I have a 23 hp kawasaki that is hard to start. Have to crank for about 30 to 45 seconds before it will fire. Black smoke emits then it will smooth out and run fine. It keeps burning the plugs sooty black. I had it in the shop and they couldn't figure it out. I have replaced the plugs, air filter, pre filter, new carb, two new coils, the diode wiring harness and the lead wire. Still no better with starting. Very frustrating. Can anybody help.