Fuel/air mixture and engine temperature

[Stingray70]

I wanted to start a thread about the effects of fuel air mixture and engine temperature in the Model A. I have read numerous other threads which touch on this. Many contain erroneous info, uneducated opinions presented as fact, and conflicting data.

My background is in aviation so I am very familiar with the use of mixture controls and internal combustion engines. I know we have many engineers, mechanics, and fellow aviators here so I am curious what you all think about this.

I recently had a discussion with a couple of very knowledgeable members of our local Model A club who gave me some suggestions regarding use of the GAV which are counterintuitive to me.

First, one local Model A expert asserted that the GAV only affects mixture at idle. My car certainly acts like the GAV changes the mixture at all speeds. Which is it?

Second, I was told by another to not run the mixture slightly more rich when driving in the hills because it will cause the engine to overheat. I thought a slightly rich of peak egt mixture gave one more power and cooler engine temperature?

Am I wrong in thinking of the GAV as being similar to the mixture control in a light aircraft?

[Gene F]

In my car the GAV definately affects idle. If it didn't affect show RPM, then why are we supposed to open it 1/2 to 1 turn for cold start up even when using the choke?

[alexiskai]

I believe it's the reverse: the GAV affects the fuel/air mixture except at idle.

Per Tom Endy, the GAV "ports fuel direct to the cap jet, bypassing the comp jet and secondary well." When the car is idling, air is pulled in through a vent near the top of the carb rather than through the main intake. This flow route goes nowhere near the cap jet. Thus, adjusting the GAV shouldn't affect idle *if* the car is truly running solely off the idle jet.

It's actually one of the ways I've been told you should be checking your carb function: idle the car and then turn the GAV. If the character of the sound changes, you're not idling low enough.

[CWPASADENA]

STINGRAY 70, You are correct. The GAV does effect the AFR at all speeds except idle.

Also, when pulling a long hill, richening the mixture slightly will increase power a little and lower the coolant temperature, Just like an airplane.

Open the GAV a little when the engine is cold and close it down when it warms up.

Chris W.

[Patrick L.]

Yep, similar to aircraft engines, except for running lean of peak. [ smiley face]

[Mad Mac]

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Changing the GAV definitely affects idle speed on my stock 29RPU. If its too rich it "rolls" until I lean it off.

[whirnot]

Originally Posted by Mad Mac

Changing the GAV definitely affects idle speed on my stock 29RPU. If its too rich it "rolls" until I lean it off.

If that's the case, you are probably idling over 400 rpm, which would introduce the mid range GAV. At correct idle, the GAV is not in use as it does only feed the cap jet.