There is no need to vent the boost port to atmosphere - equalization is not needed. The opening in the case was the result of corrosion/erosion. At one time the port connection was sealed to the assembly case. In application, both ports will see both vac and press depending on the state of the intake manifold pressure. Remember, pressure on the vac port has no effect and vac on the pressure port has no effect.

ok - thanks for the explanation. When I looked at the picture of the hole, the steel edges looked quite thick and I therefore discounted corrosion but I missed thinking about force on the diaphram. Lights are on now! :)

Yes, I do believe this was contributing to the tuning problems I was having. I could get normal power on boost or off-boost by adjusting the timing, but I could never get good power throughout the range.
Pelican list this OEM part and I have since ordered one ($174).
To get the unit apart I used a hack-saw to split the outer housing. I did this fairly carefully because if it was fixable, I wanted to be able to put it back together again - probably a job for liquid metal. However, as you can see it is beyond repair. One reason why it wasn't working on boost was because there was so must rust in the boost side that it had physically stopped the boost side diaphragm from moving (the part second from left with spring in centre).

You are probably correct, however there is no sign that this area should be, or ever was sealed. Perhaps someone could have a look at theirs and confirm that where the pipe goes through the outer housing it is or isn't sealed. However, as you say, it is a bit hard to see how it could work with this not sealed. Maybe it should have a rubber seal around the pipe?
I guess this is just something else to watch out for, especially if you are having problems getting correct timing throughout the range.

Thanks for your input guys.

Bill
 

I took off the distributor and the vacuum advance was at 2 full turns on each screw driver loose.

Also, these are my questions.

1) Look at the rotor (on top) it looks like there is a crack in the middle of that. Should I get another? Would this affect anything?

2) Beneath the rotor there is a black plastic thin cover. It looks a little worn out toward the bottom of the photo along the edge and was curious to see what that does, or is it just a dust cover of sorts?

Lastly, would you still like me to check the vacuum for you? How do I check this?

Thanks,
Bryan

http://forums.pelicanparts.com/uploa...1219280305.jpg

Bryan,
1. Replace it before it falls apart at 6000 revs (or less!).
2. Dust cover I think. You could probably save weight by leaving it out! (but don't)

I would be interested to see if your unit has the gap in the housing around the boost port (the one nearest the distributor) like mine.
Refer to 356-930's comments in a post above where he describes how the unit works with boost and vacuum.

http://forums.pelicanparts.com/uploa...1219284746.jpg

My vacuum advance canister looked the same. Worked one way, but not the other. Lots of rust inside as well.

The hole around the vacuum port was so even on mine, I assumed it to be factory, not rust and the source of water into it (or maybe where the two stamped pieced joined together?).

Got a new from Pelican.

Answerr
 

Bill,

I do have the hole around the boost port. It looks identical to yours. Mine looks factory and this car has been in the desert with with absolutely not a spec of rust on the car. Is 356-930 saying that this gap is not factory? If so, I could not see how that is possible on mine.

Do you know if that crack in the dizzy/rotor could be affecting anything or would it affect anything. How would that show itself as a symptom?

Thanks,
Bryan

Underneath that crack is a resistor. The hotter your CD/Coil system is, the hotter that resistor gets. If running a stock ignition, replace it. If the resistor open circuits, no spark to the plugs. An MSD ignition will eat the resistor for breakfast and if running MSD, cut the resistor out and solder in a piece of solid 12awg wire, add new epoxy.

356-930
 

Thanks for the info. Everything from an ignition stand point is stock. When you say replace it, your referring to the rotor itself correct? that hole around the boost port is normal correct? Mine looks like it's been there for ever.

I just ordered a cap (mine looks fine but oh well) and rotor from our host tonight so I will replace.

Question, Do you think that this could have affected my cold running issues and slight missing while under load when cold? Also, my vacuum advance was loose as well, so I tightened that.

Thanks again for your help,

Bryan

Yeah, from what I'm reading here, that gap around the port is meant to be there. Bum steer there from 356-930. ;)
It's just a dumb design that allows water to collect inside the housing.
On closer inspection, I can see that the hole around the port is actually stamped in the housing - it isn't rusted out. Maybe there's supposed to be a rubber grommet in there or some other sealing gunk.
I don't see any harm in sealing the space around the pipe as the inside void is also open to atmosphere via the hole where the actuating rod comes through into the dizzy.

I doubt that the cracked rotor would actually be causing any performance issues (until it completely falls apart :eek: ).

Thanks
 

Billjam,

Thanks for the info. I may actually put some gunk or sealant in that space just for the future, but mine looks pretty clean with no signs of rust from what I can see.

Has your car always been in Australia? If so, is it the salt in the air that has that affect? based on your pictures I would guess the car cam out of the Midwest (only based on your distributor of course) or somewhere that it gets very cold.

Bryan

Yes, referring to the rotor.
Seriously doubt cold running issues and slight missing under load would be brought on by a high resistance rotor. Resistance of most resistors increases with heat. If the resistor were the running problem, problem would get worse as the engine warmed. I don't have a clue about what could be causing your cold running issues but with heat comes expansion. Perhaps a loose ground that tightens with heat? Add some good grounds from the CD box and engine case to the chassis. No change, ground isn't the problem.
I made a bad presumption about the hole around the boost port tube as my '79 distributor's boost port is fused to the assembly's housing. Must be in later model distributors, that changed. It's not obvious from the photo of the disassembled piece parts just how that (newer) assembly seals. Perhaps an 0-ring? I'd love to see that end piece of the housing further dissected. Then we'd all know for sure just how Porsche sealed the boost port section.
Best to test to assure yours is leak free. If you've got something like a Mighty-Vac, pull a vac on the boost port, If it holds a vac, it will hold press. Or, make a + press pump using a blood pressure cuff bulb, rubber tubing, a tee and a 15-psi gage. Pressurize the port, monitor the gage for system leak. With the engine running, it's a great tool to learn how much timing your boost port pulls out and at what boost levels it occurs. (My '79's boost retard pulls 8-deg by 5-psi - that's it.)

Absolutely/obviously a bum steer. Humble apologies. Can I still post or am I relegated to sidelines as an observer?
Chris

Heck no
 

356-930... Yes sidelines only for your from now on. Heck no, you have been extremely helpful for Bill and myself and I have seen many posts from you in the past that have been very insightful... you know your stuff.

I'm taking a guess here, but are you a doctor/surgeon? Just a hunch.

Bryan

Neither. Spent most of my working life building military & medical electro-mechanical devices. And, cars and girls were always more fun than school.
Glad I get to continue to post!
Chris

What are the symptoms of a bad/blown resistor? I've been using an MSD 6AL for years on the track without problems.

Maybe I'm just lucky?

You've just been lucky. The high current/power generated by the MSD causes excess heat which will (and should have by now) blown the resistor.
A resistor eats power, power intended for the plugs. Suggest you dig the resistor out, solder in a piece of solid 12awg copper wire (found in typical household wiring) and re-epoxy.

Thanks.I'll give it a try.

One reason I asked was that my car is breaking up at high rpm and a wacky tach. A check of the battery shows a little over 12v at idle. I assume it's an alternator or voltage regulator issue.