Gas Adjusting Knob

['31 Patina]

I have a "newbie" question. I'm trying to understand the gas adjusting knob underneath the passenger side dash on my '31 coupe.

How do you know when it's open or closed? The starting procedure says you're supposed to turn the knob full clockwise and then back off 1/4 turn. How do you know when it's closed or open? I can turn my knob clockwise or counter clockwise and it never stops. It just keeps turning and turning either way. Should I be able to turn it counter clockwise and it should eventually stop...meaning it's now closed? Is something not right where it attaches to the carb?

Thanks for the help.

[newshirt]

Check the linkage. It should not turn continuously. Mine has about a 270 degree range of motion.

[Glenn C.]

Turn to right....Closed Turn to left.....Open I usually run about 1/4 turn open.

[Tom Wesenberg]

I've heard that some of the repro knobs can come loose and keep turning. You might need to call Bert's for an original knob and shaft (which is molded together and tight).

[Tom Endy]

The control is known as the GAV knob (gas adjusting valve). It opens and closes a little valve inside the carburetor that ports fuel direct to the cap jet, bypassing the comp jet and secondary well. Its function is to richen the gas mixture. You will get better mileage the more it is closed, however, the more it is closed the leaner is the mixture and you have less power. Most cars run well with it 1\4 to 1\2 turn open. If you are climbing a steep grade and the engine starts to ping one usually retards the spark slightly and opens the GAV a little more. Anything more than a full turn open and the valve is completely open.

The actual setting of the GAV is dependent on how well all the jets have been sized. Each car has its own set point determined by driving experience.

Turn the knob clock wise to close it, counter clock wise to open it.

Tom Endy

[Patrick L.]

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If you open the hood you'll see where[or if] the GAV rod connects to the carburetor.. The connection is made thru the sliding sleeve and spring.. Slide the sleeve back and you'll see how the rod connects to the needle.. There is also linkage for the choke, you pull or push the rod for choke operation..

[Sparky]

It should stop turning in the clockwise direction when the valve is fully closed. If it doesn't, I can think of a few causes:

1. Knob is loose on the shaft, as Tom said

2. The shaft is not connected to the carb correctly (this should be obvious)

3. Threads are stripped on the valve needle or the housing; you may have to replace one or both parts

4. The valve needle is backed out too far and the threads are not engaging the housing at all.

[Brentwood Bob]

You can tighten the GAV too tight, so easy does it. Early and late carbs have a bronze seat and you can bind the GAV if you force it against this bronze seat. Bob

[J Franklin]

Have a helper turn the knob as you watch at the carburetor it should become more obvious what the problem is. You should look at all other aspects of your car in a similar manner to get to know how your model A works. Time spent now will save time and grief later! my thinking is you turned the knob too many turns open and unthreaded the needle from the housing, you only need 1 1/2 to 2 turns open to start. close as it warms up.

['31 Patina]

Thanks everyone for helping me understand how the GAV works and giving me trouble shooting advice. I went out there today and I realized I had turned it too many turns open. So I turned it clockwise and it eventually stopped in the closed position.

Thanks!

[heneste]

I also experienced one of those repro GAV adjustment knobs that would loosen on the shaft. I would get some play in the knob without the shaft moving so I made sure to Loc-Tite the knob to the shaft prior to installing the choke rod in the car.

[msmaron]

I used a bit of JB Weld to stop the turning before i got an original one