CIS Pop Off Valve Problems, Help!
 

When I had my engine rebuilt, they also installed a CIS Pop Off Valve in my '77 911 2.7L engine. Recently, normally while accelerating under 2000rpms in 1st and 2nd gear, I can hear the valve opening making that poping sound from time to time. Twice now the valve has opened so far that it got stuck on the air filter I guess and I had to take the cover off to let it loose again. What causes these frequent backfires? What do I need to do? Thanks.

Check your ignition timing and ignition points dwell (gap). Is your tach needle bouncing? This is a sign of dwell being off. Then consider your valves; were their clearances readjusted after your rebuilt engine was "broken in"? Jim.

Replace yoru cap and rotor, and make sure your mixture is not lean. Lean mixtures cause intake popping. Worn distributor caps and rotors cause crossfire.

If the Pop off valve is oriented wrong it will get stuck on the air filter when it "pops". As far as why it is popping driving down the road I will leave that to the pros.

it sounds as if the valve may be on backwards? it should relieve internal presure and move out away from the air cleaner to expell presure. it sounds as if yours may be opening when the cis really starts drawing air, creating a negitive presure in the air box that will draw open the valve.

Your valve is most certainly installed backwards, allowing vacuum to open it.

it's bad in two ways:
1. it won't save your airbox from destruction when backfire happends
2. it will allow dirty/unfiltered air to be drawn into engine.

It shouldn't open inwards but outwards.

Even more trouble. It did it again today. But I guess the epoxy or whatever the rebuilder used to hold the pop off valve in wasn't stong enough, because when I opened up the air box, the valve was rolling around in there. What does this mean? Was there a really hard backfire that just blew it out? CRAP! I need to make an appointment with the mechanic ASAP. BTW, has anyone else ever had a similiar problem?

I just observed Doug's at the engine pull.
The valve is installed in the air box just under the filter at the left of the housing.
If you take off the filter top cover....pull out the filter...you should see the valve on the left side of the box with the hinge facing toward you.
The flap opens upward...so that the flap of the valve would hit the filter in a flat position...therefore can not "catch" the filter element.
And yes...it is epoxied into position.
Bob

Ok, so when I make the appointment should I tell them what is happening and have them check the valves and the timing? Are thoise two things the most likely cause?