Idle Mix Screw Adjustment Not Change Vacuum - Ford Muscle Forums : Ford Muscle Cars Tech Forum
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post #1 of 7 (permalink) Old 01-07-2017, 02:51 PM Thread Starter
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Idle Mix Screw Adjustment Not Change Vacuum

I am trying to get my idle mix screws adjusted properly on my Autolite 2100 in my 1967 Galaxie 500 390. The car would always die after starting up cold and would hard restart (I would have to hold the choke open and give it some throttle to get it started again).

Anyway, I have a vacuum gauge on the nipple for the distributer vacuum advance on the carburetor. The vacuum gauge won't move from reading around 13psi when in park (seemed to stay around 4-7psi in drive). I can adjust the screws all I want and the gauge will not go up or down.

I had been running the car with the screw 2.5 turns out.

I read that a bad float adjustment or a too high of an idle can cause this issue. I turned the idle down and it didn't solve the problem. When I rebuild the carburetor I set the float at around 7/16" from the top of the bowl just like this:

Can anyone help me understand what is going on and how to get this adjustment set correctly?

Thanks again Ford Muscle Forums!
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post #2 of 7 (permalink) Old 01-07-2017, 03:25 PM
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Re: Idle Mix Screw Adjustment Not Change Vacuum

The distributor vacuum is located under the choke assembly. This source does not show actual manifold vacuum (especially near idle), and is considered "ported" or "timed", to allow retarded distributor ignition timing at idle. The first task is to locate and use a direct manifold vacuum source for your gauge. Some models have taps on the manifold near the front, rear, or plenum side for operating other vacuum devices. Take your new readings, make adjustments, and then post what you find so we can help diagnosis on good vacuum readings.

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post #3 of 7 (permalink) Old 01-08-2017, 05:51 AM
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Re: Idle Mix Screw Adjustment Not Change Vacuum

A good starting point is 1-1/2 turns out.
k4apy and jhayes91 like this.

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post #4 of 7 (permalink) Old 01-08-2017, 07:02 AM
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Re: Idle Mix Screw Adjustment Not Change Vacuum

pull the hose going down to the transmission & hook your gauge to that. I don't use a vac. gauge. I use a tach & adjust the carb screws for the highest rpm.
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post #5 of 7 (permalink) Old 01-08-2017, 12:19 PM
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Re: Idle Mix Screw Adjustment Not Change Vacuum

Whats a good technique to get in there to adjust the driver side screw?

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Putts drives a 1965 Galaxie. Rebuilt 390, 4100 on a Performer, MX, AC, PS, PB w Scarebird front discs on Cragar 17" S/S.
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post #6 of 7 (permalink) Old 01-08-2017, 05:28 PM Thread Starter
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Re: Idle Mix Screw Adjustment Not Change Vacuum

Quote:
Originally Posted by puttster View Post
Whats a good technique to get in there to adjust the driver side screw?
I just use a small 3" flathead screwdriver and reach in over the distributer/spark plug wires.
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post #7 of 7 (permalink) Old 01-08-2017, 08:32 PM
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Re: Idle Mix Screw Adjustment Not Change Vacuum

Stick a short length of small fuel hose on one or both screws, Putts. Adjusting by rpm can work but can couple mechanical advance into the process, faking a good mixture if you're not watching for it. When you re-set your idle speed, you also lose that advance, and the mix is still off. Vacuum gauge reading works even if it's so slight that rpm does not change. Not trying to split hairs, but it depends on the engine and curve in the distributor, your skill, and is partly why I suggest it. Just sayin'. Do your thing.

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