Yes, it is interesting and someone is likely to post a god explanation. But for now, I'll say that the connections for the two vacuum hoses end on opposite sides of the throttle body. I believe the vacuum advance connects with the air cleaner side of the TB, and the vacuum retard connects just after the TB. When you are idling and the TB is closed, there is virtually no vacuum on the air cleaner side of the TB, so there is no vacuum advance. At that time, there is vacuum on the engine side of the TB, so timing is retarded. This is appropriate for engines idling or starting.

When you open the throttle plate, vacuum advance begins to happen, which is appropriate. But at this stage, if you open the throttle all the way there is likely to be a whole bunch of vacuum advance, more than you need. There is still vacuum retard at this point which helps prevent pinging during low-rpm, WOT situations where pinging is most likely to occur.

Also, when you're cruising and you let off the gas, vacuum advance disappears and vacuum retard goes FULL. This is to avoid pinging while the pistons are moving fast with only a little fuel.

Then of course the centrifugal advance has its own curve, too. It's pretty interesting. Tyson taught me some things about these systems once, but I forgot them. I think.

I've been putting off rebuilding my dizzy for long enough. Anybody know a good place to send one for a quick turn around? Can any local speed shop do it?

There probably isn't much that your distributor needs. I have mine apart and I find it does not need bushings. And now I'm told that even if they were worn, it would not affect ignition much. You see, the old ignition systems which had points used to have problems with dwell angle changing (and therefore timing) when the bushings were worn. Ours have electronic dwells, so they are not as subject to problems from bushing wear. And as I said, my bushings are tight as drums anyway. The only other thing to do is check cleanliness and free movement of the centrifugal advance weights. And they seem fine too. Which makes sense since the ignition system was not acting up.

Bill

I did a search and printed out a list of over 20 Auto Electric shops in the Seattle area. What I found out is that most of these shops don't rebuild, but simply provide a swapout. YMMV in all events, but that's my experience here.

Also, not a single one could supply Bosch part number 123-7010-00; the rebuild kit that I understand is used for the Bosch distributor (though possibly not for my SC dizzy). I was able to find Porsche part number 930 602 922 00 at the Porsche shop, so this pinion to shaft pin is available.

John

If I buy a rebuilt one, can I expect that it has gone through some kind of testing to verify the timing curves for both RPM and vacuum? Or do these shops just rebuild using the specified parts, and assume the timing curves will be correct?

Okay, I'm going to say it again. You may be planning to rebuild or replace a distributor that works perfectly well, and will work perfectly well for many more years. Mine has 183K miles on it and it looks as though it is new.

If you exchange it or have it rebuilt, you may be RAISING your changes of having trouble, rather than DECREASING those chances.

And finally, yes the timing curve on a distributor can be checked and verified, though you may need to do some searching to find a shop that knows what it's doing and has the proper test equipment. I would STRONGLY suggest you go to a good local mechanic and have the advance looked at along the rpm spectrum, and compared to your car's designed advance curve. This can be done with a darned timing light and it will tell you whether you should be doing ANYTHING to your distributor.

I know that the vacuum advance on my dizzy doesn't work. I can see that with a timing light while I apply a vaccum to the dizzy. It needs to be rebuilt. My options are to :

1. Rebuild it myself and hope I get it right.
2. Rebuild it myself and find a shop that can test it.
3. Buy a rebuilt unit that has already been tested.

I like options 2 and 3 because I will then have a good feeling that things are working as optimum as possible.

I guess I should contact some local speed shops and see what they can do for me.

maybe it's just the vacuum unit that's tits. they just bolt on.

Hi ,

the techbook calls for detached vacuum line, 900+/-50 rpm and 5 deg BTDC, as already mentioned by several, but it also tells you to check the advance at 6000 rpm still with the vacuum detached and that should give you 24-28 deg BTDC for the US 911 SC 1978-79 model. This checks that the flyweights work as intended.

Bjorn:cool:

What is the best technique to test the 6000 rpm advance, since there is no timing mark?

Olivier

The best way I know of is a timing light with adjustable advance. I got mine at Sears, for about $60, IIRC.

Tom

I saved? $30 by going with the standard model. Is there another way other than just estimating or using a protractor?

