This ahs been ocvered before. THe weber meeds a lower fuel pressure than the internally regulated pressure from the mitsu fuel pump. The mitsu pump has two outlets and one suction. One outlet is regulated to around 7-8psi and goes to the mikuni float bowl (high front center of the carb). The unregulated bypass goes to the lower dside front (iirc) of the carb, thru the auz accel pump fuel reservoir, and then to the fuel return line to the tank. You need to put another pressure regulator on the first regulated line set to the pressures noted above.

What is happening is that the fuel pressure is pushing the needle off the seat and overflowing the fuel bowl into the carb throat. The constantly over rich mixture is carboning up the combustion chamber. The carbon deposits get white hot, and ignition source, and the leaking fuel and the ignition source make the engine diesel. Fix the fuel pressure, and then fog water spray into the carb throat with the engine at high idle to remove the carbon deposits, preferably with the exhaust disco'ed from the cat.