Is it probably the ECU?

nm9stheham said
What coil are you using?

BTW, when you originally changed timing for the orange box, how much did you have to change it from the MSD setting? It would be no surprise at all for there to be timing differences, based on circuitry differences. They could easily have different internal delays or even trigger off opposite edges of the pickup pulse.

Any way that you could have flipped the polarity of the pickup connector on the distributor when you installed the orange box or the new box?

1. Off the shelf from parts store. Not sure of the brand.

2. Timing was several degrees out. Would barely start and ran like crap. Fellow with dyno was the one who realized just what you said... MSD was triggering on opposite edge of pickup.

3. Don't know how to flip polarity. Just simply unplugged and unbolted orange box from inner fender, then plugged in and bolted down replacement box.

The stumble is at low rpm from standstill. At 20 mph, kickdown does its job and it goes.