Nissan Repair: 200sx, shaft seal, computer scanner

Question
i have a s14sr20det and it has a slight miss at low rpm mainly between 2000 and 3000 rpm but when the turbo kicks in its  mostly fine but a very slight miss at high rpm.
iv just had it serviced and checked over.the mechanics couldnt find the problem.the car is stock standard.can you help me?

Answer
Jake,

Check the distributor for the presense of oil on the optical shutter wheel.  This is under the rotor post and cover.  If you have someone with a dyno you could run the car and observe the engine's performance with a computer scanner.  That would verify what is going on.  With the optics getting oil on them the dwell time changes and even cuts ignition pulse.  If there is oil on the optics you change the distributor with a rebuilt unit.  Now, if there is oil and since it is a turbo you might do a leakdown test and check the condition of the piston righs.  Blow-by is the most common cause of the shaft seal blowing in the distributor.