After 160k miles it would be sensible to have a look inside the distributor to see how it is bearing up. The belt needed doing at the very least as there was no evidence of it being changed and no vent kit in place.
The main things to check are the belt which transfers the rotation from primary to secondary shaft as it can wear and snap causing a load of problems and the bearings as they can dry out and seize up. There are 3 identical bearings and 1 unique one which are all available to buy if you hunt hard enough.
Pin removed 
Evidence of failing belt… 
…hmmm 
Stamped ‘Z’ so has already been rebuilt
Teardown is pretty well documented so no need for further detail here but this shows all the parts separated and ready for refurb.
As I started regressing / reassembling the dual distributor I noticed that only the primary centrifugal advance seemed to be working, the secondary shaft advance seemed to not be working correctly (if at all). To investigate I tore the distributor down further to bare parts.
Looking at the various parts and comparing the working advance against the non working one I suspect the lubrication had failed due to heat and centrifugal force so I’ll clean everything up and re-assemble with fresh grease.
View the parts in more detail and its obvious that the secondary advance hasn’t been working for quite a while as it has much less wear on all the components compared to the working side.
Also worthy of a mention is that Porsche must have adjusted the design when sizing the secondary shaft spindle as they have had to sleeve it up to the correct dimension using what looks like a plastic shrink-on type modification – might also explain the secondary advance seeming to be a bit lazy!