Oliver, I would have to go downstairs to garage and get spec book to be sure, but I think that full mechanical advance comes on much lower than 6000rpm w/ our cars. Only cars w/ 6000 test-spec are earlier cars, I think.

Bill, Do you have a Miti-vac pump? Best way to check diaphram is w/ this, it only moves plate very little in each direction; you can also do it w/ mouth and clean short length of vac-hose, if you have OE lungs. :D It sounds like you might just need new advance unit, dist. on SC's are pretty stout from reading other posts. If new one is expensive, good used one is almost as good, just make sure you get right one for your euro car. If this is what you need, fixing it will put big smile on w/ new ponies and MPG. :cool:

Never had a problem with the mouth technique. Any male should be experienced enough to perform it.

http://www.pelicanparts.com/support/smileys/wat3.gif

Don't know about your US cars but on our Euro cars there are two timing marks on the belt pulley. One for 5 deg and one for the more advanced, so we just hook up the strobelight check one mark at idle (900+/-50) and the other mark at 6000rpm.

Bjorn:cool:

I checked my Spec Book and it provides no information about advance curves. It simply says 19-25 degrees BTDC at 6000 rpm with vacuum lines disconnected. Again, I am going to place a mark on my crank pulley at 25 degrees BTDC, and perhaps one at 19 degrees.

I checked the vacuum advance by using a short length of hose and my OE lungs (I was born near Stuttgart). Rather than look for movement inside the distributor, I used a timing light to look to see if I could detect any movement in timing. I couldn't.

The vacuum unit seemed to hold vacuum fine, so I don't think I've got any leaks. It just seems to be froze up. I figure if it's froze up, then the rest of the dizzy is probably a little bad too, and could use a good cleaning and new grease. I need to get off my ass and just do it.

Pot hole season is approaching soon, so that would be a good time to get the car off the road for some work.

Jdub -- How's that tech article coming along???

I am in process, with a digital camera on hand at last. The "problem" is that the dist. I am working with is not an SC unit and I do not have on hand/intend to remove the shaft bushings...that useful tidbit is out of my realm. It is also very frustrating to find that many of the parts we will need to replace are unobtainable by the common peon like myself. It is a throwaway world...

One thing Bill: dumb thought, but is there any possibility that the vacuum unit is NOT hooked up internally? You can quickly determine this by unscrewing the vacuum unit and GENTLY pulling it away from the body of the dist. If it pulls away easily, without ANY resistance then the hook from the advance unit you are holding is not hooked to the small pin on the advance plate proper. Rehooking this is a matter of tilting the advance unit up and, with a bit of light, ID'ing the pin and mating the two up. Not easily done with the unit in situ.

If you will be pulling and cleaning the dist., do have on hand another set of the semi-clear plastic "bushings" that sit on the shaft-side pins. The springs ride on these bushings and I found one of mine completely gone, the other intact. I cannot even say that there was a pair; only that I saw only one in my dist. and given that the springs are intentionally different, I am hesitant to declare that both spring posts must have these bushings.

Sorry not to be of more help. Bill: my "beta-level" writeup could use your review I am thinking...

John

My distributor is in a box. Perhaps I'll bring it tomorrow in case you want to take pictures, John.

Bill, I'd do what John's suggesting. Two screws hold the vacuum advance unit onto the distruibutor body. Remove these, and see if the advance unit comes right out easily into your hand. If it is hooked up, you will only be able to move it away from the distributor perhaps less than an inch. If it has become disconnected, it will come all the way OFF.

While it is dangling there, you can squirt penetrating oil into the distributor, with the hope of freeing any frozen mechanisms. If you just mark and remove the distributor, You'd be about half way done with the 'rebuild' you are seeking. It's pretty straightforward.

In a past thread, Jdub has already posted an excellent explanation of the rebuild procedures.

Am I right in assuming that the Z1 mark is TDC for timing purposes?

So the "5" mark to the right of Z1 is BTDC?

I just want to confirm this so I make sure to make a mark at 25 BTDC in the right direction, which seems like clockwise measured from Z1.

This seems to make sense, but when I rev the engine to 6000 to check the advance, the timing marks "move" counter-clockwise, so it almost seems like advance is in the clockwise direction on the pulley.

Olivier