want my babbit to last.
You beat me to it. and don't forget about the Pistons and general overall engine health

Last edited by Mitch//pa; 04-03-2016 at 08:27 AM.
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Old 02-03-2019, 11:36 AM   #16
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Default Re: Poor gas mileage

I’m haveing the same problem. About 8 miles per gallon. Going to check the gva
Think I know witch screw that is. But what does GAV stand for????
Woodrow, Did you fix your problem?

Last edited by Richardt2k; 02-03-2019 at 12:08 PM.
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Old 02-03-2019, 11:59 AM   #17
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Default Re: Poor gas mileage

GAV=Gas Adjusting Valve You turn the choke rod to make adjustments.
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Old 02-03-2019, 12:04 PM   #18
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Default Re: Poor gas mileage

Quote:
Originally Posted by Bob C View Post
GAV=Gas Adjusting Valve You turn the choke rod to make adjustments.
Though that was the fuel mixture in the cab. Lean/rich
Been playing with that one. Turn it too far closed and then I lose power. Got it adjusted about 3/4 turn out. Runs tops.
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Old 02-03-2019, 12:06 PM   #19
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Default Re: Poor gas mileage

Bob what does the adjust screw do at the carb??
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Old 02-03-2019, 12:26 PM   #20
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Default Re: Poor gas mileage

How long has it been getting bad mileage? Is your odometer correct?

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