Dual carbs?

[Old Crow]

Hey you guys running 2 carbs on a 6 cyl....what did you do with the distributor vacuum? I was thinking of just running the vac line to the front carb and plugging the port in the rear carb.
What say you?

[Dan in Pasadena]

I'm no expert so others may chime in with a better answer.

The engine doesn't know if its being provided fuel by one carburetor or two. It needs it's ignition timing advanced based on total vacuum so I THINK you'd need to put a T in the vacuum line so vacuum from both carbs is "provided" to the distributor.

[1project2many]

Most guys looking for an advance signal use manifold vacuum from below the carb plate. This is equalized automagically. Ported vacuum from above carb plate can be different bet. carbs. If carbs are synchronized properly they will be set to have throttle angle same based on ported vacuum or carb inlet vacuum readings, which means vacuum line at either carb should give same reading. One connection is fine.

[Old Crow]

I'm of the school that wants a ported vacuum signal, and besides, I've already got power brakes and a PCV valve hooked up to the port on the intake.
I was looking at the carbs after I got the linkage modified and installed. Because of the linkage, I'm not sure I could get a hard line hooked up to the rear carb and make it look halfway decent. The customer is all about hard lines to retain some semblance of stock appearance.
I guess I'll try it with the one carb and if there's a problem change it

[OrrieG]

Are both carbs opening at the same time or progressive linkage? I have always thought to dual or tri carb setups one is used as a primary for off idle and slow crusing around town with the others coming on line as more throttle is applied. In that case use the primary carb (usually the one with a choke) for vaccum.

A few years ago I got a box of carbs at a yard sale just to resell. One was a Rochester 2bbl, 2gc I think, that was missing the choke. I tossed in on CL and a guy called right away. I sold it to him for like $10 as a core He then told me it was a relatively rare one used as the second or third carb on 50's GM tri power setup (he had an Olds J2). It came without a choke.

[Old Crow]

both carbs are identical with chokes and they operate together, not progressively.

[curmudgeon]

I have Tom Langdon's dual progressive Carter/Webers and I tried various combinations, dual ported, manifold vacuum, and the engine ran best with a single ported connection to the distributor. (I am also using the Langdon's HEI as well). Not a stock appearance, but the Carter/Webers replaced the dual Rochesters that I had before (leaky, chokeless PITA stock looking carbs). Hooking up either H2O or exhaust heat to the manifold helps with driveability as well.