911SC Poor Running & Distributor Signal Phasing Problem
 

Over the years a numbers of Porsche mechanics have experienced poor running
conditions at RPMs below 3000 and incorrect distributor alignments relative to
Z1 on the damper pulley on 911SC engines. This usually results when the distributor
has been rebuilt or the green wire replaced.

The problem is caused by the incorrect polarity (phasing) of the distributor signal to
the CDI unit. Since the unit triggers the spark on the negative going edge of the
distributor signal, that edge must go negative in a short time relative to the spark
timing. If the signal polarity is incorrect and the triggering signal changes too
slowly, the timing will be incorrect relative to the low and high RPMs. The problem
is easily corrected by just reversing the pins of the green wire.

The ideal method for determining the correct polarity is by using an oscilloscope.
The negative going edge must be a faster voltage changing edge of the signal.
Once the signal has the correct polarity, the static timing relative to Z1 and the
dynamic timing (timing light) will be correct and the engine will run smoothly
and rev very responsively as the RPMs are increased.