Model B carb on stock Model A

mcorrell

mcorrell

Senior Member
I am wondering what jets you who have installed a Zenith Model B carb on your stock Model A. I recently rebuilt a Zenith carb that was originally sold as a service replacement for an A as it had the 20 mm venturi. Assuming I need the added flow for a power increase I installed a new 22 mm venturi along with all new jets (not flow tested) that I soldered and drilled to the size specified by Ford for the Model A. My car runs ok but seems a lot lean above 25 mph in 3rd gear. Opening the GAV about 1/2 turn helps a lot.
If I am using the larger B venturi should I also be using the B jet sizes?
 
You will need to enlarge the A manifold to match the increased size of the B carburetor. Or use B intake manifold.
 
I understand that the B carburettor likes the GAV open more than the earlier A carbs so what you are saying makes sense.
 
Same here. I've got a B carb on a stock A engine and it runs best at a full turn open. Stock intake also, so that may explain that? I do have a B intake, but never got around to installing it.
 
Seems like practical experience does not agree with the Ford Service Bulletins
on the B carb GAV. June 1932 Turning the button in a counter clockwise direction enriches the fuel and air mixture. The valve should be turned back (clockwise) as soon as the engine has become warm. Advise owners the car should never be operated with this adjustment open. (Bold letters from service bulletin.)
 
I've never run a B carb, but all carbs mix the air & fuel to attain a certain ratio. This means a bigger carb, of similar design, will need bigger jets. You should be running B jets in the B carb.
 
The cap jet in a B is huge. 362-472mil per min the main jet is smaller. I run my B closed. I can post the actual sizes later today if you want. They are in Gordon Biggers book.
 
Thanks to all who have replied. I did open up the bore of the "A" intake to match the carb's throat.

What I am going through is what you get when you do things over a long time. I rebuilt the carb and resized the aftermarket jets (from Snyders - the jet sizes where all over the place!) to match the information found on the "fordgarage" website (I hope I am not breaking any rules by mentioning their name) about 2 years ago and then installed the carb on the modified intake last Spring (2021). I had drive-ability issues but had some medical issues that kept me from working on it until now. I vaguely remember thinking I should use the smaller jets listed for use on the Model A (with smaller venturi) so that they could be drilled out if needed.
My next step is to confirm adequate fuel flow as I just replaced the leaking original fuel shut-off valve and sediment bowl screen. Once that is confirmed I'll pull the carb, measure the jets, and redrill as needed to get to the Model B factory jet sizes.
Note that the car ran great with a Tillotson X carb, but the carb leaks badly due to warped gasket surfaces and cannot be repaired.
 
A big mistake that some folks make is to assume that the currently available aftermarket jets should be used to replace any original Zenith jets in an old carburetor. I have found that they tend to be much oversized, sometimes wildly so. I keep an eye out for the old Sorensen carb kits (Part # CK-1 for the "A" and CK-2 for the "B"), which typically have numbered jets probably sourced from Zenith itself. Otherwise, good originals or the flow-tested jets are the way to go.
 
Yeah, I did enlarge the new float needle's seat to the match the gravity flow application for the Model A. Once the fuel enters the float bowl the pressure is the same for both the Model A and Model B carbs as the bowls are vented to the atmosphere. The Model B carb must be more efficient which could account for smaller jet sizes.



I disassembled the carb and found that I did size the jets to the Ford spec for the 20mm venturi version. I have drilled out the jets to match the 22mm venturi application and hope that will work.
Unfortunately the temperatures here are just too hot for a test drive. Hopefully it will cool down soon...
 
Another thread (query) I reference Vince's page discussing the B carburetor "variants" used by Ford. https://www.fordgarage.com/pages/zenithBcarb.htm

The differences include venturi size and jet sizes as well as (apparently) "float weights." One imagines one encounters other aspects including the fact that when mounted on an A manifold the B carburetor has a decided "tilt" to the rear.

It is not entirely a "swap" - although one can get lucky.

Joe K
 
I finally got a short test drive in this evening, even though the temp was 84 deg. The correctly sized jets did the trick, though I think she runs best with the GAV open about 1/4 turn. When it cools down a bit I'll do a longer test drive.
Next I'll have to replace the repop 2 core radiator and rebuild the 7 tooth steering box, but those are stories for another day.
 
Anyone who has questions and wants to talk about any of these carburetor topics feel free to call Renner's Corner during normal weekly working hours. 734-428-8424

I'm reading a lot of these comments and so much of this is outdated, folklore, wives tales not true today information. None of this is good to promote and pass on... when working with carburetors you must have facts, data, testing equipment and good parts to start with.
 
Fuel makes a difference. With ethanol in the gasoline the engine will need a slightly richer mixture. The gasoline in 1930 may have been very different than what we get today.
 
Lawrie said:
Are not the runners larger on the B manifold as well
Lawrie
Click to expand...

Lawrie, I can't point to any source for this but I have been told that the runners are the same size as the A manifold. That makes sense to me because they feed into the same sized ports in the block. I'm sure you know that the vertical part of the manifold is a larger ID than the A manifold. That also makes sense because the smaller A carburettor doesn't need it to be any larger.
I have also read that Henry wanted 40 hp from the new engine and sent the designers back to the drawing board when the first effort was well short. I suspect that they made a number of small increments to increase power and once they reached 40, they went no further. A better carburettor and larger bore in the vertical riser would have taken them past 40hp.
If I am wrong about the size of the runners, I look forward to hearing about it.
 
The runners are different ,on the A manifold the ports neck down from where they mount to the ports, the RHD B one I have does not.
Lawrie
 
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