Distributor query

[billjam]

While I have my motor in bits to fix some oil leaks and the clutch I thought I'd just check over my distributor and it seems that the vacuum advance/boost retard might not be working properly.
When I suck on port A, the arm pulls in and out as I would expect. It holds vacuum until I release it.
However, when I suck or blow into port B, nothing happens. There is quite a restriction to airflow but it doesn't hold vacuum or pressure - its as if it the diaphram has a small leak.
What is normal for this type of 930 distributor (1986)?

One of the reasons I pulled the motor was to look for any issues that might be causing very low off-boost power. I suspected cam timing and found that it was set at 0.2mm (instead of 0.65-0.8). This may have been partly the cause of my low power problem, however if the advance and retard mechanism is kaput, is this also likely to be a factor?

If the vacuum advance unit is stuffed, can anyone steer me towards a replacement source (new or used) without having to buy a complete distributor?

[NathanUK]

Telling us your engine type may help too. eg 930/66 etc etc

[billjam]

Engine type is 930/68 = 1986 Turbo USA.

[dsiegel360]

My US 88 works as follows:
pressure applied to right port: timing remained the same.
pressure into left: retards timing (boost retard)
vac into left: advances timing
vac into right: advances timing

Call Barry hershon to do a dizzy rebuild: 313 532 5350.
I have a rebuilt one that will be for sale soon.

[NathanUK]

Billjam, is there a part number on the vacuum unit at all?

[billjam]

Nathan,
Absolutely no numbers on the vacuum unit but plenty on the body of dizzy.
Bill

[NathanUK]

That's annoying as I would of compared it against mine for you. Mine is a euro car.

[billjam]

Bump - I am getting closer to putting this engine back together and I still really didn't identify if my vacuum unit is stuffed or not ....

Originally Posted by billjam

While I have my motor in bits to fix some oil leaks and the clutch I thought I'd just check over my distributor and it seems that the vacuum advance/boost retard might not be working properly.
When I suck on port A, the arm pulls in and out as I would expect. It holds vacuum until I release it.
However, when I suck or blow into port B, nothing happens. There is quite a restriction to airflow but it doesn't hold vacuum or pressure - its as if it the diaphram has a small leak.
What is normal for this type of 930 distributor (1986)?

Is there anyone out there with a vacuum unit from a 930/68 motor that they can test for me?
I would like to know what happens when you blow and suck in each of the ports. On mine, port A seems to work, but pressure or vacuum at port B doesn't seem to do anything. Mind you, I was only blowing with my mouth - perhaps at boost pressure something happens?
I'd rather be sure it's stuffed before I order a new $175 part from our host.

[356-930]

Originally Posted by billjam

Is there anyone out there with a vacuum unit from a 930/68 motor that they can test for me?
I would like to know what happens when you blow and suck in each of the ports.

My '79 930 distributor has a similar looking vac/boost unit. It is unlikely Porsche altered the operation of this accessory over time.
Note that it's very easy to draw almost a full vacuum by mouth, near impossible to generate even +1-psi by mouth. Get yourself a hand pump, even a bulb and rubber hose off a blood pressure cuff. Easy to generate 15-psi with a blood pressure cuff bulb.
Port A is vac. retard (vac pulls timing out of distributor). A positive pressure on port A will do nothing. Port B is boost pressure retard (positive pressure on port B also retards timing). Vacuum on port B will not affect timing.
Both ports should hold vac or pressure.

[x98boardwell]

Question,

I have had issues and rough idle when the car is cold. I noticed this post and went outside to look at mine (87-930) and it is very loose, meaning it will toggle up and down.

What does this part do and will it affect anything while it is loose?

If so, I will be happy to take mine off and test for you in the process when I fix/tighten.

Any ideas on the question above? Not trying to hi-jack post, just seemed relevant since you are on the topic.

Thanks in advance,
Bryan

[billjam]

Originally Posted by 356-930

Port A is vac. retard (vac pulls timing out of distributor). A positive pressure on port A will do nothing. Port B is boost pressure retard (positive pressure on port B also retards timing). Vacuum on port B will not affect timing.
Both ports should hold vac or pressure.

Thanks, that is what I was looking for. My vacuum side seems to be working OK, but the boost side won't hold pressure or vacuum.
This surprises me a little as I would have expected there to be just one diaphragm in the centre so I don't understand why it holds pressure/vacuum from one side and not the other.
This is just the excuse I needed to cut it in half to find out what is really going on in there!

Bryan, if yours is loose you obviously need to look into it.
What does it do? It alters ingition timing under boost and vacuum.
Taking the vacuum unit off the dizzy while it is in place could be a bit tricky - getting it back on again will be even trickier as you need to hook the end of the linkage from the diaphragm over a pin inside the dizzy. It is really best to pull the whole dizzy, then you can see what you are doing in there.
If you do take it off, I would be interested to hear if port B (nearest the dizzy) holds vacuum and pressure.

[x98boardwell]

I may have to pull the entire dizzy out to tighten this thing up properly. Would it being loose affect how the car runs when cold and only when cold? When she's all warmed up it seems fine. When cold, she hesitates under any kind of load until boost kicks in. It seems to have been getting worse slowly that is why I think this may be the issue.

I would be happy to check the pressure/vacuum for you if I need to take it off.

Thanks again for your help, I realize this is a little off your original topic.
Bryan

[356-930]

Originally Posted by x98boardwell

What does this part do and will it affect anything while it is loose?

This part works/acts on the part in the distributor that houses the spring controlled centrifugal advance plate. Its action is additive to any timing change that results from centrifugal advance.
If it's loose and moves up and down, not likely to affect operation. If it can move in and out, possible to affect timing.
This said, every degree of proper advance yields power. I'd fix it if were mine.

[billjam]

Bryan,
There are a host of gadgets on these motors that affect hot and cold running and I am far from being an expert in this so I am afraid I can't really offer any useful advice on your cold running problem.

[x98boardwell]

Thanks guys,

I will fix. If I have to remove, billjam I will let you know regarding the vacuum question.

Bryan

[billjam]

If you were wondering what is inside a vacuum advance unit, wonder no longer ...



Mine is a bit stuffed. It seems like it has had its fair share of water over the past 21 years. Considering that it is actually exposed to the elements, it's surprising that there isn't a drain hole in the housing. Water can enter the boost side of the housing via the hole around the connection pipe, but it can't drain out. It's not hard to imagine water trickling down the outside of the hose and dripping into the housing.

[boostfix]

Nice dissection Bill! Did it become obvious why the vacuum pull was working but the boost functionality was not?

[x98boardwell]

Was this giving you problems prior to the engine rebuild? Have you found a source for another one? How did it come apart.. was there threads between the two halves?

Great job on the pics,

Bryan

[356-930]

Originally Posted by boostfix

Did it become obvious why the vacuum pull was working but the boost functionality was not?

Yes, from the excellent dissection work, it's clear why the vac retard port (A) did not leak but the boost retard port (B) did. The housing's vacuum section did not leak. The diaphragm sealed it from the housing's boost section which was open to atmosphere and thus leaked.
For the assembly to have ever worked properly, the B or boost press port had to be sealed where it entered the housing.

[boostfix]

IMO - the opening, in the case around the boost connection pipe, is there to allow the diaphram to equalize to atmospheric pressure.