SC pinking and wierd timing light issue

[911mot]

Here's one for you...
My friends 83SC (Euro) has started pinking at around 4k. I wanted to check the ignition timing, so got out my digital snap-on timing light. Connected it up as normal but it was flashing waay too fast and the rpm meter said about 5400.
I discovered that if I have the engine running and the timing light connected to 12v , then as soon as I put the inductive pickup *anywhere* in the left side of the engine bay (without touching any part of the car or engine) it starts flashing like mad and reading 5400rpm. Right side of engine bay no problem, I can put the inductive clamp right next to a lead and nothing unless you clamp round it.
We are going to try a new set of HT leads, cap and rotor next week, but until then if anyone has any thoughts as to what might be causing this and if it could also be the pinking issue, I'd be very grateful. In 10 days time the car is going on an 800 mile trip to the Nurburgring. The last thing we want is pinking

[Early_S_Man]

Well, you have some contrasting issues there ...

The '5400' rpm signal pickup sounds like an open or intermittent Beru spark plug connector issue, and I wouldn't normally believe that could cause knocking or 'pinking' ...

A change of the cap, rotor, and cables should bring everything back to normal.

He wouldn't happen to be running an MSD box, would he?

[Paulporsche]

Warren,

Does the MSD box have some problem that can cause this?

[Early_S_Man]

Paul,

Below the 3000 rpm - 3500 rpm switchover point ... MSD output does cause erroneous tach readings if they are triggered by the sparks from the coil. If the sparks aren't getting to the spark plugs, and are just running around the 'ether' in the left side of the engine compartment, who knows???

[Paulporsche]

Got it. Thanks.