So, we seem to have found the source of our blown pistons and ring lands! Our distributor (off a fresh rebuild I might add), does not have enough vacuum retard at WOT. Turns out we were way too far advanced (about 38-40 degrees) under load.
My question is, how do you adjust the vacuum unit? The advance and retard work when you pull a vacuum on them. However, I suspect that the adjustment screw is not adjusted properly. It is almost all the way out. Does anyone know how to get this speced right?

Thanks for your help,
Chris

Shops used to have a machine that you could put the dizzy in to adjust the advance curves. Hmm, I haven't seen one in awhile soo look around for a shop who does/can.

Geoff

Yes, mine was tested, but the machines only test the mechanical advance. That tested out righ ton spec of 22 degrees. I am looking for some method of setting the diaphragm steady state so I get the proper advance and retard. There probably is a machine to do this, but I thought we could probably do it in the car some way...

Chirs

According to the stuff I have read, the vacuum advance should be full on by 2500rpms and not affect the advance after that. Brad Anders did some testing last year on this and what the difference having the vacuum advance hooked up or not did.
Hell, just plug the vacuum advance hose and tune the engine to get it running right, then plug the hose back in. The car should run great without it as it is supposed to help only between 1500-2500 rpms.
Geoff

The problem is I have an adjustable vacuum canister on my distributor. At load, the retard should kick in, but it does not. I am looking for something like this:

1) Remove the hoses to the vacuum advance/retard and plug hoses.
2) Run engine to 3500rpm and set timing to 27 BTDC
3) connect ONLY the advance hose
4) DO SOMETHING TO THE ENGINE HERE
5) You should see XX BTDC now. If not adjust screw 2 turns clockwise to retard ore counterclockwise to advance.
6) Once you have set advance correctly, hook up the retard hose.
7) Disconnect the advance hose
8) You should see XX BTDC now.

etc.

Does anyone know this process?

Chris

Both my dizzys have the adjustable vacuum canister too. Like I said you don' t need to run the vacuum advance or even the retard. I have plugged the retard hose and only use the vacuum advance as it seems runs alittle better this way. The retard is for emmisions I thought. Just use the mechanical advance to set up the engine for now. Someone will chime in on how to adjust the advance.

Geoff

Actually, the advance is for emissions...
http://www.gofastforless.com/ignition/advance.htm

At part load, there should be an equilibrium set up across the diphragm that sets the proper advance. The retard is needed for this to occur. Right now, we are seeing too much advance at WOT. This means that the retard is not working properly because at WOT, it should basically be the mechanical advance as master....

I hope someone does chime in....

Chris

I cant focus on this right now. I wish they would have made an attempt to adjust this when they had it on the scope..

The best I could do is tell you how many threads are showing on the one in my truck.

I'm on eternal hold with Dell support.


B

We can turn it into a "How many threads are you showing?" thread....

let me guess, the network card in the laptop is still not working?

man, i almost went through a full bottle of Myers Rum since you called
Andy

The retard works at idle when there is high engine vacuum below the throttle plate. (plate closed). The vacuum advance works when you are just coming off idle and the engine vacuum is high and the throttle plate is open, (port is above)

The mechanical advance works by engine rpm, centrifugal force works on the springs and advance plate.

Porsche disabled the vacuum advance on 914's after 73 to help the emissions, that is what I understand.

You don't need the vacuum advance hooked up now, plug the hose and the car should run fine after setting the timing.
If it can't be timed right, then the dizzy is out of adjustment or broken in some way and you should get another dizzy.

Geoff

would there be a notible difference in power in a throttle bottle that has the vac. adv. line over the throttle body without it? my 75 does not have the little pipe comming out, while my friends 75 cali car does. both are 2.0s

Only the 73's had it so he has swapped one in his car. Yes, there is a slight improvement in off throttle response. You can add the port as its not hard to do.

Maybe I'll do a photo shoot showing how soon.

Geoff

Geoff,
It is only the vacuum canister that is screwed up. There is an adjustment on it and I am still trying to figure out how to set it. The dizzy is just fine as it was just rebuilt and curved.
I believe it was the retard that was removed in the later model cars, not the advance.

Chris

All the dizzy I have seen have the retard function. The vaccum advance is what Porsche deleted. Here is a pic of a 74 Throttle body.

Interesting. Wonder why they would go with a retard port. I know it is in the same spot, but I wonder if it is actually an advance port. At idle, where some advance is needed to help get off the line, the weights are not spinning fast enough to get any appreciable mechanical advance. Is the dizzy completely different? Seems like it should be, but looking at the specs, it isn't all that different. Leaving the retard and deleting the advance doesn't make sense to me....

Chris

As I said the retard only works at idle where the engine vacuum is high and the throttle plate is closed. Open the throttle and the vacuum drops so the retard doesn't function (so you add 5 degrees of advance). As engine rpms increase the mechanical advance takes affect.
The vacuum advance only works between 1500-2500 on the documents I have seen, so it helps only "get you off the line". Crusing speeds are usually around 2500 to 3000 rpms.

As I said, try setting the engine up without the vacuum advance and use the retard function to set the correct idle. The engine should run great like that.

Geoff