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Faulty Vacuum Valve  Rating:  Rating
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 Posted: 10-08-2006 12:23 am
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wtberks
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On the inboard Stromberg, there is, what I think is a vacuum check valve.  It sits in line between a vacuum line coming from the intake manifold and two lines leading to each carb.  It is aluminum, round with a black cap on top held in place with a cotter pin.

On my GT I heard a hissing coming from it and felt a small vacuum from the hole in the black cap on top.  I removed a disassembled it and found that the diaphragm inside was old and torn.  Currently I simply removed it, running the line without the check valve. 

So, here are my questions...  First, is it OK to run without this valve?  The engine seems to operate OK.  And Second, is there a rebuild kit or replacement available?

 

Thanks.

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 Posted: 10-08-2006 04:58 am
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Mark Rosenbaum
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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.

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 Posted: 10-08-2006 04:30 pm
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Paul Koehler
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Physical Therapy departments use different gradients of a strechy rubber elastic material, that is given to patients for exercise purposes. I had a piece of the thinest (least resistance ) laying around, and, using the old diaphragm as a pattern, was able to create a replacement. It seems to work fine. Only time will tell if the contact with gas fumes and emissions will deterioate it, but it is easy enough to make a new one, if the need arises. PK 

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 Posted: 10-08-2006 04:41 pm
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wtberks
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Unfortunately Delta no longer has them in stock nor do they carry a rebuild kit.  In fact, they say they never even heard of a rebuild kit for them.  Their suggestion was to simply bypass it.  However, the arguments in favor of keeping it sound valid.  I just haven't driven the car much since bypassing it to see what effect it has.  I have looked at rebuilding it as it really is quite simple.  The only thing stopping me was not having a suitable diaphragm material.  the original almost looks like paper, but obviously is not.  The gas vapors within this system certainly does limit what kinds of material can be used.

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 Posted: 10-09-2006 05:09 pm
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jcdean
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No laughing please.

I have used condoms on numerous occasions to fix magnahelix gauges. Common sense must prevail here.  No ribs, bumps, colors, or lubricants.  The cheaper the better. Slit them down the side, lay out your pattern, cut, and repair.

Joey

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 Posted: 10-09-2006 05:32 pm
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John Kimbrough
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Nothing surprises me any more about the ingenuity used by JH owners to keep their beloved cars on the road.  This is one more great example.  Salute!

John

 

jcdean wrote:
No laughing please.

I have used condoms on numerous occasions to fix magnahelix gauges. Common sense must prevail here.  No ribs, bumps, colors, or lubricants.  The cheaper the better. Slit them down the side, lay out your pattern, cut, and repair.

Joey

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 Posted: 03-30-2007 03:32 am
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s16454
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Here's an idea I'm going to try. I just finished rebuilding the two carbs on my car and I have a good used throttle bypass diaphragm/gasket. I noticed it is very similar to the vacuum switch diaphragm in size and pattern. I'm simply going to drill a hole in the brass center for the rod and trim and/or modify the gasket portion to match the original. Has anyone tried this yet?

Last edited on 03-30-2007 03:33 am by s16454

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 Posted: 03-30-2007 06:37 am
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wtberks
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I finally just bypassed the vacuum valve.  Every thing that I try to use as a gasket fails within a few days.   Good luck and be sure to let us know how it turns out.

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