Modifying the Holley 4000 Carburetor for a Modern Distributor

Modifying the vacuum signal of a Holley 4000 is simple with only three details required.

What is the Big Deal?

If you have been following the most recent blog posts you are fully aware that much of the content lately has centered on ignition and carburetion. The most recent article included technical advice concerning the vacuum signal modification of the Holley 94 (and 2100's) two barrel. This customization of the carburetor switches the vacuum signal from venturi to ported vacuum, therefore "working nicely" with modern carburetors. ("Modern" being carburetors that have both vacuum and mechanic advance.) The original setup for mid-50's Ford carburetors mounted on Y Block engine intake manifolds was paired with a vacuum-advance-only disributor. Without going into a lot of technical information, this meant that when compared to a 1957+ carburetor (much like those readily available off the counter in the present day) the Holley 4000 puts out a lot more vacuum at its original advance line port than what is necessary. When the engine speed would increase the vacuum signal would stay quite strong since this was the only means of advancing the breaker plate. Simply put, if you plan to upgrade your distributor and continue to run the Holley 4000 you will need to modify the vacuum signal to ported vacuum.

Preparation and Process

The process that follows is best performed when the carburetor is both completely disassembled and in the middle of a rebuild (or at least a jiffy kit). It is highly recommended to have the carburetor broken down all the way to the base. You will need a Holley power valve plug, a short piece of 1/4" metal tubing, and a plug for the original vacuum advance line. I also use epoxy to help seal the 1/4" tube when making the new port for the vacuum line.

1. Replace the original Holley power valve with power valve plug.

2. Remove the soft metal plug of the corresponding passage. (see photo below)

   
   

   

   
   

   You can remove the plug by drilling it out or using a tap and a threaded screw to pry it out.

3. Remove the small brass stem plug from beneath the base. (see photo below)

4. Ensure the ported vacuum hole is open to the bore at the butterflies. (see photo below)

Although out of focus, the light shining through the hole to the carburetor bore proves success.

5. Using epoxy to help seal the tube, drive a 1/4" piece of metal tube into the bore. (this tube will be the new port for your new vacuum line to the distributor.)

6. Cut off the stem of the brass stem plug that was removed from underneath the base. (This will ensure no interference once reinstalled.)

7. Plug the original threaded port for the distributor advance hard line. This is a 3/8-24 female port, but can either be plugged with a matching set screw or you can tap for 1/8 NPT and plug the hole with a fitting.

8. Use a 1/4" to 3/16" reducer for your vacuum line to your new distributor. (Most of the aftermarket and remanufactured OEM 1957-64 Ford Y Block vacuum advance diaphragm ports are 3/16".)

Credit Due

Much of the content in this article was spurred on by my friend Ted Eaton in Lorena, Texas. His website, Eatonbalancing.com, has a wonderful archive of technical articles for engine balancing, Y Block Ford engine builds, and general engine building advice. The orignal article can be found at this link: Modifying the Holley 4000

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