Carter bb1 technical question.

[Trained Monkey]

I have been using a friends bb1 on my series 5 Mercer for the past few years and have recently acquired enough parts to make my own identical carb.  Unfortunately someone "customized" the manifold flange on the upper portion of mine with a grinder.  In the photo, my carb is on the left and my friends on the right.  My friends carb does not have this notch and the gasket seals the small port shut entirely when installed.

My technical question is this...

What is the name of the small port which has been ground into, and how will allowing manifold vacuum to reach it change the operation of the carb?  As a note, there is no governor 

[Bloo]

It's *probably* a critical vacuum signal for enrichment control, and *probably* used a gasket about like this to connect the vacuum signal originally. I even think I see it in the rust shadow on your friend's carburetor. @carbking will know for sure.

 

[Trained Monkey]

That's sort of why I phrased the question the way I did.  The port would apply vacuum to the step up piston lifting it and presumably "shutting" the step up valve in the float bowl.  The gasket that was installed on my friends bb1 had no opening for this when it came to me.  I have never had a chance to street drive my car to know if this would have a significant impact on high speeds (high speed circuit assembly not closed ever) or if it's causing issues in tuning my idle (high speed circuit stuck partially open impacting idle settings).  I'm still very much learning how these carbs work.

[Bloo]

Typically having no vacuum on that port would cause it to run super rich all the time except high speed full throttle. I doubt it could have run well.

 

Edited April 9, 2023 by Bloo (see edit history)

[Akstraw]

I am curious on this subject as well. I hope that Jon will weigh in.

[carbking]

The issue here is that the BB1 is a TYPE of carburetor, not a specific identification.

 

Carter built 68 different type BB1 carburetors.

 

The very first versions did not have the vacuum port as pictured.

 

Soon after the initial production, Carter started placing the vacuum port in this position. As mentioned, it was a source of vacuum for the vacuum piston, which ultimately controlled the power enrichment valve. A slotted gasket (as pictured) allowed engine vacuum to be applied to the piston.

 

However, of the 68 different versions, MOST had truck applications. Many of the truck applications did have governors. For the versions specifically designed from trucks, Carter supplied TWO different mounting gaskets: (1) as pictured, with 4 slots, (2) one with 4 closed holes in place of the slots. The closed hole gasket was to be used with governor applications, where the vacuum source for the vacuum piston would originate from a port in the governor, and pass through the hole to the port for the vacuum piston.

 

Jon

[Gunsmoke]

Appears the second carb has not been modified at all, all you need is the correct gasket as shown above with a "finger" cut out to the small hole and Bob's your uncle. Should be able to make one easily.

[Trained Monkey]

Installed the freshly rebuilt (notched body) carb on the car last night with the idle mix and main needle set identical to my friends carb and was immediately impressed with the difference in tuning ability.  The idle was noticably different and all sputtering issues had disappeared.  It seems like the blocked port was in fact leaving the step up circuit in a sort of limbo state between the ball check spring and the piston's spring.  Thanks for all the input.