Gm hei

Big D said
Try swapping the distributor wires and rechecking timing. Oddly, one works and the other doesn't. In the HEI conversion on my Duster, the correct connection never let the ignition rev more than 1300 or so, and by switching the connection, allowed free revving and starting.

the reason for this is that it changes relative rotor timing, called "rotor phasing." The change in the pickup pulse moves the timing and the spark now longer fires when the rotor/ plug towers are aligned.