Carburetor Idle Adjustment

[ampico-kid]

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?

[Tom Endy]

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

[ampico-kid]

Originally Posted by Tom Endy

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

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

[nkaminar]

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

[Bob Bidonde]

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.

[ampico-kid]

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Thanks for all of the advice and information. I see I've got some work ahead of me. Assuming I find the issue, I'll report back and let you all know what I found. Thanks.

[old31]

Originally Posted by nkaminar

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

Aren't you making the fuel mixture leaner by turning it in (clockwise)? By turning it out you are adding more fuel, making it richer, I think.

[alexiskai]

Originally Posted by old31

Aren't you making the fuel mixture leaner by turning it in (clockwise)? By turning it out you are adding more fuel, making it richer, I think.

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.

[Patrick L.]

Try some spray Deep-Creep thru all the passages and then compressed air.

[Bob Bidonde]

Here's my practice driving with the GAV:

[CT Jack]

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.

[ampico-kid]

Originally Posted by CT Jack

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.

Yes, I hear you and have already replaced the throttle plate shaft. Seems that the shaft was worn much more than the hole it passes through. It's got a relatively airtight fit now. Thanks.

[Tom Endy]

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

[ampico-kid]

Originally Posted by Tom Endy

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

Thanks Tom. That document was my guide when I was going over my carburetor. I removed my carb again today and dismantled it. I went through ALL of the passage ways again, and everything is clear. About the only issue I could find was the float height was off a bit. I had to use a shim to get it to the right level. I haven't tested it yet (bad weather today). Hopefully we'll have a break in the rain soon and I'll be able to road test it. Thank again.