B carb

[Lawrie]

I have been setting up a B carb for a friend, it has not idled smooth , , after a full strip and clean , including all the passages, and flow testing all the jets,
I found it was still lean at idle, and needed the idle screw hard in, even after fitting a larger idle jet.
Here is what I found, the idle jet screws into the bottom of the carb and registers into the drilling in the upper housing, there is about .006 clearance between them
, so the air is able to leak around this gap, on one of the spare tops this part has a machined face on it, all the others are just as cast. I fitted a small viton O ring as a trial and ,hey presto a smooth idle and adjustable with the idle screw.
My question , does any one know why they machined the face, almost as if they thought a seal should go here.
Lawrie

[HarleyFLHS]

I have several Zenith B type carbs, but I don't recall any gap there. Are you sure this one wasn't butchered in the past?

[Lawrie]

No, I have about 4 carcases and they are all the same ,except for the one with the face machined.
I will post some pics tomorrow
Lawrie

[HarleyFLHS]

Would like to see that for reference, thanks.

[Marshall V. Daut]

Photos, please?
Marshall

[ursus]

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A couple months ago someone posted here or on the VFF site that the fellow at Renner's Corner recommended adding a seal of this sort to improve the idle on B carbs. The idea was that air leakage around idle jet at the interface of the upper and lower halves was the culprit. Lawrie's findings seem to support this proposition. I plan to test this out in the future.

[Gene F]

Now where is thi9s area in question? Anyone have a diagram, or photo? PLEASE This sounds very interesting.

[Marshall V. Daut]

Although it is often recommended here not to drill out the brass plugs in order to clean out the Zenith passageways, sometimes it IS necessary. For weeks I was fighting a poor-idling B-2 carb with new Renner flow-tested jets, including the idle jet O-ring. No amount of blowing compressed air through orifices or running paper clips through them helped. Finally, I did drill out the plugs only in the upper casting and run small drill bits into the passageways. Maybe my carb is the exception, but the bits kept churning up compacted crud until they bottomed out where most "How to" carb rebuild books said the depths would reach. Reassembling the carb after installing new plugs, it idles 1000 times better, albeit still not perfectly. That's why I would like to see the size of the Viton O-ring Lawrie recommends and where he places it. Maybe that's all I need to make this B carb idle the way I want it to?
Marshall

[Lawrie]

The carb is now fitted to my mate 30 Tudor for a week or so real life testing, I will have it back in my shed then, so will post some detailed picks of the area.
I will also post pics of my permanent fix,
On the tops that have no machined face ,just the as cast finish, I will counterbore and fit an Oring in there.
Nice to know others have had similar issues.
Lawrie

[ursus]

The "machined face" noted by Lawrie is a relief 0.475" wide and 0.040" deep, centered on the idle valve. It only seems to appear on the upper half of later Zenith B-carbs made post Model-B Ford production. I have no idea what intention prompted this production step.

[AL in NY]

I've been adding this o-ring for years to my rebuilt B-Carbs. I have posted it either here or on the "other" forum before. A also remove ALL passage plugs and clean ALL passages during my rebuilds. Attached is a diagram showing the placement of the o-ring (labeled gasket in drawing) which GREATLY improves the idle speed sensitivity of the idle screw and eliminates a lean idle mixture.

[Gene F]

Interesting. Thanks.

I wonder why when we order a B kit why there is no O-ring in there??? In fact, I do not ever recall seeing one offered in the A suppliers catalogues.

[Synchro909]

Now that I have seen AL's diagram, I realise what Lawrie is talking about is what I read somewhere else a while ago. When I put my B carburettor together, I used a piece of plasticine to measure the thickness of the gasket I would need. I cut one out of a piece of neoprene sheet I have and added a small "O" ring to get a little crush. The improvement was immediate and better than I had hoped.
It beats me why Ford did what they did. Surely their carburettor experts would have been onto this issue.

[Gene F]

Guess it needs a nice snug seal to make the suction needed.

[Lawrie]

OK, here are some pics
One of the top with the machined face on
one with another top without the machined face
And one with my fix.
O ring used was a BS008 viton
I also machined a spacer to fill the gap between the O ring and idle jet face
Lawrie

[HarleyFLHS]

Okay thanks for the photos, I will look at all my B carbs again, but I think they are all like the second photo.
The one I am using now will surge up and down at a stop light when warmed up, and the GAV has to be played with, the biggest problem seems to be poor gasoline in California here.

[AL in NY]

You don't need to machine the upper half for an o-ring. Using a small piece of clay, place it on top of the idle jet and assemble the carb. Now disassemble the carb and measure the thickness of the compressed clay. Using that measurement, find a gasket/o-ring combination that is .005" to.010" thicker and place it on top of the idle jet. This will solve the problem.

[Gene F]

So that O-ring seals around which jet?

[Benson]

idle jet

[Synchro909]

Originally Posted by AL in NY

You don't need to machine the upper half for an o-ring. Using a small piece of clay, place it on top of the idle jet and assemble the carb. Now disassemble the carb and measure the thickness of the compressed clay. Using that measurement, find a gasket/o-ring combination that is .005" to.010" thicker and place it on top of the idle jet. This will solve the problem.

Refer to post #13.