Turning the Choke knob

[bartmcneill]

So what does turning the choke knob do? I turned mine all the way to the right and the car did not start. Turned it to the left a couple turns and she fired right up. Is there a best place to leave it, like two full turns from closed?

[TerryH]

Turning the knob controls the air-fuel mixture in the carburetor. Turning it all the way to the right closes it, and most A’s won’t run with it closed. Every car is a little different....most A’s seem to like it 1/4 turn open to the left. Opening it too far will make it run rich. The owner’s manual discusses how to use it. Most folks open it a whole turn when starting, then close it down to about 1/4 after it is running. Every car is a little different, so I would suggest closing it as much as you can while it is still running well.

[Ruth]

Basically it enrichens the mixture the more you open it. I leave mine at ~½ turn open.

[Mike V. Florida]

Great animation here to help visualize what TerryH said,


www.modelabasics.com

[30 Closed Cab PU]

The GAV knob is useful for only idle and very low speeds. When over those speeds internal Carb jets take over, which are not adjustable by the operator.


Turning the knob to the right makes the fuel air mixture leaner, turning to the left makes the fuel air mixture richer.


Typically most As will not start if the GAV is set fully and up to a quarter turn to the left when the motor is cold, the mixture is too lean. Most A require a full to 1/2 turn to the left (is dependent on the car)..

[johnbuckley]

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Originally Posted by 30 Closed Cab PU

The GAV knob is useful for only idle and very low speeds. When over those speeds internal Carb jets take over, which are not adjustable by the....

Whoa! Just the opposite ! (if we're talking the same controls) ....the GAV ( controlled by the knob in the cockpit by the knees ) enriches the cap jet flow which comes into play at above idle speeds. At idle ( below about about 500rpm) only the idle jet is in use - the mix for the idle is adjustable by the small screw at the base of the throat of the carb. The GAV control mix should have barely perceptible effect on idle mix if set up correctly. ( I'm talking warm engines here!- not start up procedure on a cold engine or fast idle when cold)

[30 Closed Cab PU]

If I am incorrect, thanks for correcting me. I want to confirm, Seems I may need to reread/research carb info again. Am not saying you are wrong, just need to reinforce this info, part of my learning process.


Am fairly sure I read a procedure with motor warm, set the idle lever to half speed (cruising RPM) and turn the GAV, and it should not have any effect for 3/4 to 1 turn.



Comments, correct or incorrect?

[rotorwrench]

It's a pita to reach over and adjust it while driving at cruise but I only do it once and leave it at that adjustment until a major change in outside air temperature. When driving distances where altitude changes, I would readjust for every 3000 Ft of altitude change but I seldom drive it that far.

[johnbuckley]

Originally Posted by rotorwrench

It's a pita to reach over and adjust it while driving ...I would just for every 3000 Ft of altitude change but I seldom drive it that far.

Easy adjusting on RHDs! Find that best setting varies with ambient temp, and also with fill ups of different gas . Rarely get above 900ft over here. Best setting for my cars when warm is ususally from zero to 1/2 turn ... all cars different , but i reckon if over 3/4 turn needed on an A the carb or manifold probably has a air leak/weak mix/partially blocked jet somewhere

[100IH]

Mr. John Buckley is correct. look at the fuel flow animation, it is clear that idle is independent and not affected by GAV. This is dependent on what the idle speed is though. If your idle speed is faster, then your idling with fuel from idle circuit and mid range circuits and GAV affects the mix of the latter. Also, the needle screw for idle mixture is controlling only air and the idle circuit is only fuel. Turning the needle screw (idle mixture) in takes air away from the fuel supply making it richer not leaner. Slowing the idle gives a whole new beast where the GAV not involved at all.

[katy]

Does the GAV work the same on Tillotson carbs as it does on Zenith carburetors?

