Bill wrote:

> Understood, however since the vacuum advance port is above the
> butterfly It's the transition from high vacuum (intake manifold ) to
> atmospheric pressure which allows a momentary vacuum signal to the
> advance port in the throttle bore. The key to remember is that it's
> momentary, not sustained as would be the case using an actual intake
> manifold take-off (i.e. like the vacuum brake port on a later bus.) On
> a dual vacuum dist. the advance port is above the throttle and the
> retard port below the throttle valve.

It's been so long, I've forgotten some of the details, like the
exact plumbing details of the vacuum source. Thanks for the
reminder.

I do remember those dual (advance and retard) distributors. And
combinations with both vacuum and centrifugal advance.

> SA type distributors are best suited to the industrial motors which
> tend to spend most of their time at constant RPM or competition
> engines which are usually in the high rpm range. This is why the 009
> is best suited to competition use and can't provide the initial
> advance at low rpm need for smooth acceleration. Also, aggressive
> camshafts often don't produce enough vacuum for SVDA type distributors
> to properly function at low rpm.>

I used a distributor from a 126 industrial engine on my rally
beetle (1600 Mahle cylinders with domed pistons) back in about
1970.

John