John Walker and all you experts read the linked thread please?
 

http://forums.pelicanparts.com/porsche-911-technical-forum/77882-close-but-no-cigar-post519303.html#post519303

Here's my problem. Razer has his too. Similar, but on opposite ends.

Here we go.

The vaccum unit is only for vacuum advance, right? This one has just one vacuum connection on the dist. When I apply vacuum (motor stopped and dist cap off), I can see the pins on the outer ring move clockwise. The mechanical advance allows me to move the inner pins c-clockwise (centrifugal advance). The mechanical advance springs back after about twice to three time the amount of rotation as the vacuum advance. I assume this is correct since advance curve from the factory manual shows about 3.5 degrees of vacuum advance max and about 12 degrees of centrifugal advance max. I have not received my new vacuum unit yet, but the current one seems to be working fine. It advances with vacuum and springs back on release.

To get the motor to idle smoothly, I have to set the idle timing at about 10 degrees. This is with the vacuum line disconnected and plugged (more on this later). When I run the revs up at this setting, number one plug fires at 26 or so degrees. So I assume I can leave it like that.

Now, when I go to connect the vacuum to the vacuum unit, I get surges and general rough idle and it dies, I can start it up again, but it runs rough and dies unless I play with the throttle. If I advance it more (with the vacuum line connect now) by turning the dist, it will idle, but again, I have to advance it over 10 degrees and it surges a bit. I can see the vacuum amount change a but with the surge (vacuum guage tee-ed into the dist vacuum line).

If I leave the vacuum line disconnected and plugged, with idle set to about 1100RPM, I can drive the car fine. It is full of torque and runs great at speeds. I don't think I have any detonation (atleast I can't tell). Isn't the vacuum advance for detonation prevention when the throttle starts to open? At some point the centrifugal advance exceeds the vacuum advance and it is out of the picture, right?

So the big question is, does the dist. have vacuum advance at idle? That is, there should be vacuum pulling the vacuum unit diaphragm back at idle. (A) Then the vacuum line should be connected to a vacuum port below the throttle plate. (B) If there should not be vacuum on the diaphragm at idle, then the vacuum line should be connected above the throttle plate.

With option B, I can get the car to idle fine with vacuum disconnected and plugged, but have to set it higher than the 950 RPM spec, more like 1100 RPM. Driveability is great. With the vacuum connected (above throttle plate), it stumbles and sags off throttle (10-20 degrees throttle opening).

With option A, I can't get as good an idle idle. I tried this today, but since I couldn't get it to idle well, I didn't drive it with this option so I have no idea what driveability is like.

I think I have more than one problem due to the high RPM requirement for a good idle, but the biggest problem at this time is the distributor.

Pheew! Sorry for the length. So what is going on?

Souk H.

I didn't see a mention of what year/type 911 you have.

On my '78SC the exhaust was changed to SSIs and all smog crap removed. The vac line you spoke of was also eliminated and both the dizzy pot and throttle body plugged. From what I could gather the vac retard at idle and advance features were emmision items. The mechanical advance worked perfect for me, the car ran great all the time. On the highway she got from 22-25 MPG and managed 14.2 quarter mile times. Start up was a one-hit deal from 0-100 F.

My motor has no emissions pum and related equipment, but I do have a cat.

When I plug the vacuum to the dist, the car runs great like your car did. From what you say, there does not seem to be a negative long term effect with no vacuum at the dist.

And are you sure it is vacuum retard at the dist? Reread my last lost in the linked thread.

Thanks.

No, I was speaking in generalities. Different years had one or two "vac" lines. My '78SC is a Euro with one vac port connecting to the throttle body. There is so little detailed information out there on the particularities of CIS systems for specific models that I wouldn't begin to venture a guess as to what most of this junk does. I looked at Euro schematics. If the Euro didn't have it I pulled it off. This is also what I am doing with the 930. Seems I haven't pulled enough off it yet.

suck on the hose to the vacuum unit when the engine is running. if the idle slows, it's a retard, and the hose can be kept disconnected and plugged.

Souk, see my reply in the other thread. ;)

John and all,

Please read the linked thread for the latest.

Thanks,

Souk