tach

on my 71 240, the previous owner put in a L28, and a Mallory Promaster coil. how should I hook up the wires to make the tach work?

this is how i have it hooked up now;the car runs, but the tach doesn't work:

------I

n...-.I====B==to distributor==

.......I

.coil.I

.......I

p..+.I====BW==to wiring harness==

.......I

------I

1=Covered BW to harness

2=GW to harness

I---1--------2---I

I......................I

I......................I

I...Resistor......I

i used a multimeter to check for continuity, and found that the covered black and white wire goes to the tach, and the uncovered one goes to the ignition switch. however, if i connect the wires like the wiring diagram says (switching the two BW wires), i get no spark @ the coil. what's going on here??