How does the vacuum advance unit work, what does it do? What happens if its not connected?

It works by drawing vacuum of the carbie from a port which is closed off when the butterfly is in the idle position, this vacuum is piped to the distributor vacuum advance unit.

This has a spring and rubber diaphram in it. As vacuum is applied, it draws the diaphram in to the vacuum, working against the spring.

Off the back of the diaphram is a linkage to (usually) the backing plate in the distributor. This has the points mounted on it. As it is pulled around by the vacuum advance unit, it does so in a direction oposite to that of the rotation of the distributor shaft. By doing so, it advances the heel of the points and so advances the actual ignition timing. Note that it will only do this when there is vacuum present, within the range for which the unit is designed to work.

It is fitted as an ecommony device and will only work on part throttle. On wide open throttle and full closed throttle (idling and over-run) it does not (or should not!) operate.

For the most part, your car will drive and work OK without it, but you might use more fuel. If your planning on disconnecting permenantly, I'd suggest locking the backing plate in the distributor.

Thanks. Na im not going to disconnect it.

Thats OK.

I actually like them on road cars and use them where I can.

I had one fitted (a Fraud unit actually) to an Aldon Yellow dizzie and fuel consumption improved by around 30% (yep...three zero).

Cheers, Ian

They don't work with sidedraft Webers (or Dellortos). Nowhere to properly connect to.
Nor do they do anything if you have a big cam in it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thanks for the info. 30 per cent, that awsm.