Running a Model A Zenith carburetor on a Model B intake manifold

[Joop]

..
Happy new year.

The Model B intake manifold has a larger opening then the opening in a A Zenith carburetor.

So does anyone on the board run a Zenith carburetor on a B engine with B intake manifold.

The reason is that the B carburetor is running very rich so like to try a Model "A" Zenith on it.

Thanks

[Herb Concord Ca]

Not your question but similar. I have run Tillitsons and Marvels on a Model A intake bored to Model B size for 12 years, car runs great.

[Gene F]

How thin does that make the updraft tube on the A intake? Does Bill Stipe or someone (machinist) do that modification? I have a B manifold and carb, but always thought I might like to have the A-manifold machined bigger.

I drive my car, so what are the odds it will bust down the road?

[Herb Concord Ca]

Can't answer your question. Work was done in my company's machine by the shop supervisor, a car guy.

[Bob Bidonde]

Hi Joop! The last time I saw you was at a New England Meet.



Thinking about the physics of the case, the larger volume of the Model "B" intake manifold will likely slow down the flow velocity of the fuel, maybe enough to reduce throttle response.

[AL in NY]

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Do you know the size of the main jet on your Model B carburetor? For a stock engine. it should be a #19 jet. If you have a mildly modified A or B engine, then a #20 jet should work just fine. I would check the jet sizes in the "B carburetor before you change to and "A".

[29spcoupe]

Originally Posted by Gene F

How thin does that make the updraft tube on the A intake? Does Bill Stipe or someone (machinist) do that modification? I have a B manifold and carb, but always thought I might like to have the A-manifold machined bigger.

I drive my car, so what are the odds it will bust down the road?

I have been running a B carburetor on a ported A manifold for 3000 miles with no trouble at all. Don't know how much added hp I have, but the car seems to fly. LOL John

[Gene F]

Where would I buy a #20, Individually I hope?

[AL in NY]

Renner's Corner

[BRENT in 10-uh-C]

Joop, I think you are looking at two different issues.

If the B carburetor is running rich, then maybe repair the B carburetor first. Once you have a properly-functioning B carburetor, that is where the GAV comes into play to optimize the fuel mixture.


While I don't typically use a B manifold on restorations I do (because of aesthetics and the flange's offset mounting holes), I often bore a Model-A manifold to 1.3000" in the draw tube to allow an A manifold to flow better than a B manifold. (A stock Model-B manifold has a 1.250 draw tube bore size.) I have not seen ANY ill-affects in low-end performance, nor any lack of throttle response that would be contributed to the larger draw tubewith an A carburetor. The only thing I have seen is a stock Model-A Zenith with run out of airflow with a B-sized intake at near WOT. The smaller size of the draw tube on an A manifold makes this less noticeable.

Originally Posted by Gene F

How thin does that make the updraft tube on the A intake? Does Bill Stipe or someone (machinist) do that modification? I have a B manifold and carb, but always thought I might like to have the A-manifold machined bigger.

Gene, to answer your question ...yes the Model-A manifold can be modified to match the sizing of a B manifold. The costs of this are not with the actual boring process -but in the setup and mounting. An A manifold is hard to hold to begin with, and they are difficult to indicate the setup because there are not any machined surfaces that are parallel with the draw tube. I typically try to machine 5 or more at a time, and then I go thru my collection to find 5 (-or more) manifolds with the bore size within less than a 0.001" differential. Then I machine a piece of bar stock as a mandrel that has one end turned to 0.750" so I can insert it into a ¾" collet on my mill, then machine the opposite end to be slightly larger than 1.000". This allows me to slide the mandrel up into the tube. This allows the alignment of the manifold to the boring head to be very close where I can bore the draw tube to a little over 1¼" size without going thru the side of the manifold. Not saying it still doesn't happen, but this has worked for me for quite a few years now.

[Joop]

Sorry about this late reply.

Thanks for the great tips and will work on it.

[Terry, NJ]

I was boring out A manifolds for a while, but no more. A few reasons! I didn't have a large enough drill press so I used a friend's Bridgeport. However the friend lives about 60 miles away.Kind of a haul. I was making a locating pin kinda for all them too. I found you can wrap the pin with tap and accomplish the same thing. If the pin and the bore are on different axis, you shim the back of the manifold until they match. The main thing is not see or hear a Click when you hear the pin come up out of the runner. I used the rear jaw of the vice to locate off of. The purpose of all this is to avoid coming out the side of the runner when your drill gets near the bottom of the hole. It's about 4 inches long and should be about 1.281 -1.300 dia. My 1.250 drill cuts slightly oversize. Once it's drilled, you can start polishing the bore. I use 1" 80 grit cylinders backed up with a 1" rubber cylinder. on a long shaft. Get into every place you can with it' Knock down all those casting imperfections. I like to widen the bottom of the bore (Actually the top) at the entrance to the two runners. This aids somewhat the gases enter the runners. Finally! last thing! Take a 1' ballbearing and roll it through, back and forth, for ten minutes. This will knock down all the casting imperfections that you couldn't reach with the polishing. About the finish, This is known as porting and polishing (With out the porting) and there are two schools of thought about the finish. 1 The highest mirror finish you can get! 2 Leaving a even, but with somewhat coarse finish that the molecules of raw gas can bang into and be smashed even finer for a better explosion inside the cylinder. I'm not sure which is better, I want to see some Dyno numbers before I commit to one way or the other. Good Luck! By the way, All my loc. pin, drill, and polishing stuff is for sale if anyone is interested.
Terry