[Ruth]

Originally Posted by johnbuckley

Whoa! Just the opposite ! (if we're talking the same controls) ....the GAV ( controlled by the knob in the cockpit by the knees ) enriches the cap jet flow which comes into play at above idle speeds. At idle ( below about about 500rpm) only the idle jet is in use - the mix for the idle is adjustable by the small screw at the base of the throat of the carb. The GAV control mix should have barely perceptible effect on idle mix if set up correctly. ( I'm talking warm engines here!- not start up procedure on a cold engine or fast idle when cold)

Thanks for catching that, John. I read that last night and it sounded incorrect but I was too sleepy to respond...

There was a discussion in the past about this where someone stated that a Model A should not idle with the GAV closed. The consensus was it should if the idle circuit circuit and speed was adjusted properly.

[duke36]

The different brand carbs (including the Marvel/Allstate) are similar with the GAV method of adjustment, easier to describe for some of us clockwise or ccw rotation. If the needle tips are scored, a varying amount of rotation may be noticed.

[katy]

Originally Posted by duke36

The different brand carbs (including the Marvel/Allstate) are similar with the GAV method of adjustment, easier to describe for some of us clockwise or ccw rotation. If the needle tips are scored, a varying amount of rotation may be noticed.

Thanks for that bit of info.

[johnbuckley]

Originally Posted by katy

Does the GAV work the same on Tillotson carbs as it does on Zenith carburetors?

I find that Tillotsons generally need the GAV open much more - usually about 1 to 1.5 turns.

[30 Closed Cab PU]

Sorry to hijack the thread, after this post will start a new post/string if necessary.
Just looked the link. Missed it 1st time, should learn never to post that late in the day.
www.modelabasics.com
Thanks for your info and patience with me, is appreciated.
Still would like verification of operation, maybe I missed the info and is posted.


When motor is warm, at mid speeds, say 35mph and up, does GAV affect carburation?
Info above seems to indicate no. I do not think no also, but I want to make
sure.


Also, when at full retard, throttle all the way up, setting the GAV the last 1/8 turn to seated affects the idle. From the explanation above GAV should have no effect. So why do I observe this?


GAV seated at idle spark lever up makes my motor run slower but I have never seen the exhaust emit black smoke. Not rich enough to emit smoke?


If the GAV is turned to the right makes it run richer according to the above, why does typically the motor run hotter when GAV adjusted to the right at lower rpms?
My understanding is richer makes the motor run cooler, leaner makes the
motor run hotter.


I am basing this on my idle is set with my motor up to temp, set as low as it can go for even idle without stalling with the mixture screw set for best idle. GAV set 1/4 open

[100IH]

30 cc, !. mid range is anything off idle (700 RPM) up to whenever high speed circuit kicks in, Load is a determinater. So GAV has an effect. 2. The lever position up might not get the throttle plate as closed as is needed for that really slow idle that leaves the GAV out of the picture. Adjust the idle speed screw with the link rod disconnected and make sure there is still spring tension to close the throttle to be pulled closed. . 3. CCW to the left will add more fuel to mid range and high speed, richer but not at idle unless it's a bit too fast. 4. Too lean= hotter burn until lean misfire is created. Then one or more cylinders are dumping unburned fuel. Likely cooling that hole but burning in the exhaust manifold (red hot). Over rich can cause misfire due to fouled plugs otherwise I would expect cooler until it burns in the exhaust. 5. 4 cycle engines are heat engines. A hot combustion chamber is required to be efficient. Tune means efficient. Cooling system just needs to uniformly control heat in surrounding areas. Monitor temp but cold won't do well. Measuring the exhaust manifold might be much more revealing.

[30 Closed Cab PU]

100IH - Thanks for taking the time to provide the details.


Will need time to let this sink in, reread over a period of time to fully understand with referencing the link above.

[bartmcneill]

Thanks guys, I can start it now without flooding it and it starts easily when all is adjusted right. It has been a couple of years between Model A ownership and I forgot a lot. As soon as I sell this Cabriolet I have a coupe and a roadster I am working to buy.