Walt @ CE, Jerry, and Bruce are all correct; Porsche installed a batch of defective outer valve springs in the 79-81 model years and this has been well known problem for many many years now.
I started seeing those in 1980,...
I tell anyone with an SC of that vintage to press on each valve with your thumb at valve adjustment time. If you can push the assembly down manually, the outer spring is broken and requires replacement. This can be done with the engine in the car,.....
You'll hear a ticking (much like a loose valve) when a spring has failed and its VERY important to fix this right away.