1980SC Mysterious Issue

[lada03]

I have a 1980SC, in April i got it serviced. Normal oil change, and a slight air/fuel adjustment by a reputable Porsche mechanic.
I drove the car home and it ran amazing, parked it in the garage and did not touch it until this past Saturday. So here is the issue:
It would crank over and then die, crank over and die after a second. I tried continuing to crank by turning the key/giving it gas and it would cough, crank, and the RPM would go to MAX rapidly and then come down and die. It scared the crap out of me. The RPM went up without me pressing the gas pedal.
I called my mechanic, he said to check if the mechanical throttle was stuck and move it back and forth. Also to check if the oil cap was properly tightened. I did and both were fine. He then told me to remove the air filter and check the pop off valve. Everything seemed fine. Before putting the air filter back, i cracked the car and it started right up.
It idled perfectly at 900. I put the air filter back on and the again started fine.

Everyone is stumped including my mechanic. He thought maybe the pop up valve base was loose and the old glue wasn’t holding, but it wasn’t the case.

Any thoughts?

[timmy2]

Best guess...
Fuel distributor plunger stuck due to no use.
Freed up and fine now.

[lada03]

But how did it free up? All i did was remove the air filter
Before that, i spent half an hour trying to get her started

[proporsche]

take a picture and show us how is your pop off valve installed? if it is facing the wrong way it could get stuck open and the car will start and die again...once you removed the air filter it got closed.
the opening has to face the front of the car not towards the back..
Ivan

[lada03]

The pop off valve faces the air box like in the picture below.
In my 7 year ownership i never touched it or had this problem

[timmy2]

Backwards

[Bill Douglas]

Yep, backwards, backfired and got stuck against the aircleaner and didn't come back down until you lifted out the aircleaner.

[lada03]

Hmm.. i will leave it as is, maybe it was a freak occurrence getting stuck. If it happens again, i will address it

Thanks Everyone!

[Bill Douglas]

Ha, it sounds like you have the fix in place, lift the aircleaner and pop it back down. Easy

[proporsche]

u r welcome;-)
Ivan

[mhackney]

Ideally, the pop off valve should not have popped off. Sounds like you need to sort the reason for the backfire. Also, I noticed you said you pumped the gas while starting, isn't that a no-no with these cars for exactly this reason - backfire?

cheers,
Michael

[lada03]

Im not sure if i did more damage or good when pumping the gas. It seemed to help it crank over easier, and when it did, loud RPM rev

[john walker's workshop]

Pumping the throttle on a CIS car does nothing but open and close the throttle valve. No fuel squirt like a carbed engine. It does increase the possibility of a backfire though.

[lada03]

And it did backfire pretty loud

[mhackney]

Mystery solved.