Proper GAV valve usage

B

bdentremont

Member
I recently flow tested the carburetor jets in my Mom's 28 Roadster, found them to be a bit rich, and soldered them back down to specifications found on model.org. The car is now running well and now exhibits good response to adjustment of the idle air screw. However, I'm still not sure where the GAV valve ought to be for maximum fuel economy on the road. We've been starting with it about a 1/4 turn open. Between there and fully closed I don't detect a significant change of engine sound or power on the road. We've been getting about 14 mpg in mixed town and country driving with a standard compression head, standard rear end, and non-ethanol fuel.

What signs should I be looking for that indicate proper positioning of the GAV valve? I'd expect that I the GAV valve ought to be openned farther when we are forced to buy E10 or E15 fuel. Does anybody have a good idea how much more is appropriate?
 
I start with the GAV set for best idle. Position varies with different cars. 1/4 turn open seems pretty close for most.Some times I open it a little going up hills or close it for better mileage,but I don't know if it makes a difference.If the engine idles smooth and revs up good,it is set fine.

As for 10% ethanol,I do not notice that it has any effect on my A.I think any gas today is far better than 1930's gas. Others certainly differ in this and think ethanol is the cause of all kinds of problems. See what works for you.

Good Luck,John
 
1/4 turn open is typical, I have noticed different carbs will be more responsive to adjustments than others (closing the GAV on some will stall the car and others will not change at all)
 
From Page 206 of the Service Bulletins:

To Start Engine - Open a full turn

For Warming Up - Should be ½ turn open

As Engine Warms Up - Close off adjustment to suit

For Maximum Economy - Can be turned off

For Traffic Driving and Power - Should be ¼ turn open
 
From Page 208 of the Service Bulletins:

Dash Adjustment-The dash adjustment does not control the entire fuel supply. A minimum amount of fuel is constantly drawn from the float chamber through small fixed openings even when the dash adjustment is fully closed. For best operation under usual driving conditions, the dash adjustment should be backed one-quarter turn off its seat. Running with the adjustment more than one-quarter turn off its seat may be necessary on new stiff engines, but otherwise this will result in poor economy, carbon and crankcase dilution. The dash adjustment may be turned less than one-quarter turn off its seat to obtain a lean mixture suitable for high altitudes, high test fuels, or when driving at steady speeds on level roads. Under normal conditions, however, too lean a mixture causes uneven running at low speeds and slow pickup. Do not force the adjusting needle down on its seat as this will score the parts.
 
Thanks to all. I was afraid running too lean might damage the engine, as I've heard on other cars, but it sounds like I can run it as lean as it runs well.

RonC: I particularly appreciate the quotes from the Service Bulletins.

John in Illinois: I've yet to be convinced that ethanol is the root of all evil and am glad to hear it's worked well for you. While working the bugs out of the car, I've felt it was worth the effort to avoid the potential for extra complications, but it's running well now, so I'll probably try a tank of E10 soon.
 
You must log in or register to reply here.
Back
Top