There are some really old posts about this and what the problem was. You will have to do some digging in the old "B" archives.
OK, I did some digging for you and came up with this.
"I believe there is a resistor inline at the start panel that has failed. I remember reading post(s) with the same problem. When you release the start button, it cuts off the ignition system. The resistor tells the "board" to keep the unit running."
This is what my service manual say's.
1. Low oil level.
2. Defective low oil level switch.
3. No field flash volage due to:
a. Open circuit in wiring.
b. Brushes not making good contact.
c. Slip ring surface is rough or pitted.
4. Defective generator, control assembly
A1, or voltage regulator VR1.