Anybody test a vacuum cut-out? ’57 Lark

[mas]

Just wondering what spec vacuum PSI is the threshold to activate the vacuum cut-out switch on a ’57 Lark? I’m not having any issues, just trying to kill time in the winter cold. I can easily "bench test" the unit inside where it is warm.

Mas

[frankr]

Interesting question. In the past, I’ve applied suction to see if the switch closes. But I just now went through three factory service manuals to see what the actual vacuum spec is, without success. Somewhere in the back of my mind it looms that the spec isn’t the same for all years (??) Good question.

[frankr]

Moving forward a bunch of years (1972), the manual tells how to test the switch by applying a vacuum, but still doesn’t give a spec. Maybe it is a non-issue, works or doesn’t work? But I’ve seen those diaphragms become rock hard petrified. Maybe that falls into the doesn’t work category.

[jeff-register]

Frank,
If I may interject, What kind of vacuum is generated from the engine in the normal operation? Please educate me as well as what the cutout switch performs in it’s dutys.. I have never applied a vacuum gauge to an outboard…..has anyone performed this test? I would think with two big pistons at 4K would have a substantial reading….or not?

[joecb]

I just did a bit of research on the function of the vacuum cut out switch and was surprised to learn that the switch functions to momentarily cut out one cylinder when the motor tries to overspeed in NEUTRAL during idleing. This in NOT a high speed, in gear wide open throttle cut out. The engine sees highest vacuum at idle when the throttle is closed, lowest vacuum at load and throttle open. However still no info on what vacuum lever closes the switch. I’ll attempt to attach the tech article with full explanation, here…

Joe B

[chris-p]

The cut out is often overlooked. WHile it is rare that they fail, when they do, they are often not thought about, and the owner is left struggling to determine why they are only running on 1 cylinder. I personally can only think of a couple that I have ever had to replace. I do dismantle and inspect during my restoration process however.

To test, I install my vacuum pump (same one I use for testing gearcase). I connect my audible continuity meter between the center post, and the ground. Increase vacuum, and you should eventually get continuity. Of course, if there is continuity without any vacuum at all, it is bad or shorted somehow. If you cannot short it out even with full vacuum, the diaphragm is usually petrified as Frank stated.

I cannot remember what amount of vacuum activates the plunger fully to short out, but I can go down and run one quickly to find out.

[chris-p]

I would think one of their main functions would be to prevent runaway after a shear pin is sheared by shutting down 1 cylinder. (Think hitting a rock, stump or alligator for you florida folks)

[frankr]

If I may add in simple terms to what the book says, it probably has been experienced by most people that have worked on the alternate firing Big Twin series motors, that if you somehow increase the RPM when there is no load on the engine, the darn thing then doesn’t want to slow back down. It is commonly called runaway. This usually happens if you simply advance the throttle too much in neutral. But it can also happen if you break a shear pin or lose a prop. Whatever, or whenever, is is a hair raising experience. The vacuum cut-out switch senses the condition and shorts out one cylinder until things return to normal. I should emphasize that the switch does absolutely nothing under normal conditions.

[mas]

Very interesting indeed. But if the shear pin breaks, the throttle would still be open…would this be enough vacuum to close the cut-out?

Mas

[chris-p]

Yes it is

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