Idle Mixture Screw

[shew01]

I swapped out my carburetor for a rebuilt one, and I’m trying to adjust it. I’m a newbie at this. I’ve been working through the Les Andrews book, but I’m still unsure which direction is more lean and which direction is more rich for the idle mixture screw.

I’m thinking counter clockwise would make the mixture more rich. Do I have this backwards?


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[AL in NY]

shew01: the idle mixture screw is an air bleed adjustment. Opening (CCW) the idle mixture screw allows more air to mix with the fuel and closing (CW) it decreases the amount of air in the fuel mixture, Hope this clears things up.

[shew01]

Originally Posted by AL in NY

shew01: the idle mixture screw is an air bleed adjustment. Opening (CCW) the idle mixture screw allows more air to mix with the fuel and closing (CW) it decreases the amount of air in the fuel mixture, Hope this clears things up.

Thanks. I’ll be really frank. I don’t see much difference between fully closed and three turns open. This carburetor was rebuilt by a well known rebuilder on this forum; so, I’m assuming it was rebuilt correctly. Maybe I’m doing something wrong…


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[P.S.]

Originally Posted by shew01

Thanks. I’ll be really frank. I don’t see much difference between fully closed and three turns open.

That usually indicates a vacuum leak.

[Brentwood Bob]

Or a worn throttle shaft.

[Gary WA]

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Originally Posted by shew01

I swapped out my carburetor for a rebuilt one, and I’m trying to adjust it. I’m a newbie at this. I’ve been working through the Les Andrews book, but I’m still unsure which direction is more lean and which direction is more rich for the idle mixture screw.

I’m thinking counter clockwise would make the mixture more rich. Do I have this backwards?


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If turning all the way in your engine should start to stall out, the same as opening more. adjust till it starts to not run you should be able to hear it and then back out till the same moving in till it runs smooth. Three is way to much, usually 1 1/2 turns out to start.

[Jim/GA]

Originally Posted by shew01

Thanks. I’ll be really frank. I don’t see much difference between fully closed and three turns open. This carburetor was rebuilt by a well known rebuilder on this forum; so, I’m assuming it was rebuilt correctly. Maybe I’m doing something wrong…

Idle mixture should be set with the engine fully warmed up and at normal low idle speed (~400-500 RPM). If you are not seeing any change with changing the mixture screw, your RPM is probably too high.

As soon as the engine RPM gets above about 500 to 600 RPM, the main jets start kicking in and the idle mixture screw doesn't really do anything (but the GAV will).

I hope this helps. I start out with the mixture screw open 1-1/2 turns and make small adjustments from there.

[Gary WA]

Originally Posted by shew01

I swapped out my carburetor for a rebuilt one, and I’m trying to adjust it. I’m a newbie at this. I’ve been working through the Les Andrews book, but I’m still unsure which direction is more lean and which direction is more rich for the idle mixture screw.

I’m thinking counter clockwise would make the mixture more rich. Do I have this backwards?


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Info here,Thanks to supplier of siteSite Index (Model-A.org)

[shew01]

Originally Posted by P.S.

That usually indicates a vacuum leak.

Hmmm… I have a vacuum wiper that never has worked all that great. A vacuum leak could be a possibility.


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[shew01]

Originally Posted by Gary WA

If turning all the way in your engine should start to stall out, the same as opening more. adjust till it starts to not run you should be able to hear it and then back out till the same moving in till it runs smooth. Three is way to much, usually 1 1/2 turns out to start.

Yes, I did start at 1-1/2. I need to check on a possible vacuum leak.


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[shew01]

Originally Posted by Jim/GA

Idle mixture should be set with the engine fully warmed up and at normal low idle speed (~400-500 RPM). If you are not seeing any change with changing the mixture screw, your RPM is probably too high.

As soon as the engine RPM gets above about 500 to 600 RPM, the main jets start kicking in and the idle mixture screw doesn't really do anything (but the GAV will).

I hope this helps. I start out with the mixture screw open 1-1/2 turns and make small adjustments from there.

I’ve had the car for almost 5 years, and the GAV never has done much; the car always ran fine without having to adjust the GAV. (That had me scratching my head because I’ve read quite a number of posts about setting GAV.) The carburetor has been on the car since I bought it until I swapped it for this rebuilt one today. I swapped the carburetor because of a leak while parked—that leak has been growing worse lately.

According to my tachometer, the idle is set around 525. The car wants to stall randomly while stopping when set lower. I’m not sure if that is a float level issue or not. The old carburetor didn’t seem to have that problem.

Yes, I started at 1-1/2 turns out.


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[shew01]

Originally Posted by Gary WA

Info here,Thanks to supplier of siteSite Index (Model-A.org)

Thanks for the link.


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[Big hammer]

My vacuum wiper supply line was a very small copper line, it cracked near the manifold and caused issues. I just removed the fitting into the manifold and plugged it !

[Y-Blockhead]

Originally Posted by shew01

I swapped out my carburetor for a rebuilt one, and I’m trying to adjust it. I’m a newbie at this. I’ve been working through the Les Andrews book, but I’m still unsure which direction is more lean and which direction is more rich for the idle mixture screw.

I’m thinking counter clockwise would make the mixture more rich. Do I have this backwards?


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You don't mention what carburetor you have. A Marvel is the opposite of a Zenith. The idle mixture screw adjusts the amont of gas at idle on a Marvel so turning it CCW will enrichen the mixture.

[shew01]

Originally Posted by Y-Blockhead

You don't mention what carburetor you have. A Marvel is the opposite of a Zenith. The idle mixture screw adjusts the amont of gas at idle on a Marvel so turning it CCW will enrichen the mixture.

Yeah, I missed that. It’s a Zenith. Thanks for pointing that out.


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[shew01]

There may be a slight vacuum leak in the wiper. When I warmed up the engine and idled at 500, I took this video. Is this helpful? This video is with the idle mixture screw 1-1/2 turns open. With the wiper disconnected and the vacuum gauge connected directly to the manifold, I could hear a difference in idle when the idle mixture screw was turned.

https://youtu.be/JeYbABtRKp0


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[shew01]

This is 3 turns open.

https://youtu.be/pzpxulmkFEQ?feature=shared


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[shew01]

The wiper vacuum line itself does not seem to have a leak. I don’t know how to test the wiper body itself. This is 3 turns open measured at the wiper vacuum line at the wiper body.

https://youtube.com/shorts/nW-BxHJIy...NodHCP3DoLur1Q


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[katy]

Originally Posted by shew01

I don’t know how to test the wiper body itself.

Put a length of "wiper hose" on the wiper motor, suck on the other end.

[shew01]

Originally Posted by katy

Put a length of "wiper hose" on the wiper motor, suck on the other end.

Hmmm… That’s a thought.


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