vacuum advance

A better check is to use a timing light and see if the flash point changes when you "goose" the throttle. Remember there is also a centrifugal advance mechanism.

The function of the two advance systems is little understood by most here. I'll explain if you wish.

crushnut said:

Thanks SD, that would be great if you could explain it to me

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Both devices, vacuum and centrifugal, are devices that advance ignition timing in order to achieve maximum engine power and efficiency.

Vacuum advance: a device (usually diaphram) which advances ignition timing in response to increased engine vacuum. For example, when going up a steep hill vacuum drops and the advance RETARDS the timing. The signal to do this comes from a ported vacuum source in the venturi of the carburetor. Once up the hill, vacuum increases and the spark is again advanced.

Centrifugal advance: a mechanical way to advance spark timing by using flyweights in the distributor that advance timing in response to distributor shaft speed increases. Cruising at highway speed, for example, it such a case. When the engine is running with both high vacuum AND at higher speeds, BOTH advances are in operation.

Just why the engine needs spark adjustment under these conditions is another story which require a through understanding of internal combustion engines.

crushnut said:

Ok, i understand what your saying. So a non-functional/poor vacuum advance could account for bad gas mileage, yes?

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Yup, largely because the operator tends to compensate for of the lack of power by using more throttle to compensate. "Pinging" would also be evident. Even the Model T and Model A had an advance on the steeering column. (left side)

Here's a question for you: If you were hand cranking a Model T Ford, would you advance or retard the spark before turning the crank? Why? (Many people learned this the hard way!)

crushnut said:

I would have to say advance, but i know nothing about Model T's.

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I chose the example to illustrate that FUNDAMENTALLY there is not much difference between a Model T and a modern car; they don't get much simpler than a T.

If you were to advance the spark you'd stand a good chance of breaking your arm when the car fires early and reverses the direction of the crank. Ouch!

Like a modern car experiencing detonation (or "pinging") the spark is arriving prematurely with the result that the valves make a horrible noise. Functional advance mechanism "adjust" the timing appropriately and the valves remain quiet. The initial timing is also a factor.

I use an adjustable timing light to check initial timing, vacuum advance and centrifugal advance. My light is made by SunPro and, once I learned how to use it, it became very useful. Having the right tool and knowing how to use it is one of the things that I love about things mechanical.