I had the same problem with my kz 650 many years ago. The problem lied in worn out ignition coils. The bike would run fine for about five minutes then just die. Making things worse was the fact that it would show spark and fuel and compression but still not run. Purchase a new set of coils, Accell or Dyna, recheck all settings in timing and carbs. The problem is that old coils may have overheated and literally split open, allowing the dielectric oil to spill out, rendering the coils useless. Try that. Unless the bike makes terrible mechanical noises, it just needs more bolt on parts(har har). Good luck