Modulator hook up

frickenbored · Sep 16, 2010

I recently swaped in a th400 into my 1988 k5 blazer with a 383. I noticed that my shifts are a little late so I know I have to adjust my vacuum modulator but before I do I wanted to make sure that I have it hooked up to the right vacuum port. Right now its hooked up to the carb which is a holley 650 street avenger:

I've been reading that it should be hooked up to the intake (i have a edlebrock performer intake) but the only vacuum port I can see on the intake is the brake booster port and I don't think I can tee into that because that line is a lot bigger than my modulator line. Will that carb hook up be enough? Also this engine makes less vacuum because of the cam (not sure how much less because I've never tested it)

frickenbored · Sep 16, 2010

actually its a holley 670 street avenger

ericnova72 · Sep 16, 2010

You have it hooked to ported vacuum, it won't work right there. You can put a "T" in the vacuum advance line going to the bottom of the carb(full manifold vacuum) without a problem.

AndersonRacing44 · Sep 16, 2010

It needs to be hooked to manifold vacuum not the ported.

frickenbored · Sep 17, 2010

I actually messed up, that line in the picture is the vacuum advance line coming off the distributor. My modulator line is acually hooked up to a port on the base plate of the carb, I downloaded a 670 manual and found out I have it hooked up to constant full vacuum so its hooked up right. I'm going to adjust the screw counterclockwise and that should solve my problem.

joe_padavano · Sep 17, 2010

You can get a vacuum "tree" that screws into the manifold and has both the large port for the power booster and a small port for the modulator. There are a lot of factory vacuum tap configurations. Take a quick look at any late 1970s car in a wrecking yard.

cobalt327 · Sep 17, 2010

Here are a few that you may find:

joe_padavano · Sep 17, 2010

cobalt327 said:
Here are a few that you may find:

:thumbup:

That's quite a collection.

Modulator hook up

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