Sunday, September 20, 2020

Carburetor - Adding Ported Vacuum

On my carburetor, there is no ported vacuum for the vacuum advance on the distributor.  Many hot rod folks say to just use manifold, but manifold and ported are actually inverted.  Using manifold vacuum for your vacuum advance would mean that you get MORE vacuum advance on idle than you would on open throttle.  So, I decided to remedy that by installing a nipple on my Holley carburetor fuel metering panel.

You have to make sure that the fuel metering panel supports it, and that the carburetor has the input, too.  Ported vacuum is truthfully the vacuum produced by the venturi effect, which comes from the carburetors venturi (not all carbs have them, but mine as a venturi carburetor does [surprise, surprise]).  It also had the vacuum channels required to support venturi vacuum, so I know I'm good.

I was actually surprised to find that the vent hole where the vacuum nipple attached was not sealed.  Venturi vacuum would pull air directly from the outside.  That was more shocking.  Most folks drill out the metering panel a little bit more and install a 1/8"NPT nipple.  I wanted a more "stock" look, so I measured a few things.

I used some brass hex rod that I had laying around, and drilled it out, then shaped it on the lathe for the nipple that I had on my old carb.

Parting it off, I had a functional vacuum tube.  The fuel-metering-panel end had a taper so that it would be a press fit into the existing hole.

It was then only a matter of pressing it into the fuel metering block and ensuring that it did not cover the hole into the venturi vacuum chamber.

Now to get the carburetor installed.

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