Voltage reg melted

[Joker2]

The other day while underway doing I don't know how many RPM's because my Tach started jumping all over the engine just quit, look back see smoke coming out of the vents in cover, smell plastic melting, pull cover see little flames coming out from under the plastic cover that houses the voltage reg. shur fuel valves get fire ext. squirt, squirt again, not out, squirt again unscrew cover squirt till out, big plug on top of volt. reg. and all wires down right side of motoor melted, volt.reg. toast. My mech. changed it last week, ( put back old one ) overcharging ( 18 V ) with the new CDI that was in there. Also when he changed the reg. we had the control box apart to try and get the fast idle lever to work ( no luck with that ) While I was underway and shortly before the engine quit I lightly touched the wire harness where it goes into the control box and the voltmeter ( new ) went to 12 ( normally 14 - 15v ). Mech. told me he had trouble getting the Tach to work when he originally gave me the motor. He had wiring trouble. I think there was a short in there somewhere, actually I don't know what to think. any ideas