Choke Adjustment Needle?

Houston Rodder

Houston Rodder

Member
By turning the choke tube in the cab of my A counter clock wise, am I adding air or gas to the carb? In other words, am I leaning out the carb by opening up the adjustment or richening the mixture with more fuel? In the attached picture I circled the adjustment needle in question.
Thanks
 

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Keep turning the valve CCW and it should run rougher due to too rich a mixture.

Turn the valve CW and it should slow and run rougher due to too lean a mixture.

Mine runs about the same 1/4 open or closed, so I just leave it closed except during warm up.
 
When assembling the carb b y the instructions, they say to bottom out the needle and back it off 1 and 1/2 turns. That would be too rich, correct?
 
Houston Rodder said:
When assembling the carb b y the instructions, they say to bottom out the needle and back it off 1 and 1/2 turns. That would be too rich, correct?
Click to expand...

That's where I turn the knob for cold starts, but I close mine after a minute of running. Most people leave the valve 1/4 turn open.
 
i run 1/4 open at all times and never really touch it... sometimes at cold start up you can give it more then put it back. all this depends on a properly functioning carb so your results may vary...
to ck mixture,
ck spark plugs for proper burn , a good nose can smell it or a gas analyzer
 
If you're running too rich, you'll most likely get black smoke out the tailpipe and possibly increased engine heat. Too lean and you could get backfiring and increased engine heat and lack of power.

Turning the GAV clockwise will screw the adjusting needle into the seat in the carburetor and restrict the Gas flow to the carb (leaning out). Turning it counter-clockwise will open up the passage and allow more gas into the carb (richening the mixture).
 
Bob C is correct in post number 10 The idle air adjust screw on the upper housing adjustment starts at one and one half turns off the seat. The model B carb runs with the GAV on the choke rod closed, all the way gently to the right after warm up. I usually open the GAV 1/2 turn to the left with a B carb when cold . After it warms a bit the engine will begin to lope and I then turn the gav knob all the way to the right. The model A zenith carbs usually run best with the GAV open 1/4 turn to the left , off the seat.
 
No needle valve will function as designed unless it closes at full CW rotation, and what's more frustrating than an "adjustment" screw or knob that doesn't adjust anything? So... I have four observations to add:

(1) Make sure that both the GAV needle assembly and the idle mixture screw are the correct length and actually do bottom out in the seat (I've seen versions of both that were so short that they "bottomed out" in the threads long before the needle hit the seat.)

(2) Some repro idle mixture screw springs are too long, and also prevent the needle from seating. Try the screw without the spring, see how much space you have for the compressed spring, and cut the spring to fit. Oh, and make sure that the spring collar (ferrule?) is not too deep, as well.

(3) Make sure that both brass needles are polished, true, and free of scoring.

(4) Make sure that both needle seats are clean and smooth.

Running these checks is a fun, easy, and rewarding workbench job. Get it right and you're good to go for your lifetime.
 
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