Vacuum canister hooked up to throttle linkage 1957

[wearymicrobe]

This is weird, been chasing down a vacuum leak in the manifold of my 57 de ville. Near the firewall and at the top of the throttle pedal linkage there is a weird looking vacuum canister.

There is an idle adjust screw that touches the throw on the canister as well. But when you put vacuum on it with the mitty it retracts out of the way and I have no idea why.

I was going to pull the whole thing out as there are leaks everywhere because of it. Any idea what this does.

[wbdeford]

That exists to prevent the throttle from closing too quickly and stalling the engine when you suddenly release the accelerator.  It is called the "Throttle Check" in the shop manual.

[TJ Hopland]

Does it have a vacuum line attached to it?

[wearymicrobe]

That exists to prevent the throttle from closing too quickly and stalling the engine when you suddenly release the accelerator.  It is called the "Throttle Check" in the shop manual.

That makes sense. It works just fine on the vacuum but he adjustment knob is way way way out of wack then its not even touching with no vacuum so its not stopping anything.

Looks like the PO just backed it off and used the CARB butterflies as the idle setting which is odd. Never had any idea with decel but man do I have leaks downstream of that in the wiper system so its all cut out now. Just going to us it as the stop

Finally got my idle set right though, and got the mitty gauge out to set the idle screws running much much better now.

New rule, never trust the PO ever.

[wbdeford]

A '57 slow idles with the throttle plates completely closed.  There is an air bypass valve + the two mixtures screws that are used to set the idle speed....in theory. 

On my '58, there is enough wear in my throttle shaft bushings that even with the air bypass valve completely closed, enough air gets by the throttle plates for my car to run slightly above spec idle speed.  Eventually, I will have to have to get those bushings replaced. 

"never trust the PO"....yes, BUT, dumb things the PO did were often to mask a real problem and so when you put it back the way it is supposed to be, it is likely you will have a "new" problem.

[Jon S]

DO NOT TURN the screw adjustment - It will break!  It is an anti-stall solenoid known as a dashpot (vacuum controlled).  If you open the throttle in Park and release the linkage, the plunger should extend out and then retract and bring the throttle linkage back to idle slowly.

[TJ Hopland]

Is this a Carter AFB?  Rochester?

[Jon S]

Doesn't matter. They both use dashpots.

[TJ Hopland]

I was more interested in the idle bypass valve thing.

[wbdeford]

TJ, I was describing the Rochester 4GC system.  The '57 manual does not tell you anything about the single Carter AFB (though it does tell you they exist and are used on some late-year cars.).  The only Carters it addresses are the dual WCFBs, which have an idle by-pass as the throttle plates are completely closed at slow idle.  The '58 shop manual supplement describes the AFB and it also uses an idle bypass system, so I would guess '57s do too.

[Jon S]

The idle bypass screw on the 4GC does the same function as the air horn screw on the AFB.

[wearymicrobe]

This is a Carter AFB factory unit. Amazingly the throttle check valve actually was still holding vacuum so I was able to get it all hooked back up and working reasonable well. You can absolutely adjust the adjustment screw but it took a a torch and some kroll to get it loosened up. Reset the idle and got it tuned back up. Much, much better now. Did have to remove the reset of the vacuum lines to the wipers and a few other things to keep the system sealed up to the manifold as well as I would like. 

[Jon S]

You were lucky. Those dashpot screws normally break when adjusted.