Air Fuel Mixture Adjustment

Rex Smith

Rex Smith

Member
Still messing with carb issues, but making some headway. The issue now is that no matter where I set the air/fuel mixture from inside the cab, there's no difference. Engine still runs even when it's fully closed.
The needle valve looks to be in good shape. Perhaps the seat is worn or something is out of spec? Not sure where to go from here.
thanks
 
Rex, can you clarify which needle you are referring to? The statement "air/fuel mixture from inside the cab" has me confused. Are you asking about the air/fuel mixture screw or the GAV?
 
Sorry, I'm referring to the GAV. It doesn't seem to want to seat, nor does it have any effect on the operation of the engine. I start with it out one turn. When I turn it in clockwise, nothing, even when it's bottomed out.
 
If the cap jet is drilled out too large, the GAV valve will have no effect. That is why it is important to have all the jets flow tested. The GAV does not have to seat because you normally run with it open from half a turn to three quarters of a turn.

Tom Endy
 
Make sure that the serrated knob on the choke shaft is not spinning without turning the GAV. The knob on some repo choke rods can come unscrewed and while you think you are turning the choke rod, you are only turning the knob. Either have someone turn the choke rod while you watch the carb to see if the choke rod is turning, or from the passenger compartment, pinch hold the rod beneath the gas tank and turn the knob. You should feel the rod turn between your fingers as you turn the knob.
If the choke rod acts properly, a loose knob is not the problem. Time to move on to Plan B.
Marshall
 
Unless it's in a former post, you have not given a lot of info on the proceedings so far. While I can agree with Tom's suggestion, it could also be that your lower carb body is the 28/29 style that has a removable seat for the GAV needle. If this is missing, the needle will never seat and fuel flow to the cap jet will be massive. Then, there have been several different manufacturers of both the GAV needle and GAV housing over the years. The needles vary in length and threaded area length. SO, in some cases, even if the lower seat in the body is good, the needle will not seat. Are the parts you're working with new or used? Do you have an extra carb so you could interchange these parts from one to the other? Are you missing that removable GAV needle seat in the housing?
 
All good things to look for. I have two carbs, both of unknown origin. They came with the two A's I purchased long ago. Both have worked well in the past, but they sat too long with old gas in them and now I'm trying to get them going again. I'm learning as I'm going.
thanks
 
Tom Endy said:
If the cap jet is drilled out too large, the GAV valve will have no effect. That is why it is important to have all the jets flow tested. The GAV does not have to seat because you normally run with it open from half a turn to three quarters of a turn.

Tom Endy
Click to expand...

Tom, I respectfully disagree...... The cap jet can be huge flowing 300 cc or more, it's just the pathway/tube that connects the idle well and GAV well to the venturi. If you look at the model B carburetor, they eliminated the orifice in the cap jet all together because it really was just limiting / restricting what the GAV could provide. If you flow test an original model A cap jet you'll see that it can only flow about 170 cc and the compensator jet that feeds the idle well flows about 150 so really opening the GAV one full turn can only add about 20 cc of extra fuel, which is not enough in a lot of cases.
I also always start off recommending that you try and drive your car with a GAV closed as it was designed that way and only open the GAV enough to provide extra fuel as needed for the current conditions. Running around with the GAV half to 1 turn open is a red flag to me that you have a vacuum leak as no engine needs that much extra fuel typically. The main and compensator jets would have to be severely under sized to require that much extra fuel. Max flow is achieved through the GAV orifice at one turn open it is pointless to open it more than one turn. The GAV is a high speed adjustment active only above 600 rpm. It should have absolutely no effect at idle rpm's, 600 and less. David ( I would gladly discuss this carburetor theory with anyone who is interested, feel free to call me. )
 
Rex Smith said:
All good things to look for. I have two carbs, both of unknown origin. They came with the two A's I purchased long ago. Both have worked well in the past, but they sat too long with old gas in them and now I'm trying to get them going again. I'm learning as I'm going.
thanks
Click to expand...

Rex, check out my post in this thread about the GAV. Feel free to call me if you'd like to run through some carburetor theory of operation. David - RENNER'S CORNER
 
RENNERS CORNER said:
I also always start off recommending that you try and drive your car with a GAV closed as it was designed that way and only open the GAV enough to provide extra fuel as needed for the current conditions.
Click to expand...

Would you recommend this for the Marvel carbs as well? I assume so but wanted to check. I run my Marvel with the GAV open 1/4 turn on startup to help the engine power through the high-torque upshifts needed to get on the main roads, but I close it once it's warm.
 
alexiskai said:
Would you recommend this for the Marvel carbs as well? I assume so but wanted to check. I run my Marvel with the GAV open 1/4 turn on startup to help the engine power through the high-torque upshifts needed to get on the main roads, but I close it once it's warm.
Click to expand...

Yes, the Model A and B Zenith, the Marvel Schebler and the Tillotson were all Ford released with Ford part number carburetors all specked out to operate and perform the same.
 
Dave, you are a wealth of knowledge on carburetors. Thank you very much for sharing.

Over the past few years I have spoken with you and I learn something every time.
 
RENNERS CORNER said:
Tom, I respectfully disagree...... The cap jet can be huge flowing 300 cc or more, it's just the pathway/tube that connects the idle well and GAV well to the venturi. If you look at the model B carburetor, they eliminated the orifice in the cap jet all together because it really was just limiting / restricting what the GAV could provide. If you flow test an original model A cap jet you'll see that it can only flow about 170 cc and the compensator jet that feeds the idle well flows about 150 so really opening the GAV one full turn can only add about 20 cc of extra fuel, which is not enough in a lot of cases.
I also always start off recommending that you try and drive your car with a GAV closed as it was designed that way and only open the GAV enough to provide extra fuel as needed for the current conditions. Running around with the GAV half to 1 turn open is a red flag to me that you have a vacuum leak as no engine needs that much extra fuel typically. The main and compensator jets would have to be severely under sized to require that much extra fuel. Max flow is achieved through the GAV orifice at one turn open it is pointless to open it more than one turn. The GAV is a high speed adjustment active only above 600 rpm. It should have absolutely no effect at idle rpm's, 600 and less. David ( I would gladly discuss this carburetor theory with anyone who is interested, feel free to call me. )
Click to expand...


You are absolutely correct. My apologies. I identified the wrong jet. Memory is hazy, I should have referred to my own sketch. What I meant to say, if the comp jet is drilled out the same or larger than the cap jet, the GAV has no control. The sketch is attached.

https://www.santaanitaas.org/wp-content/uploads/2023/11/Zenith-Passageway-Diagram.pdf

Tom Endy
 
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