CIS Pop-off valve

I have seen these mentioned in technical articles and advertised at Pelican and Automotion. My 81SC does not have one, but has not since I owned it ever back-fired.

Mine is a European spec SC and I was wondering if this was an issue only with the ROW models, or have I just been lucky???

Any advice appreciated.

Neil

According to Bruce Anderson, changes in the '81 injection eliminated 90% of the backfires.

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Warren Hall
1973 911S Targa

many thanks and a happy Easter. Is it worth the expense (1NZ dollar is only 50c US)?

Neil

Hey Neil, I can relate to your conversion rate woes!

Neil,

I have a 1981 911SC ROW and even though Bruce Anderson says that 90% of the back fires what happens if you are the 10%. I was and luckily I had the pop-off valve and it saved me a chunk of change. I think that it's a good idea.

Dave
1981 911SC
1984 Alfa Spider

I installed a pop-off valve in my 83SC.
I considered it a cheap insurance policy
against being in that 10% that has the
problem.I did it myself, it was fairly
easy.The valve was about $50.00,new airbox
$300.00,not a hard choice for me.

I'm putting one on my '83 SC this week. It has 145k miles and no explosions (I believe), but I'm not going to gamble. Met someone two days ago that blew an airbox. He says I'm doing the right thing. I would not agree that the airbox is an easy thing to remove and replace, on top of the $300 cost.

For $50 and 1/2 hour easy labor, the popoff valve is cheap insurance, even on the later SC's which don't have the blowup as much, but can and do still have them. Any change in your FI components (i.e. wear), vacuum lines, atmospheric conditions, etc., can spark a backfire!

A blown airbox is a MAJOR pain to replace! And, the cost of a new airbox is high.

Its a cost/risk analysis, even if the risk of blowing the box is fairly low, the cost of the valve is also low, and the cost of replacing a blown box is very high. So, I would not feel comfortable driving an SC without the valve!

Thanks for all the views (pretty consistent!)

I will be sending my order off this weekend!!

Paul, have you found any good (inexpensive) suppliers of parts in Australia?

Plus, did you see the Kangaroos whip our b###s in the league last nite. You guys are just too good!!!.

Neil (I wish I was paid in $US)

Let me ask a question regarding the question of wheather or not the "pop off valve" may or may not be required.

When I first purchased my porsche (1978 911 SC Targa 3.0), the car wouldn't run right and it would backfire from time to time through the air box. I did not have a pop off valve installed.

Looking at the design of the mixture control unit, there is a flat metal leaf spring (haynes manual pg 256) which the operating lever rests on. If I am looking at this correctly, the adjusting screw for air flow raises or lowers the leaf spring and in turn this raises or lowers the operating lever.

It it possible that this leaf spring acts as a releif when the engine backfires through the air box. That may be the reason why I have not had any damage to my air box when the engine backfired.

I have had quite a few occasions where it backfired but it never exploded. Since then I have added a "pop off valve" but I now I am wondering if it was really needed?

Regards


Steve

Interesting. A relief spring. Well, I too am going to install one. Apparently, these things do blow, and they really come apart. I read somewhere that some guy thinks he can remove an airbox in 45 minutes. I'd buy a ticket to see that.

I hesitate though, because I am (engine removed) working very hard to eliminate all possible vacuum leaks everywhere because I consider that important. If the seal on the popoff is not perfect, I will have air bypassing the airflow plate. I'm not worried about the glue joint. You can bet I'll keep the o-ring clean and lubed though.

Also, erratic idle and other stuff can be caused by a displaced Sensor Plate (haynes p.256). Fuel injection parts there are delicate.

By the way, I think part #33 is where the mixture adjusting screw is. that independent arm touches both the big arm #32 and the actual Fuel Distributor Plunger (the parts in his hand on p.260).

You are right, the screw that is shown on the end is adjusted through the air flow sensing housing.

The operating lever (#32) does raise and lower the plunger in the fuel distribution block.

As far as the pop off valve goes, I installed mine and the way it is set up, you shouldn't have to worry about any leakage. When I did the installation, I used two part epoxey to seal it to the air box. Used a hole saw to make the hole, and used a dremel to grind down the ribs and smooth it out. Havn't had any problems at all.

Regards

Steve