Distributor vacuum tubes.

[T.Hanson]

The '79 distributor has the advance and (I assume) retard connected to the top and bottom little vacuum tubes on the throttle body.

The '81 uses only the top throttle body vacuum tube, attached to the top diaphragm tube on the distributor. The bottom distributor diaphragm tube is unused, open.

The lower vacuum tube on the throttle body uses a vacuum hose cap, plug.

So, what happens if both vacuum tubes are connected on an '81 ?

What happens if the lower throttle body plug isn't there ?, both ends not connected ? Vacuum leak ? No vacuum retard ?

I.e., what's the big deal if all the distributors look exactly the same, have two vacuum tubes, top and bottom of the diaphram ?

[Mike W.]

Basic answer, not a lot.

More complicated. 79 was 79 only, then 80+ was at least a different part number. Since timing is done with vacuum disconnected, timing would not be a problem. I forget exactly what now, but when my 79 had a bad retard diaphragm, it just wasn't quite right, mostly idle, IIRC but it may have been transition. Unplugged vacuum port? A small vacuum leak, it's a small port in the throttle body. Biggest problem is if the hoses were hooked up wrong. Ported vacuum is funny, it took me a while looking at the throttle body and how the butterfly opened to see how it worked, but it did make sense. And it only works on small throttle openings, at large openings there is essentially no vacuum, so no effect.

And the two vacuum ports are on opposite sides of the butterfly at idle, so how they are hooked up is critical.

[Lenny D.]

What happens if the lower throttle body plug isn't there ?, both ends not connected ? Vacuum leak ? No vacuum retard ?

I.e., what's the big deal if all the distributors look exactly the same, have two vacuum tubes, top and bottom of the diaphram ?

1 - Vacuum leak and no retard since the tube is not connected to the advance/retard mechanism attached to the dizzy.

2 - Distributors, particularly, Bosch can look the same but they made a plethora of different dizzys by way of different spring tensions on the mechanical weights to achieve different advance curves (and their part numbers tell them apart). Check the dizzy part nos. to verify their 'exactness'.

+1 with Mike's explanation of what the difference amounts to.

Vacuum retard (at idle) back in the 70s was a means to achieve fewer emmissions (VW did it, too, and Bosch made many different models in the span of a few years). With the advent of fuel injection and a closed loop (Lambda) system to continually monitor idle exhaust the necessity for vacuum retard at idle was superfluous. BMW just used the same dizzy on the 528 models and capped the port on the manifold while leaving the retard port on the dizzy open to the atmosphere, no harm done.
What you wind up with is a an advance curve on the dizzy that doesn't have the 'window' of having to recover from retard to get to advance when you hit the pedal. A minor but noticeable effect of softness in acceleration.