Idle Mixture Screw

[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-BxHJIylE?si=wYNodHCP3DoLur1Q


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This is the first time I’ve ever used a vacuum gauge as well as the first time I’ve done anything with a Model A carburetor. (The fuel system is not my forte.). Does 45-50 with the idle mixture screw 3 turns out seem reasonable?


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[shew01]

How many miles will it likely take before I should check the plugs for carbon? If it’s rich, will it show up immediately on the plugs? Or will it take a while?


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[Jim/GA]

This is the first time I’ve ever used a vacuum gauge as well as the first time I’ve done anything with a Model A carburetor. (The fuel system is not my forte.). Does 45-50 with the idle mixture screw 3 turns out seem reasonable?


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My vacuum gauge doesn’t go to 50. I think it goes up to about 29 or 30.

What are the units on your vacuum gauge?

 

[Y-Blockhead]

This is the first time I’ve ever used a vacuum gauge as well as the first time I’ve done anything with a Model A carburetor. (The fuel system is not my forte.). Does 45-50 with the idle mixture screw 3 turns out seem reasonable?


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My vacuum gauge doesn’t go to 50. I think it goes up to about 29 or 30.

What are the units on your vacuum gauge?

You need to read the inches of mercury scale (inner scale on your gauge). You have ~20 inHg.

A vacuum gauge is a handy diagnostic tool.

 

[shew01]

You need to read the inch of merury scale (inner scale on your gauge). You have ~20 inHg.

Oh, okay. Like I said, I’m a newbie on fuel systems. Thanks.

With the vacuum gauge attached to the manifold, the engine did want to stall when closed to near zero. I didn’t see signs of stalling at 3 turns. I thought 3 turns might be a bit much; so, I stopped there.

Does 20ish sound reasonable with 3 turns out on the idle mixture screw? Is it reasonable to go more than than 3 turns out?


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[Y-Blockhead]

Oh, okay. Like I said, I’m a newbie on fuel systems. Thanks.

With the vacuum gauge attached to the manifold, the engine did want to stall when closed to near zero. I didn’t see signs of stalling at 3 turns. I thought 3 turns might be a bit much; so, I stopped there.

Does 20ish sound reasonable with 3 turns out on the idle mixture screw? Is it reasonable to go more than than 3 turns out?


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20 inHg is good, If your Idle Mixture Screw is 3 turns out, you may have a vacuum leak somewhere. A lot of good suggestions where to look in previous posts.

Edit: My mind was still on Marvels... disregard above if you have a Zenith.

 

[Big hammer]

When I adjust my zenith carburetor it is after a long drive of 10-20 miles. Engine speed at a low speed as so the GAV has no effect and with the timing level up. IMHO your idle speed sounds high and maybe timing is advanced. Fords owners manual covers how to set the idle speed and how to adjust the carburetor mixture , maybe someone with skills on posting photos can post here ??!!

 

[Fullraceflathead]

With a vacuum reading of 20 that's excellent but 3 turns out on your mixture screw tells me you must be compensating for a vacuum leak.

 

[shew01]

When I adjust my zenith carburetor it is after a long drive of 10-20 miles. Engine speed at a low speed as so the GAV has no effect and with the timing level up. IMHO your idle speed sounds high and maybe timing is advanced. Fords owners manual covers how to set the idle speed and how to adjust the carburetor mixture , maybe someone with skills on posting photos can post here ??!!

Yes, I had the timing advanced. I thought I read that somewhere. I’ll need to read the owners manual again.


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[shew01]

With a vacuum reading of 20 that's excellent but 3 turns out on your mixture screw tells me you must be compensating for a vacuum leak.

I sprayed WD-40 around the obvious joints (carburetor mount, intake manifold vacuum port, intake manifold/block joints) but I didn’t see any change in idle speed.


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[Rob Doe]

X2 on your post #11, Shew01. Stalling at stops has a list of possible causes, but in my brief Model A experience, a proper setting of the base idle air mixture is the first place to start. When one gets down to the details there are differences in the descriptions of how this is done.

Some iteratively slow the idle speed and make air adjustments until an rpm that allows the throttle plate to become nearly closed is reached. This allows idle mixture to be controlled by the idle air screw and the idle jet via the tiny hole in the upper carb intake. Very little air is passing through the venturi, thus the Cap and Main jet cease flowing fuel. Some set the base idle with the GAV fully closed as it has no impact at the nearly closed setting.

