View single post by Mark Rosenbaum
 Posted: 10-08-2006 03:58 am
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
That is the vacuum switch, which controls the vacuum applied to the throttle bypass valves.  It's an emissions system, and its purpose is to minimize emissions when the engine speed is high and the throttle is abruptly closed.  When the manifold vacuum is sufficiently high, the switch functions, applying vacuum to the throttle bypass valves, which open to allow a limited amount of fuel/air mix to bypass the closed throttles.

In addition to the above, the system prevents evaporation of the thin layer of fuel which is normally present on the walls of the intake manifold runners.  The absence of this layer results in an excessively lean condition, and may cause engine hesitation or stumble, when the throttle is next opened, and some of the fuel from the new incoming fuel/air mix is deposited on the manifold walls.

The Delta Motorsports catalog lists replacement vacuum switches, but the price is very high and I don't know whether any might remain in stock.  As far as I know, the part was unique to the JH, and no service parts were ever produced (could be wrong on either or both of these, though).  It might be possible to replace a ruptured diaphragm with some local expedient material, and if you discover how to do that, I'd love to know how you did it.  The vacuum switch and throttle bypass valves were discussed in detail in the Stromberg Carburetor Overhaul article I wrote, that was published in the club magazine from March-June 2005.  Contact me directly if you don't have access to those issues.

If I understood your post correctly, you connected the manifold vacuum line directly to the two throttle bypass valves.  This won't hurt anything, but may result in an unstable idle speed and an imprecise throttle -- both highly undesirable during emissions tests.

If the vacuum switch is leaking, I advise sealing off the vacuum hose that feeds it until you can get the switch repaired or replaced.  Since you have a GT, and still have to deal with emissions tests that include a visual inspection, this should be done in a non-obvious manner.

Absent a working vacuum switch, you might be able to partially or completely compensate for the stumble prevention function by using a thicker damper oil, but doing that would also adversely affect acceleration from a partially open throttle.  How much, I don't know.