Model B venturi Question

P

PRG999

Member
I have 4 model B carburetors I have been taking apart and rebuilding.
They all have different venture sized openings.
They are marked:
1. .866 with a raised number 4 on the opposite side,
2. .866 with a raised number 3 on the opposite side
3. One marked 22,
4. One wide opened Venturi not marked.

I understand the larger openings were for the B Carb for the B engine and the smaller openings were for B Carbs on the A manifold and engine.
Which one would be the best venture for a A engine with a bored out A manifold?

Thanks
Paul
 
How do you bore out an A manifold? I have a B carb, on a B manifold, because the bore is larger (engine has the vales improved, B/F head, etc) I'd be interested in having my March 28 A manifold modified. I have a double venture Zenith, and the B carb seems like a lot of nothing to me. I'd like to try to put my A manifold and Zenith back on. Bet it's not a thimble full of difference if I can get enough air through the manifold.
 
I have used an adjustable reamer to enlarge the model A intake manifold . Reaming by hand is a slow process . Most have them bored at a machine shop . If the manifold isn't set up just right the boring bar can run out the side and destroy the manifold . I think that the model B carb with the large venturi gives around 3 horsepower if used with the C head , B cam and model B size intake ports . The larger B carb will give a little help on the top end . Don't expect to get a boost that you can feel in the seat of your pants . The B carb has a high speed jet that cuts in at near full throttle . The B carb is more complicated that the model A Zenith carbs .
 
Virtually all Zenith-made B-carb venturies are marked for bore. Zenith was a French company so bores for the Model-B were originally marked as 22 (22mm = 0.866 inches, so those marked 866 are the same size). The venturies used in B-carbs intended as replacement units on the Model-A had a 20mm bore.

B-carb sized Zenith venturies were used in a variety of non-automotive industrial carburetor applications and varied in size from 17mm up to 26mm. These odd sizes turn up occasionally.
 
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