Another knowledgeable person suggested having the GAV 1/4 turn open as the procedure is done, as this would allow the GAV to have an affect upon the idle mixture. I assume this method might be used and adjusted at idle rpm settings that allow some fuel to be supplied by the Cap and Main jets as Jim suggested in post #7. There is a transition period / overlap between low idle and low initial road speeds.

I ordered a rebuilt carb from Berts when our car first came to us and installed it. It came with a note that said the carb was tested on a running Model A over a designated test course. The idle speed and air mixture was set at 470 rpm and 25% advance. I monkeyed with the idle air once as an experiment to see if the plugs would burn tan rather than gray, then promptly forgot about making the change. Weeks later the car was stumbling at stop signs. I reset the base idle and the problem disappeared. This is thousands of miles ago.

X2 on post #27. Spark lever full up, engine at operating temps, start at 1- 1/2 turns open, idle speed screw incrementally adjusted down resetting idle air for best position as stated above.

X2 on reading the inner scale of the vacuum gauge. Your gauge is not dampened. On a 4 cylinder the power strokes are too far apart to get stable needle readings. (I learned that here on this forum a couple years ago) Using your first two fingers and a thumb, try pinching it closed and back toward open until your needle calms down.This worked to stabilize the needle for me.

 

[Big hammer]

Yes, I had the timing advanced. I thought I read that somewhere. I’ll need to read the owners manual again.


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Both levers up I just reread the instruction manual, slow idle speed to keep from stalling, idle mixture 1 1/2 to 3 1/2 turns is normal ( I didn’t believe this ) each carburetor maybe different, once you get it set screw in the idle screw until you get a slight increase in speed. Idle mixture 1 1/2 to 3 1/2 turns out is on the next page in the photo of a carburetor ! I couldn’t believe 3 1/2 turns could be normal ! this is in 1931 manual

 

[Ayers1]

Start off by putting a 1/8 pipe plug in the vacuum port on the intake. If your car runs better then you know where to look for a vacuum leak. I use a manual vacuum pump to test the wiper and lines. If it still doesn't adjust properly then you could have a leak where the intake bolts up to the motor. Again this is assuming that whoever built the carb did it correctly.

 

[shew01]

Hmmm… Part of my problem was that the rebuilt carburetor bottomed out on the idle screw setting, and I could not set the idle lower than 550 (or so) RPM.

I had to bend the control shaft to get closer to 500.

I didn’t notice that until yesterday.


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[TomInCologne]

better to put a little pressue on it and use leak detection spray (used for checking gas (natural gas, propane) pipework for leaks. Should be readily available in any diy-store for a small amount of money. Or you use water and washing-up liquid.


Don't put too much pressure on, just a tiny amount is enough...

 

[Rob Doe]

Am I confused? 3.5 turns out on the idle air mixture screw would be allowing additional air into the idle circuit. Wouldn't this compensate for a rich mixture, rather than a lean one????

 

[Gene F]

Ok, 1) idle it down if you can. 2) set the air fuel mix screw to 1/1/2 turn off it's seat (be gentle) 3) open you GAV 1/8 to 1/4. 4) use your wiper ports as a full vacuum port for your vacuum gauge. Set the air fuel mix with the vac gauge. Highest vacuum is what you want. 5) re-check your idle speed screw.

In carburated cars, if you drive around a corner and the car dies the float is too low.

We are however assuming that the inside of your float is dry....

 

[shew01]

Somewhere I read that it’s better to run the carburetor a little rich instead of a little lean. Can someone explain the rationale behind that?


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[Synchro909]

shew01, A lean mixture will burn the valves because of the high temperatures and there will still be some oxygen in the exhaust gasses. A slightly rich mixture means there is no oxygen - it has all been burned.
It doesn't have to be very rich, just a little. The big test is the colour of the spark plugs after a run. The centre electrode should be grey, not black.

 

[SAJ]

Am I confused? 3.5 turns out on the idle air mixture screw would be allowing additional air into the idle circuit. Wouldn't this compensate for a rich mixture, rather than a lean one????

I agree with Rob. Turning the idle air screw out for better idle leans the mixture and does not indicate an inlet air leak. If there was an inlet manifold leak the idle air screw on a zenith would need to be screwed in not out for better idle mix.
SAJ in NZ