MSD Rev limit
 

Just installed a new MSD unit into my 78 911 SC. Also installed the Tach adapter. Car runs real smooth and the tach works find. The problem is the engine will not rev over 4000 rpm. It seems to hit a rev limit. I have changed the rev limit chips from 6000 to 7000 rpm with no change, still 4000 rpm limit.
Any ideas?

Check your wiring.

and check the type of and age of the coil

Got that problem taken care of. Removed the tach adapter and everything works find. I guest you really don't need the adapter

JLW,

Can you measure the resistance of that tach adapter with a digital multimeter? Something tells me it is just a pullup resistor ... from the schematics that have been posted!