Busted pop-off valve top

[npresley]

Went to start my '77 911, it backfires, and then won't start. I pulled the air filter and found the lid to the pop-off valve no longer connected. I have purchased a new one, but wonder how to replace. I am thinking of just replacing the lid instead of the whole valve. Any thoughts?

[Guardsred911]

I had mine backfire and crack the seal on epoxy. I think you can easily get a pair of needle nose pliers in there and work it back and forth to get it out. Then re glue a new one in there with two-part epoxy.... I just used a vacuum with a narrow attachment to get any plastic chunks out of the chamber below the airbox.... Hope that helps ( and makes sense)


I would say if the seal is good, you could just replace the lid. If you can get the valve body to move, just reglue using the above comments
J

[porsche930dude]

youll have to take a closer look and see if the pin comes out. if its crimped in permanent you can cut it out and use something else for a pin. seems doable. if its really glued in there well you may not have a choice but to do that. I cut the ribbs on my box so it sets down in there and glued in there with lots of surface area. i dont think it would come out without atleast some collateral damage

[M.D. Holloway]

interesting...why do you suppose it broke?

[pors1968]

probably the lid was touching the air filter

[M.D. Holloway]

hmmm now I wanna go check mine...I always wondered if it touched or not. Think I'll go measure.

[pors1968]

half time they are intalled wrong way.Small backfire only open half way Big one you blow the lid or they jamme open .Check for light burn on air filter.

[npresley]

The filter has a blown out place in it. The lid opens with the hinge to the rear of the car. The ribs do cause it to sit high. I can't break the broken one (it's already broken), I can see what happens with the hinge pin.

[pors1968]

a way to do it (before to glue ) cut by half a used air filter tape it on the air box and check if you can lift the lid whitout touching the air filter.

[copbait73]

Those damn valves cause as many problems as they solve.
I pressure tested two separate manifolds and both leaked. Wanna guess the symptoms that causes...lean start backfire.
I turbo'd my 73.5 CIS and used an aftermarket stainless box. It leaked also but I welded it shut and have never had a backfire problem since.

[npresley]

I've been thinking about going full EFI & this helps me lean even more to EFI.

[weseeeee]

I recently had to replace one that broke after a back fire (I replaced the top and bottom of the pop off valve) After posting on this site I learned that it is a common issue. One of the posts was from a guy that stated he now carries a rubber stopper that can be used to seal the air box hole if this happens away from home. I now carry one, cheap short term fix.

[npresley]

like a bath tub stopper?

[Tidybuoy]

You probably should also consider determining why it backfired in the first place. A perfectly tuned engine should not do that and there is a good chance that you replace the pop off valve and then it happens again.

[porsche930dude]

mine usto do it and i adjusted the mixture just a hair and it never did it since

[HarryD]

Tidybuoy: Several issues, for us guys with early cars like yours and mine. The design of the air box causes some of the backfires due to location of the cold start injector and sir flow. It was corrected in later (non-interchangeable) boxes.

I have had my pop off valve come entirely out. I decided to use RTV to glue it back in. Why? To my way of thinking, if the backfire is so forceful to cause my valve to be ejected, if it is glued too firmly, the box could break before the valve relieves the pressure. I would rather the glue joint fail. Also, since RTV is pliable, I merely need to push the valve back in to reestablish the seal and drive on.

[Howard M]

Temporary rubber plug for emergencies: Freeze Out Plug, available from any auto store. In my glove compartment. Buy one and you'll never need it (Umbrella Effect).

[copbait73]

The leaks I found were at the valve body to air box joint.
Locktite makes an epoxy for this type of plastic.
This is the only product I've used in similar applications that should hold up over time.
I'd still bet most of the valves out there leak.

[npresley]

porsche930dude, do you remember which way you adjusted? Leaner or richer? Ultimately, the temperature changed here. I know my WUR doesn't have the best of pressures. I need to do an oil change and valve check.