[Bob Ashlock]

The 1970 Tach needs a high-voltage pulse to operate. With the original 1970 wiring, this was derived by a filter network (intermediate/ballast unit), a small round can on the electrical panel. One side of that network connected to coil-positive and the other side went to the tach (black/violet wire). From what you describe, you have the black/violet connected to the distributor. That doesn't sound right, so maybe there has been some prior modifications.

The MSD has a specific tach signal output and unfortunately, it is a low-level (12V) square wave. So direct connecting that wire to the black/violet will not work. You need a device that will convert that 12V square wave signal to a high-voltage pulse. I'm not sure the 8910 will do that for you .. I believe it is supposed to, but I get repeated complaints that it does not always work properly. What does work is the TACH-ADAPT unit that I manufacture and is sold by our host. Pelican part number PEL-TA-V3. The output of the TACH-ADAPT unit is designed to directly drive your tach and therefore the intermediate/ballast unit (if still present on the vehicle) is not used.

Apologies for self-promotion, but if you have no luck with the current setup, and in the interest of getting your problem solved please consider the product.