Well this is the 2nd diaphragm failure in a relatively short period of time , the first was when I ran the pertronix 3 dist that one went out at about 600 miles I was limping home since I didn't have my timing light to increase initial timing . It leaned out all I could do is raise the idle and try to make it as rich as I could to get home , I heard this with the pertronix 3 from alot of people and you need to take the dist out and apart to change it .

​​​​​​Now I have a motor craft with a peronix 3 drop ign module it has the stock vacum pod diaphram ????
its a 69 F250 390 mild cam 350 CFM 2bbl ?? I'm wondering what is a high vacum reading that would put stress on the diaphram ? In park warmed it's around 20/21 HG ..in gear it's only at 13/14 HG and that goes down as you drive the engine is rebuilt and had a great compression reading at 150/ 149 dry on all cylinders maybe 3,000 total miles .

Could it be heat ? , Maybe at cold start running it at say 900/1000 to warm it up for long periods it's cold blooded I'm not getting why I'm pulling the diaphragms I mean if it was oily residue from the pcv I would have issues else were too , power brakes vacum modulater I don't get it .......

 

If it's heat or excess vacuum, then it comes right down to the parts being defective.
You can't have enough heat to harm a vacuum advance can, sitting all out there in the air stream from the fan, and not overheat a whole lot of other things.
And a vacuum reading of 21 is fantastic. It's what my 302 had and the vacuum advance lasted thirty years and only got replaced because the distributor needed replacing. Not the vacuum advance. Probably at least in part a result of such a healthy engine with such a small carburetor. A 390 with a cam should probably be running the 500cfm 2bbl. Or at least something in the 450cfm or better department I would think.

In fact I've never had one fail me yet, and I have some that are 60 years old and still working. That's just by way of getting back to my first statement that something must be wrong with the parts right off the bat.
They certainly can fail, but yours does seem a bit extreme (to say the least!) in such a short time. What's different about the Pertronix distributor? The vacuum advance still looks like a standard vacuum advance can, correct? I wonder why you need to remove the distributor to replace the canister?
If this is a common issue that you're hearing about, then as much as I generally like Pertronix products, I still say poor quality is at fault here.

Are you using straight vacuum, or ported vacuum for the signal? Sounds like straight manifold, because you mention idle and things like that.
You don't need to limp home and you don't need a timing light. Just loosen the distributor and turn it clock-wise until your idle is back up to normal. If you're using ported vacuum, then something is wrong with the carburetor adjustments and the throttle blades are being held too far open.

Turning up the idle/air mixture screws does nothing for richening the mixture above idle. Or were you just talking about the idle speed screw? If that's the case that probably helps a little, but the bottom line is the timing is still off and you'd be better off adjusting the distributor until it felt normal again.
That, or you can change to the ported fitting on the carburetor so that there is no change of the timing at idle, and when you set the base timing that's where it stays even if the vacuum advance fails again. When that happens all you'd lose is some performance from having no vacuum advance. But it would not change the idle.

Not sure what's up with having two separate brands do the same thing. I can't think of anything other than what you said about contamination. But they're used to being in a gassy environment, so the material used should be capable of handling a little fuel vapor coming up the line.
Interested as always, to hear what others think about it.

Paul

 

The pertronix 3 dist is designed in a way that you cannot release the pins that hold the advance unless you remove the gear at the bottom and take it apart , I ran it on ported for awile using different advance springs and slightly higher initial timing 10-12 intiial and bronze spring at front end using ported w actually did nothing you can remove the vacum hose and no change at all not even enough vacum to open the advance .
I used the 500 CFM Holly 2bbl , that carb is difficult to tune for daily driving I had it on for 2-3 years it's a flat out WOT carb it will flow well and give it over 5,000 rpm but it's a gas hog as a daily and needed alot of mods to have it right in all areas low mid and WOT.
I had the 350 which is a much better all around daily and mild performance carb over the 300 CFM from Ford . I seen a standard brands V31 advance it said it's tested to 34HG no way I ever hit 34 HG I maybe run 21/22 max in park on fast idle warm up in park at a stop light it's around 12-14 so it must be cheap parts I know the older ones rarely went out when mine did it acts like a vacum leak since the diaphram rips the vacum just sucks through it , I could had cracked the bolt and turned the dist I think that day I didn't have my 1/2 wrench only thing I could do is plug up the vacum line and turn the idle up and now I was only at 8 initial timing it ran way leaner compared to 26 or so with full manifold vacum so Turing the screws to make it richer did help it gets richer with more timing .
I'm going to try the standard advance pod says it's 3 yr warranty over the cheaper T series one but I like full manifold vacum for all around daily driving ported gave me max 2-5 degrees in park almost 0 advance in gear it runs hotter on ported the exhaust is 50 percent louder it's great for tire burning power although it gets better milage , sounds better , runs cooler on full manifold that is if I can find a good advance ....
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Have you talked to Pertronix Tech Support? If this is their distributor and not and not their module, they should have an answer. Sounds more like a mechanical failure, than a vacuum problem.

