GAV Newbie question

G

Gemboy12

Junior Member
All:
I my father gave me his 31 Deluxe Roadster, I have the starting and driving basics down, but the GAV is still a bit of a mystery to me. Unfortunately dementia has robbed Dad of his ability to help. The various instructions I read say turn ¼ turn to the right for this and ½ a turn for that.My newbie question is how I determine if the GAV is set at 12 O’clock. How do I tell when it is at the starting point for the various turn.
Thank you.
Jim Barringer
 
Turn the GAV initially all the way to the right until it stops. Then turn it 1/4 to 1/2 turn to the LEFT for starting. Turning to the left enrichens the fuel mixture; conversely, turning it to the right leans it out. Ideally - depending upon the carb and condition of the engine - you won't need to drive with the GAV open more than 1/2 turn to the left. However, due to variances in those items, as well as altitude and temperature, you'll have to fine tune the GAV adjustment that best suits the engine's needs. The engine will tell you if it's running too rich or too lean.
I suggest you order a reproduction owner's manual from a national vendor and study it. Most questions about starting and driving a Model A are answered in those few pages.
Marshall
 
If I understand your question correctly, then I think the starting point is all the way closed. And the couple I have been around are in different positions when all the way shut. So 12 o'clock position does not really matter. I opened a half turn to start, and then run it about a quarter turn open when it is warmed up, where the engine sounds best. Hope this helps.
 
Turn the GAV clockwise to close the Gas Adjustment Valve. If you have an original Zenith Carb then typically you open the valve a quarter to half for startup and then as the engine warms up then either fully closed to a quarter open. If you have a Tillitson aftermarket carb then you may have to open the Gav more for startup and may have to run it a quarter to half open depending on the condition of the carb. Every motor and carb are a bit different so you just have to play with it and find your carbs sweet spot. I find too many folks run the GAV more open than they should. If you pull the plugs and find them covered in black soot the you are running the carb too rich. Check out www.modelabasics.com for some great details.
 
And if your father's engine has an air cleaner, take it off!!! That will make the engine run rich unless your father balanced the two carb chambers to equalize the atmospheric pressure between them. Doubtful that he did that. Most guys don't, and they can't understand why their engine runs rich and the plugs soot up. See NUMEROUS discussions on this topic in the archives.
Marshall
 
One more hint to the list:

GAV usage additional:

If it is cold weather (usually under 40-50 F - each engine is different) AND the engine starts and promptly dies, increase the GAV an additional 1/2 turn (about 1 turn total).

Now engine will stay running. Close GAV down to 1/4 gradually while engine warms up.

Normally GAV needs to be 1/2 to 3/4 for a cold start.

Normal GAV position after warmup was designed to be 1/4 open at sea level.

An additional 1/4 turn while climbing a long steep hill will give additional power. Return to 1/4 after clearing the top of hill.

Henry's idea of 1/4 setting is that you can lean out mixture at higher altitudes with the GAV if needed. As an example I run from 5500 feet to 12,000.

Sometimes I need to adjust the GAV leaner (maybe 1/8th to closed) to suit conditions above 8,000 feet.
 
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Jim,
Close the GAV, IF it starts & runs well, just forget it. Minerva ran well with it closed, except in REALLY cold weather.
Chief, my Dad, taught me, "With a warm engine, run at a constant speed of 20 to 25 MPH, in second gear & adjust the GAV for best, smoothest running & DON'T diddle with it anymore"!
Remember, the GAV is just an extra jet that adds fuel to the MAIN jet, when it seems to be too LEAN.
AND, if the GAV affects your idle, it's idling TOO FAST.
Bill W.
 
The GAV (gas adjust valve) controls the amount of extra fuel going to the cap jet. The comp jet (the little button jet in the fuel reservoir) ports fuel into the secondary well where the cap jet gets its fuel. The comp jet is much smaller than the cap jet. By opening the GAV fuel is ported around the comp jet and adds to the fuel in the secondary well.

The positioning of the GAV has to do with the sizing of the comp jet and the cap jet. Many Zeniths on the road have had the jets molested with welding tip cleaners such that they are way out of spec. Many reproduction jets are out of spec to begin with. If the comp jet is larger than the cap jet the GAV has no effect at all.

A Zenith with properly sized jets will operate best with the GAV a quarter to a half turn open.

Tom Endy
 
Claude, (RIP) our REAL GURU, said that many Zeniths vary & you just have to feel out what yours' needs.
Main thing is to clear up mysteries of what the GAV is, & what it does.
Each car is sorta' like your Dog, you have to lurn their PERSONALITIES, UR them Dudes'll BITE YOU!!!
Bill Bleeding
 
Glad I searched GAV posts. I'm also a Model A newbie, with similar frustration sorting out my GAV usage. Thanks to all for helping better understand how to properly use my GAV, driving at elevation 2600. My 31 Pickup thanks you as well.

Billy
 
It should be fine between 1/4 and 1/2 turn, then leave it alone If you need a cold weather start, you may have to open a bit more for the start. You don’t have to keep playing with it.
 
One of the first things I do to a new to me Model A is rebuild or add a new Zenith carburetor.

1/2 to 3/4 turn to open for cold start. Than 1/4 turn open to run. Any thing less seems to cause the engine to hesitate when accelerating.
 
Also, when you are driving at 50-55 mph, you may need to open the GAV to the 1/2 turn level to enrich the fuel at the higher speed.
 
Great advice guys. That's why we newbies read Ford Barn, best explanation of the GAV I have read.
Gerry, Birch Bay WA
 
Hi folks, does nobody of you use the GAV adjustment for inner engine cooling in high load low RPM situations?


I am climbing steep hills that way and it works pretty fine. The overrich mixture is cooling the cylinder walls and prevents overheating of the engine. In these cases I open the valve one complete turn or more. Works fine to my engine on 16% steep blackwood forest roads..
 
Gerald1961 said:
Hi folks, does nobody of you use the GAV adjustment for inner engine cooling in high load low RPM situations?


I am climbing steep hills that way and it works pretty fine. The overrich mixture is cooling the cylinder walls and prevents overheating of the engine. In these cases I open the valve one complete turn or more. Works fine to my engine on 16% steep blackwood forest roads..
Click to expand...

I'd change down a gear rather than flog the engine that hard. Too rich of a mixture will at best dilute and maybe remove the oil from the cylinder walls leaving them very prone to wear, especially when the motor is working so hard. In the lower gear, you'll need less throttle anyway so the engine won't over heat
 
The manual seems to negate posts 15 and 18...

Regulating Gasoline Mixture
For economical driving, reduce the quantity of gasoline in the
mixture by turning the adjusting rod to the right as far as
possible without affecting the operation of the engine. This is
particularly true when taking long drives where conditions
permit a fair rate of speed being maintained, and accounts for
the excellent gasoline mileage obtained by good drivers.
Turning the carburetor adjustment too far to the left results
in a "rich mixture." Such a mixture has too much gasoline and
should be used for starting and warming up only. Running with
too rich a mixture causes excessive carbon and overheating,
likewise it wastes fuel.

Adjustment ol Carburetor
The method of regulating the carburetor for ordinary driving conditions is to tum the carburetor adjusting rod to the right
until the needle just seats, then turn the rod back approximately
one-fourth of a tum.
 
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