Twin plug experts, need some help
 

Just installed a twin plug, Mfi, rs spec 2.4, now 2.7 7r motor in my 72 hotrod. Car had been running a hot 3.0 cis, single plug motor with 6 pin cdi

So, I got a second 6 pin cdi and a burnham bros distributor that was set up with the magnetic pickup like the stock sc distributor. Followed all the directions and all is good, mostly. Car runs well.

The only trouble is the tach that was working with the former setup no longer works at all. I swapped in a known good SC tach and it works at idle only. Once you get above 1200 rpm, the tach bounces all over the place, and by 1500, it drops to zero and stays there.

Any idea what's going on?

I forget where you need to pick up the tach signal bit it isn't from the purple/black wire where you usually pick it up. For some reason, I think it was one of the coil wires but don't quote me on that.

I really wish Burnham would supply a wiring diagram for his system!