Distributor changed from dual vacuum control to single vacuum control?

[toddetch]

I just had a number of things done to my new (to me) 1980 SC Targa by a local mechanic. One note they added to the line item on DEQ testing was: "Note distributor has been changed from dual vacuum control to single vacuum."

I haven't done much work on cars since I sold my '66 Mustang in 1991 to get a more dependable car to get to work (one of the worst decisions of my life - should have kept the Mustang and bought a scooter or something). Anyway, I'm not sure what this note means. I assume this means I have a non-stock distributor? What would be the effect of single vacuum instead of dual vacuum?

Thanks in advance for any input and advice!

[Flat6pac]

The 80 distributor had dual vacuum. One to advance at high RPM, the other to vacuum at idle to draw the spark to ATDC.
Vacuum at idle is used to meet emissions in the day. Most of the early cars are not required to meet emissions any longer in most states
Bruce

[Bob Kontak]

Originally Posted by toddetch

Thanks in advance for any input............!

I would figure out what the vacuum unit is doing.

You need a timing light, some calculatin' time and patience.

You have to put marks on the crank pulley to indicate degrees.

I would not go over 25 BTDC on an SC. However, right now, do you know where it maxes out?

[toddetch]

Thanks Bruce and Bob for your replies.

Bruce - now it makes sense why the shop noted it under the DEQ section - here in Oregon cars much older than this one still have to clear emissions.

Bob - I don't know where it maxes out. From your description, I'm going to have to do some more digging to understand how this is supposed to work. Thanks!

[Flat6pac]

On the 80/83 distributor there is two vacuum connections.
The advance connection is on the outside of the pod on a 3.0. The connection hose derives it vacuum from above the throttle body butterfly so as RPMs increase so does the vacuum to the distributor advance.
The retard connection is to the inside toward the dist body. The hose connects below the butterfly so at idle the vacuum causes the pod to retard the ignition.
Setting timing @ 5 degrees btdc the dist will mechanically advance 15 to 20 degrees. The vacuum caused by higher RPMs will advance the distributor another 5 to 10 degrees.
Usually the advance is all in by 4500 R if you're advancing more than 30 degrees you back the original 5 degree setting back toward 0. If you don't get to 30 degrees all in, advance the idle setting so you get to the 30 degrees.
As another point, the 3.0 distributors will freeze inside and not pull on the vacuum or advance properly. The feeling is like hitting an invisible wall at 3500 and not pulling smoothly. If hats the problem, PM me.
Bruce

[toddetch]

Thanks Bruce

[emcon5]

Why?

On edit, to be clear, are they just noting that the distributor is not original, or is it saying they replaced it?