I have a Kawasaki fc420v. I removed the head to have a helicoil put in as the sparkplug threads were messed up. I replaced the head and installed new plug. (I had a problem before I removed the head. The engine shut off after running 15 minutes on one property. I was able to restart and finish mowing. A few days later I tried to use the mower again and it started, ran about 90 seconds and has not started since.) It is getting fire and fuel, but will not start. The engine is on an older Exmark 36" walk behind. Suggestions?