main wiring harness is melted and wires to bottom of carb is melted. any ideas would be great.

 

The 300 has an electric carb heater just below the carb for easier cold starting. If the (+) wire for the heater rubs against some metal around there, it usually blows the main fuse. Not knowing why fuse keeps blowing, people put a higher amp fuse or even a paper clip or metal piece, then there you go - melting the wiring harness.