It will be interesting to hear the outcome, keep us informed.

 

Yeah, I hear you on the 500cfm 2bbl. Gas hog was my experience as well, but I thought that even though your 350 might be larger than stock (did Ford really used the 300 on the 390?) the larger carb might still be a better choice.
I do prefer snappy throttle response and better economy however.

Good luck.

Paul

 

Yes I called them and asked about there warranty but the dist was already past warranty the truck is a weekender so years past over miles I say it failed around 500 to 1000 miles and I'm only pulling 22max HG I seen alot of post on you tube many failed sooner , and they sell a adjustable one and they said that failed easily as well some do some don't mine did ...I asked them about how to remove the vacume advance he sent me a PDF file sure enough you remove the dist from the engine .
Pertronix sells good stuff I just think the out of USA made items the quality is not there anymore if they used thicker rubber inside I think it last 50 years like the USA made ones ...

 

Finding out why ?? Is they key I lost 2 already I'm going to try the standard V31 it said they tested it to 34hg not to leak and it has a 3 yr warranty , I am also going to install a check valve between the manifold vacum and the vacum advance some say that helped them .
And I might use a vacum pump and install a stop in the arm of the advance I'm getting 21 hg from the vacum advance with the 8 initial it was 29/30 timing of course in drive it's less but I'd like to get it to 16/18 in the vacum advance so I can use a little more initial timing seems it like 10/11 initial with about 14/16vacum I'd be around 24/26 ...

 

Do you have the vac can hooked up to full manifold on the carb, or on an intake runner port? If the latter I could see pulsating vacuum eventually tearing the diaphragm, but if you're hooked up to full manifold on a carb port it should be pretty steady and oughtn't damage anything.

 

The 500 CFM carb had a full vacum port and a above throttle plate ported /port the 350 does not it only had above throttle plate ported and the rear full manifold big port they both failed using the 350 CFM carb the first time I had it hooked to the mainfold now I have it T into the rear of the carb with the power brake hose it might be pulsing or surging ? Back into the diagram instead of a constant vacum . I heard some use a check valve in between so it won't surge I'm going to try that .

 

I don't like to run it ported since there's such low vacum at a idle above the throttle plates, closed 0 advance from the vacum advance untill you crack the throttle your getting more but very little then to run better I need to increase my intial and actually put a weaker spring in the front end of the dist so I can get additional 5 degree or so.
that's like 10 initial and 5 springs im at 15 at idle like to be 24/25 just can't get that with ported unless I take the spring off completely heard of people doing that as well.

 

I have a little rubber oozing out the sides that's not a good sign also in the back there's a stop there so I don't think it over extended it .
i think its not dealing round the edges also that spring is stuff I think it would need min 10 hg just to move it at all that's why ported the port above the throttle blades gives you 0 advance at idle from the diaphram only once you crack the throttle do you get any advance ...

 

It's not excess oily but besides the cut marks I used to open it seems like it deteriorated around the edges now I only have maybe 300 miles on it

 

Ok I just got the new one and it looks alot heavier in the rubber diaphram area it doesn't have a stop like the other one but it's ajustable it wasn't listed as ajustable but instructions said it must be ajusted prior to install so you turn the screw clockwise to stop then turn out counter clock wise according to your dist # which I can't see



Going to try the check valve in the system it seems the more turns counter clockwise the stiffer the spring gets so I guess the stiffer it is the less amnt of advance you will get .

 

Ok just updating you all on he progress pretty easy the VC31 from standard is ajustable although it is not listed as such the more turns clockwise the less advance it gives I have it set at 11 intial and then ajusted the screw to give me 24 /25 with vacum in gear it's around 20 ....before I had 8 intial and the can would give me 21 degrees so I had 30 which I though was too much advance if I went to say 6 initial it would start like crud so I will try it this way I have no problem going up to 12 initial .
im giving it less advance with the check valve so no oily vapors can come back to the pod , I might think about getting a dist boot soon to protect it from heat but not sure if that was ever a factor anyway .

 

I find it interesting that they claim to have tested the vacuum canister to 34" of vacuum. Assuming they're talking about inches of mercury, what they're claiming is impossible. A perfect vacuum at sea level (meaning, not even one molecule of air in the canister) is just under 30" of mercury. You can't have less than no air in a container, so at a minimum, they're playing fast and loose with the facts.