The Ford Barn

cigarman · 12-13-2011, 04:44 PM

When trying to adjust the air flow adjusting screw or the GAV, there is no difference in the running of the engine. What could be the cause that nothing changes in the running of the engine when trying to adjust either of the above?

RAH1023 · 12-13-2011, 05:08 PM

Check for a air leak at the throttle shaft or throttle plate.

Jim Powers · 12-13-2011, 05:10 PM

It sounds like an air leak. Check for manifold to block tightness, carb to manifold tightness. Condition of gaskets in both areas. Crack in carb mounting flange / manifold. If you have a vacuum tapping on the manifold, remove the wiper connection or plug and temporarily install a vacuum gauge, it should be pulling a low reading with the engine idling.

Tom Endy · 12-13-2011, 05:16 PM

The idle jet has to be sized correcly. Most of the repos are not sized correctly, as well as some originals. It is best to flow test all four jets used in a Zenith.

You have to have the idle speed adjusted very low to find the high spot of the air\idle adjustment. The butterfly is then almost closed and allowing the air\fuel mixture to be provided mostly from the small hole in the throat of the upper casting. Once you find the high spot of the adjustment you can raise the idle RPM to suit your need.

If you have an air leak at the intake manifold or at the windshield wiper it will affect the air\idle adjustment.

Tom Endy

modela-carb · 12-13-2011, 05:18 PM

The Idle air screw only affects engine performance at idle. It has no effect on acceleration or operating speed, try the following to determine if the screw is defective or not. If it won't adjust as described, replace it. Some repro screws are too short and and screw tight before they close down tight at the bottom.

Before beginning carburetor adjustments close the GAV on the dash, then open ¼ turn. Start the engine as usual and run until the engine is thoroughly warm. Then:

Set the RPM idling screw on the throttle lever to a speed just faster than the point where the engine stalls when the hand throttle on the steering column is all the way up.
Retard the engine by moving the spark control all the way up
Very slowly turn the idle-air thumb screw to the right, or in, until the engine misses or stumbles and mark this position. Then, turn the screw to the left, or out, until the engine misses or stumbles and mark this position. Now adjust the screw about halfway between the marked positions. At this point the engine should be running smoothly and the idle-air screw should be about 1 ½ tuns out from full close.

FINAL ADJUSTMENT

Pull the spark lever down about ¾ of the way and readjust the RPM idling screw on the throttle shaft to the idle speed you prefer. The lowest speed consistent with smooth running is recommended for best fuel economy and performance

TinCup · 12-13-2011, 06:13 PM

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On the two I have rebuilt as stated above the idle screws were too short.

12-13-2011, 04:44 PM	#1
cigarman Junior Member Join Date: May 2011 Location: Monroe, Michigan Posts: 8	Zenith Carburetor djustments When trying to adjust the air flow adjusting screw or the GAV, there is no difference in the running of the engine. What could be the cause that nothing changes in the running of the engine when trying to adjust either of the above?

12-13-2011, 05:08 PM	#2
RAH1023 Junior Member Join Date: Oct 2011 Posts: 2	Re: Zenith Carburetor djustments Check for a air leak at the throttle shaft or throttle plate.

12-13-2011, 05:10 PM	#3
Jim Powers Junior Member Join Date: Jun 2010 Location: East Kingston, NH Posts: 17	Re: Zenith Carburetor djustments It sounds like an air leak. Check for manifold to block tightness, carb to manifold tightness. Condition of gaskets in both areas. Crack in carb mounting flange / manifold. If you have a vacuum tapping on the manifold, remove the wiper connection or plug and temporarily install a vacuum gauge, it should be pulling a low reading with the engine idling.

12-13-2011, 05:16 PM	#4
Tom Endy Senior Member Join Date: May 2010 Location: Southern California Posts: 3,134	Re: Zenith Carburetor djustments The idle jet has to be sized correcly. Most of the repos are not sized correctly, as well as some originals. It is best to flow test all four jets used in a Zenith. You have to have the idle speed adjusted very low to find the high spot of the air\idle adjustment. The butterfly is then almost closed and allowing the air\fuel mixture to be provided mostly from the small hole in the throat of the upper casting. Once you find the high spot of the adjustment you can raise the idle RPM to suit your need. If you have an air leak at the intake manifold or at the windshield wiper it will affect the air\idle adjustment. Tom Endy

12-13-2011, 05:18 PM	#5
modela-carb Member Join Date: Nov 2010 Location: Elizabethton, Tn Posts: 75	Re: Zenith Carburetor djustments The Idle air screw only affects engine performance at idle. It has no effect on acceleration or operating speed, try the following to determine if the screw is defective or not. If it won't adjust as described, replace it. Some repro screws are too short and and screw tight before they close down tight at the bottom. Before beginning carburetor adjustments close the GAV on the dash, then open ¼ turn. Start the engine as usual and run until the engine is thoroughly warm. Then: Set the RPM idling screw on the throttle lever to a speed just faster than the point where the engine stalls when the hand throttle on the steering column is all the way up. Retard the engine by moving the spark control all the way up Very slowly turn the idle-air thumb screw to the right, or in, until the engine misses or stumbles and mark this position. Then, turn the screw to the left, or out, until the engine misses or stumbles and mark this position. Now adjust the screw about halfway between the marked positions. At this point the engine should be running smoothly and the idle-air screw should be about 1 ½ tuns out from full close. FINAL ADJUSTMENT Pull the spark lever down about ¾ of the way and readjust the RPM idling screw on the throttle shaft to the idle speed you prefer. The lowest speed consistent with smooth running is recommended for best fuel economy and performance

12-13-2011, 06:13 PM	#6
TinCup Senior Member Join Date: May 2010 Location: OKC / Tonkawa, Ok. Posts: 1,977	Re: Zenith Carburetor djustments Sponsored Links (Register now to hide all advertisements) On the two I have rebuilt as stated above the idle screws were too short. __________________ Oklahoma City Model A Restorers Group.