lLow Tachometer reading/output need advice

[Adidas]

The tachometer on my 75 911 reads at least 1000rpm's too low. The engine reads at 800-900 rpms at idle speed with a hand held tach meter, however the car's tach isn't above 0. The tach is functional and increases as the engine speed increases but still is about 1000 rpm's too low. I've tried another tach with the same results. The distributor points are clean and the dwell is accurate. Could I have a bad coil, CD unit or wiring problem? I have the o.e Bosch CDunit which appears to work(it hums when the ignition is turned on). What do I try next. Thanks to everyone who replied to my last post. Bill