GAV Position for Idle Screw Adjustment

J

jeepguy1948

Well-known member
I am unclear about the exact idle adjustment.
My question is: what position should the GAV in when adjusting the idle mixture. I would think that it should be closed but don’t know for certain.
Thanks
 
If your carb is set up correctly, the GAV should have no effect when the engine is on the idle circuit, so its setting doesn’t matter.
 
alexiskai said:
If your carb is set up correctly, the GAV should have no effect when the engine is on the idle circuit, so its setting doesn’t matter.
Click to expand...

The above statement is correct, but let me add to it a bit to really help you out...

Once my engine is warmed up, I usually have the GAV open 1/4 turn, because that is where it runs well out on the road, once warmed up. That is not a bad place to also have the GAV when you are setting the idle RPM and mixture... have the GAV set to where the car runs well when fully warmed up. For your carb, that may be fully closed GAV, or GAV open 1/4 or 1/2 turn.

Set the idle RPM pretty low on the fully warmed up engine, in the 400-500 range, with the spark lever 1/2 way down and adjust the idle mixture there for a smooth idle.

Test that it does not die if the spark lever is pushed up all the way. If it dies, increase the RPM a tiny bit. Then set idle mixture again with spark lever 1/2 way down at that new, slightly higher idle RPM.

Oh, and then to tie back to the quoted post, at this very low RPM, little if any fuel is pulled into the air stream from the main jets, so it does not matter where you have the GAV. At a higher, faster RPM, it would.
 
Last edited:
I'd like to hear thoughts on setting the idle mixture screw using a vacuum gauge attached to the wiper port on the intake manifold. Okay, not okay, waste of time?
 
BButturff said:
I'd like to hear thoughts on setting the idle mixture screw using a vacuum gauge attached to the wiper port on the intake manifold. Okay, not okay, waste of time?
Click to expand...

I have heard of this procedure for many decades, and yet I have never had any luck with it when I've tried it. YMMV.
 
I find from experience that the GAV does have an effect on the starting and idling of a cold engine. I adjust the carburetor with the GAV 1/4 turn open while the engine is at operating temperature.
 
Bob Bidonde said:
I find from experience that the GAV does have an effect on the starting and idling of a cold engine. I adjust the carburetor with the GAV 1/4 turn open while the engine is at operating temperature.
Click to expand...

I would suspect that is because you have the throttle valve (butterfly) partially open when you start your car and are no longer on the idle circuit.

When the engine is warm and idling at the correct RPM, the throttle should be closed (or close to it) and on the idle circuit.

I also run with my GAV open ~¼ turn when warm to smooth out the transition between idle and opening the throttle.
 
Last edited:
BButturff said:
I'd like to hear thoughts on setting the idle mixture screw using a vacuum gauge attached to the wiper port on the intake manifold. Okay, not okay, waste of time?
Click to expand...

On my '55 Ranchwagon with 312" Y-Block with 3 Strombergs, I adjust the timing and idle with a vacuum gauge. Never tried it on a Model A.

I adjust the distributor to give me the highest idle speed and vacuum, then lower the idle with the idle screw on the center carburetor, then repeat.

537822
 
Last edited:
The GAV affects the fuel mixture for starting & idling of a cold engine.
 

Attachments

  • Carburetor - Air-Gas Ratio Adjustment 202kb.jpg
    Carburetor - Air-Gas Ratio Adjustment 202kb.jpg
    67.3 KB · Views: 75
Bob Bidonde said:
The GAV affects the fuel mixture for starting & idling of a cold engine.
Click to expand...

Your diagram is accurate but doesn't make the case you think it does. The idle jet is positioned above the secondary well. It's .021" in diameter. Fuel flow into the secondary well is provided via the comp jet, which is .037" in diameter. The secondary well feeds the cap jet via a small tube. That tube connects to another tube that goes to the GAV needle. The way it normally works is that opening the GAV needle provides a way for fuel to flow directly to the cap jet, bypassing the comp jet.

Now I think what you're imagining here is that this small tube leading off the secondary well could work in reverse – opening the GAV could allow fuel to flow up the tube into the secondary well and feed more gas to the idle jet. The problem with your theory is that the idle jet is smaller than the comp jet. It will never pull more fuel than the comp jet can provide, assuming both jets are properly sized.

So the connection in your diagram between the secondary well and the cap jet is, in practice, a one-way street. This is why the GAV doesn't affect the running of an engine that is solely on the idle circuit; conversely, if the GAV does affect the engine, you are by definition not only on the idle circuit.

hidden%20pass%20cap.gif
 
I believe that’s what’s it’s called, although there are references to discharge tube. Which in a B carb it’s a wide open tube 400 ml per minute.
 
You must log in or register to reply here.
Back
Top