Idle Mixture Screw

shew01

shew01

Well-known member
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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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.
 
AL in NY said:
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.
Click to expand...


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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shew01 said:
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?


Sent from my iPhone using Tapatalk
Click to expand...
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.
 
shew01 said:
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…
Click to expand...

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.
 
shew01 said:
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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Click to expand...
Info here,Thanks to supplier of siteSite Index (Model-A.org)
 
Gary WA said:
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.
Click to expand...


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


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Jim/GA said:
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.
Click to expand...


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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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 !
 
shew01 said:
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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Click to expand...

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.
 
Y-Blockhead said:
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.
Click to expand...


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


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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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