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09-09-2022, 07:37 PM | #1 |
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Join Date: Mar 2015
Location: Harpers Ferry, WV
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Carburetor Idle Adjustment
Interesting problem. I purchased a carburetor rebuild kit from Brattons. The old carb was in sad shape with some kind of film on the inside of the bowl, and some light surface crud all around. I sandblasted it and it cleaned up very nice. I used the Brattons carb rebuild guide on their website to clean out all of the hidden passage ways and they all seemed good. Trying out the newly cleaned carburetor wasn't such a bad experience....BUT...I found that I had to completely close the idle adjustment for the car to run. If I open it more than a quarter turn it stalls. The idle is ok...maybe a little rough, and the car seems to run fine at highway speed hitting 50mph without too much difficulty. But I can't adjust that idle setting off of the closed position. I also notice that the GAV adjustment doesn't seem to do anything when I turn it. I've kept it closed and the car seems fine that way. Any ideas what I should be looking for?
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09-09-2022, 08:25 PM | #2 |
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Re: Carburetor Idle Adjustment
In order for a Zenith to operate properly all the passageways must be open. The attached link will show you how to make sure they are open.
The jets must also be properly sized, most coming from the suppliers are usually way off the mark. It the comp jet is not smaller than the cap jet, the GAV control does not have any affect. http://www.santaanitaas.org/wp-conte...assageways.pdf Tom Endy |
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09-09-2022, 09:15 PM | #3 | |
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Re: Carburetor Idle Adjustment
Quote:
Thanks Tom. According to Brattons, their jets are the correct size....they drill them themselves. I'm going to pull the carburetor and go over it once again using your very clearly laid out method. Thanks. Bob |
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09-09-2022, 09:59 PM | #4 |
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Re: Carburetor Idle Adjustment
Twisting the idle adjustment screw in makes the mixture richer. You may have a vacuum leak somewhere. If the GAV is not making any difference then there is a passage clogged.
See https://model-a.org/default.html
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A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past. |
09-10-2022, 07:14 AM | #5 |
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Re: Carburetor Idle Adjustment
nkaminar has it right. Your carburetor is leaking air into the fuel mixture making it very lean. If your carburetor is made of pot metal, it may be warped. If it's a Zenith, look for a sloppy throttle shaft fit, a poorly fitting gasket between the bodies, a missing passage plug, etc. By the way, when you get your carburetor fixed, set the idle mixture with the GAV 1/8 - 1/4 turn open. This enables the GAV to enrich and lean-out the fuel mixture.
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Bob Bidonde |
09-10-2022, 06:16 PM | #6 |
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Re: Carburetor Idle Adjustment
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09-11-2022, 12:18 AM | #7 | |
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Re: Carburetor Idle Adjustment
Quote:
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09-11-2022, 06:56 AM | #8 |
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Re: Carburetor Idle Adjustment
No, turning in makes it richer on Zenith and Tillotson. Turning it in makes it leaner on Marvel. The Zenith and Tillotson designs place the screw in the path of the secondary flow of air, so as you turn it in, you decrease that flow.
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09-11-2022, 07:17 AM | #9 |
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Re: Carburetor Idle Adjustment
Try some spray Deep-Creep thru all the passages and then compressed air.
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09-11-2022, 07:17 AM | #10 |
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Re: Carburetor Idle Adjustment
Here's my practice driving with the GAV:
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09-11-2022, 08:06 AM | #11 |
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Re: Carburetor Idle Adjustment
Another potential area where an air or vacuum leak can occur is where the throttle shaft passes through the throat of the carburetor. Installing bushings and a new throttle shaft can result in substantial improvements in carburetor performance. A large clearance here will seriously affect carburetor performance and make adjustments very difficult.
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09-11-2022, 09:22 AM | #12 | |
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Re: Carburetor Idle Adjustment
Quote:
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09-11-2022, 10:25 AM | #13 |
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Re: Carburetor Idle Adjustment
Here is a document that may help in the restoration of a Zenith.
http://www.santaanitaas.org/wp-conte...etor-11-07.pdf Tom Endy |
09-11-2022, 06:26 PM | #14 | |
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Re: Carburetor Idle Adjustment
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