Zenith idle adjustments

[Sparky]

I rebuilt my Zenith 1 last year and adjusted it according to the instructions in Les Andrews' book and the model-a.org website. The engine runs and idles fine, but I noticed that it idles more smoothly when the GAV is fully closed, and when driving, the engine runs better when the GAV is 1/4 - 1/2 open. I thought the the idle circuit was independent from the main circuit, so the GAV setting should not affect the idle. Is this right?

 

[RonC]

The GAV purpose is to add additional fuel and can affect idle. The operation of the GAV can be affected by several factors as it compensates for the other fuel circuits. Did you flow test your jets and are they original jets? Check your float level too. Your GAV tip and seat may not be at factory spec. as it affects the amount of additional metered fuel when you turn the knob. I think your carb is running a little rich. If it doesn't stall out or run to rough when you stop after running at 1/4-1/2 without closing I wouldn't worry about it.

 

[Chris in CT]

Hi Sparky, You are basically correct, the idle circuit is independent of the CAP and main circuits, and should not be affected by GAV adjustments at idle setting. It all depends on how fussy you want to be. My offhand guess is that the CAP nozzle (jet) is set just a little too low in the venturi and is bleeding off with very little provocation from venturi velocity. As Ron has stated above, too high a float setting will also cause this phenomenon. If the jet is too low, a jet washer or two will raise the jet and eliminate your problem, and lean out the mixture at lower speeds as well. Happy Motoring!

 

[gweilbaker]

Depends where your timing is at:

-When advanced a little, the RPM is higher; closing down throttle to run via the idle circuit, the GAV should not have an effect due the throttle plate being closed. No air movement in the venturi

-When retarded, the RPM is lower; adjusting the throttle open to idle so now it runs on the idle circuit and open plate. Some air movement in venturi, the GAV will have a minimal effect.

The GAV only works when air is drawn through the venturi.

GW

 

[HoarseWhisperer]

Interesting. Of my two Zeniths, one tends to stall when coming to a stop or on a steep decline. It will idle fine with GAV closed on level ground.

The other one will stall with GAV closed at idle, but does not stall when slowing down or on declines.

Jets and air passages have been gone over on both.

 

[Sparky]

Depends where your timing is at:

-When advanced a little, the RPM is higher; closing down throttle to run via the idle circuit, the GAV should not have an effect due the throttle plate being closed. No air movement in the venturi

-When retarded, the RPM is lower; adjusting the throttle open to idle so now it runs on the idle circuit and open plate. Some air movement in venturi, the GAV will have a minimal effect.

The GAV only works when air is drawn through the venturi.

GW

It happens when the advance is at my normal driving position, 1/2 to 3/4 down.

I'm a bit puzzled by the directions on model-a.org, which instruct you to open the GAV one turn before adjusting the idle air screw. Seems incorrect if you don't want the main jet to have an influence on the idle.