Firstfives.org

One each in three cars that run. One, the '81 528i, has always had the sensitive throttle (jerky) response,...to have replies listing the primary suspect to be a distributor with a bad vacuum advance.

Enter three spares saved for parts. To the best of my (visual) knowledge and that all came from E12, E24, '78 to '81 cars I owned, they are all the same, to be interchangeable. (?)

So, is there a simple test to spin, click, snap the innards, to select the best candidate for first replacement ?

What's the deal with the '79 using both advance, retard vacuum lines, while the '81 uses only one, (advance ?) into the top nipple of the side (vacuum ?) cannister ? Bottom nipple open, with the lower nipple on the throttle body capped.

Does any of that lower, connected, open, plugged monkey business critically effect the performance of the advance/retard mechanism in the distributor ? Or does the '81 care if the lower line is connected the same as the '79.

As a general rule, distributors are not 'interchangable' per se', although you can do anything you like that makes you happy!

A Google search easily reveals Bosch dizzy p/ns and their respective stats. There are countless variations that achieve various advance curves via vacuum and spring weighted differences for particular applications notwithstanding physical application limitations. I believe I posted here about that quite some time ago.

You can try anything, but what the car was designed to see to fulfill the engineer's concept is but one distributor with a specific advance curve to work in conjunction with the rest of the car's engine management system. Make sense?

But try anything, the car will 1) run better than it did before, 2) not run at all, 3) be no different, 4) run worse. You be the judge. As in optometrist's circles, there's only 20/happy.

If you visit again to read this, the problem is, and has been for ten years, a throttle pedal so sensitive it can blow the horn with my nose. As my other, '79 528i exhibits no such behavior, somebody here advised the place to start trouble shooting was by switching distributors. Why switching between any of four collected from '79 to '81 big six engines may not be interchangeable, you're the guru.

The off throttle, low speed, fuel saver what's-it switch works. I've adjusted the plate to click the switch off early, put groceries from the back into the front seat. Currently disconnected, I've learned to work the pedals around it.

Nobody has ever said it's the throttle body, AFM, wrong brain box in the glove box, any such culprit component. Weird to me, as I would think ONE part in the throttle system would be the primary suspect when pushing the gas pedal (without riding the clutch) will either bonce my head off the headrest or blow the horn with my nose.

I once changed the dual vacuum dizzy to an advance only on my carburetted car. It was hard to get the idle down to something decent, but that is about the only change.

On the dual vacuum, the retard connection only matters when throttle is fully closed. I thought it was for emission standards only.

You are correct, I believe Bosch made a dual vacuum port dist. in that day (I know VW did) for a variety of car manufacturers.
It was another effort to reduce emissions at idle. It worked in the respect of retarding the ignition timing at idle, and the results are a very smooth idle at the expense of slightly hotter running (at idle). VW folks, upset with that, quickly found the '009' dizzy with no ports, advance or retard, that created another problem found ad nauseum on VW forums.

I think I'll stop asking and start swapping, testing.

But just so I have the basic concept, my '79 has both vacuum lines connected, while the '81 has a little red cap on the throttle body, only the top (advance) port on the distributor pot connected.

Thus, connecting the retard ports with a vacuum tube on the '81: minor idle change, mess things up if not a perfect match with specific Bosch model number distributor, or no noticeable difference at all?

Again, the whole point of swapping, distributors to AFM's to throttle bodies,...ECU's (?) is to find the cause of the super sensitive gas pedal.

I suppose you can just swap in the '79 dizzy and not connect the retard. That should make it fairly happy. Connecting the retard will change timing at idle. That would be a second thing to test, if the little red cap is easy to remove.

I suspect more is different than just the vacuum box or the retard function. I lost the retard diaphragm on my 79 and it just wasn't quite right, idle, timing, even running. Not far off, but some. A good vacuum box and I was good again, it really did make a difference. Not that they always did everything right and there is nothing to be improved on, but my experience is dual diaphragm distributor cars run best with both sides connected, going back to Bavarias and carburetors.

Now I'm not saying vacuum retard dist's as used on 530is are desirable at all, just that simple solutions don't always work.

Yes, but if you move the double vacuum dizzy in a advance only car and do not connect the retard line, I think things should be ok at least for some testing - no disaster will occur.

I just remembered that there was an older topic with some distributor info. There is some difference in between max advance, as well as RPM for max advance, and advance on certain reference RPM, but nothing that will do any engine damage I believe.

http://www.firstfives.org/bboard/viewtopic.php?p=13409