Originally Posted by Gogar (Post 7020736)

Install a popoff valve into the new airbox, on the bench, before you put it in. Easy Squeezy!

Originally Posted by theweaver (Post 7025859)

Hi, I am interested in this, as I was only told about this few days ago, because I am changing my HT leads which have the braiding fitted. Obviously the new ones do not. 1980 SC. It doesnt look like I have any mods done already to the airbox.
Is there a link which shows the position of this valve, and also what does it look like.

I mentioned the braiding to the supplier of the replacement HT leads, he said it was because in Germany, owners complained of rodents biting the leads when parked up.....The mind boggles!

Thanks

Originally Posted by scarceller (Post 7309310)

Thinking about why you get a back fire in the intake is most likely because some spark plug is igniting while the intake valve is opened and fule air mixture is being ingested into the problem cylinder. So what can cause this?
1- Spark jumping from rotor to the wrong wire in the cap
2- Erroneous trigger signal seen from the points at the wrong time
3- Or simply the CDI deciding to fire with out a trigger

Of these three possibilities I think #1 is most likely. These are CDI ignitions with plenty of voltage potential and I could see the possibility of having the rotor place it's arc to an adjacent post in the cap rather than the target post.

#3 could maybe happen if battery voltage is significantly low enough?

Just theory on my part.

Originally Posted by HarryD (Post 7309385)

Stand on the shoulders of giants and see Peter Zimmerman's posts (10 and 11) in this thread http://forums.pelicanparts.com/porsche-911-technical-forum/689763-blown-airbox-replace-prevent-while-im-there.html to understand about CIS operation and pop off valves.

Originally Posted by timmy2 (Post 7027685)

Top and bottom are glued together with unobtanium glue....
Drop the engine and remove the entire cis system.

Originally Posted by Marc Bixen (Post 7030961)

I've seen both sides of the story. In the 80's I worked for the company that held the patent and manufactured the Pop-off valve, we were also a Porsche repair shop. Every airbox car that came in either had one or got one. We shipped out 100's per week.
Then in 1995 I started working for Pete Zimmermann. Pete explained as he does in his book, that a properly tuned, well tuned, 911 doesn't need a pop-off valve. Well don't you know that 17 years later I haven't replaced one Red Line Serviced cars air box that was equipped with a late model air box, but I have had to rebuild a few pop-off installed motors for burnt cylinders obviously caused by air leaks/ vacum leaks, at the pop-off valve. Late model refers to SC and earlier replacement air boxes that have the cold start distribution manifold installed. Generally these air boxes have phillips head screws visible in the air filter chamber area. If you have slotted screws than you have an earlier airbox without the cold start distribution manifold.
If you have a pop-off valve, check the epoxy seal at regular intervals and also the condition of the rubber o-ring. I've also seen pop-off valve stick to the o-ring and basically be useless to there purpose. We've had a number of tow in's for no start with pop-off valve cars. These include valves not closing after, valves coming loose, or even valves being installed backwards and having the flip get stuck in the air filter screen.
Moral of the story, keep your car in proper tune and you don't need the gimmicks. Also as mentioned in "The Used 911 Story" Airboxes are a plastic/nylon material, they do get old, they do get brittle. If your rebuilding your motor, ALWAYS install a new air box, especially if you have no history, if it's over 60-70,000miles old,or it's an early style (slotted screws).:cool: