What does it do?

G`day sam-d

I`d suggest you have three seperate unrelated problems .

I`d also suggest you take note of what BigJon has typed as i agree with his words and would add that it does have an effect mid-range and also at part throttle pulling up hills etc .

Cheers

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Originally Posted by BigJon

The only time the vacuum advance unit does anything is at light throttle cruise. It then advances the ignition timing to give better economy. Opening the throttle to get more power will give less vacuum, and therefore less vacuum advance.

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To clarify further, the vacuum is usually taken slightly upstream (atmosphere) side of the carburettor butterfly/throttle plate. This way, there is no vacuum advance at idle when there is maximum vacuum in the manifold. Vacuum kicks in just off idle as BigJon stated.

Hope this helps.;)

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Originally Posted by Pedro_The_Swift

AFAIK, the vacuum advance, does, or should do exactly what it says,,
as revs increase the "vacuum advance" pulls on the distributer, increasing the sparks angle of advance.

basically more revs, more advance on the distributer,,

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That's controlled by the mechanical advance mechanism which are bob-weights moved outward by centrifugal force (or is that centripetal force?).

One can have high vacuum at low revs, e.g. on a trailing throttle, and low vacuum at full throttle. Have you ever driven a car with vacuum operated windscreen wipers and noticd how they slow down when the throttle is opened wide and speed up when the throttle is closed?

Ron

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Originally Posted by p38arover

That's controlled by the mechanical advance mechanism which are bob-weights moved outward by centrifugal force (or is that centripetal force?).

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Centrifugal force doesn't exist. It is centripetal force that creates the sensation of being forced outwards. Circular motion does not have a force outwards from the center of the circular path.

Physics lesson over!:D

sorry, shorty but Big John is closer than you in what hes saying but hes not quite all the way there when saying it has no effect other than at high cruise....

Insofar as spark timing is concerned hes close enough however....

Depending on how your engine is plumbed for its vac lines if you have a split advance unit you may loose all signal vacume to other parts of the engine (fuel mix controls, EGR and a couple of other minor ones)

All of these things combined will seriously hurt your economy and power...

Woops, sorry Jon. You are right about vacuum.
Low revs max vac, but no effect on ignition at idle.
I'd also damned well forgotten about the bobweights.

Been a while had to double check myself. Thank's Dave.

How about this.
From the Bosch book on dual advance distributors.

A dual advance distributor has two independently operated spark advance systems. A centrifugal advance mechanism is located below the stationary sub-plate assembly and a vacuum operated spark control diaphragm located on the side of the distributor.
As speed increases, the centrifugal weights cause the cam to advance, or move ahead with respect to the distributor drive shaft.
The rate of advance being controlled by "rated" springs.
Vacuum advance mechanism is a spring loaded diaphragm, connected via a link to the breaker or moveable points plate, the spring side being airtight and connected via vacuum hose to the carburetor throttle bore. Just as awabbit6 remarked.
Vacuum advance comes into play when the throttle plate is opened and the distributor vacuum line is exposed to manifold vacuum, causing the diaphragm to move against spring tension.
This moves the points plate which causes the points rubbing block to move against distributor rotation, advancing ignition timing.

And of course if we speak of late model stuff, with ECU's and such stuff. Which I believe aren't actually neccessary.
It get more complicated in the plumbing.

Apart from that, we both said the same thing about spark timing.

Just to be pedantic, I think I explained the need for advanced ignition timing and the way the fuel is burned, rather than exploded, in the cylinder, at the same time. Because the time it takes to "burn" the fuel, and the time the engine has to burn it necessitates advanced ignition at higher revs.
Light the fire sooner. Basically.
Makes it "ping" to.:angel:
Ah, the good old days of standard pump, and super petrol.
Now, was the ignition too far advanced for the standard petrol, or was the standard petrol too low an octane for the ignition timing?

Jon and I both explained exactly the same old fashioned test procedure, I might have used more words.:angel:

And yeah, I agree with your closing line Dave.
It is a dual control, interactive system. If half is broken? Do I really need to finish this sentence?