Help - Tachometer Not Indicating Correctly - CDI Related?

[t6dpilot]

OK, I did a search and could not find any info on my specific CDI unit, so I thought I would throw it out to the community for help. I have a '71 T that utilizes an Autobahn Permatune CDI unit for spark. My issue is that the tach seems to lag a little between 2k and 3k, but it really lags above 3k. In fact, it really does not get above maybe 3900 rpm. I am convinced that it is electronic in nature and related to the CDI unit and the translation of real rpm to the tach. Any suggestions?

Thank you.

[tt_targa]

The tach integrates an electrical signal.

If the voltage of your system goes wacky, it can throw off the tach. My regulator shorted which allowed the system voltage to climb about 13.5 V. This affected my tach performance.

The Porsche charging indicator only lights on undercharge, not overcharge

Your problem might be related. Shot in the dark.

[Early_S_Man]

I suggest monitoring your system Voltage ... with a permanently installed expanded-scale Voltmeter such as the VDO Cockpit series model.

Assuming that you are still using points ... if it isn't high Voltage due to a faulty Voltage regulator, the points can cause flaky tach operation.

The tach shares the input signal to the CDI unit, and it is very, very unlikely that the CDI unit can do anything to the signal that is presented to the tach.