Bought a used 2001 yz 426f last week and have ridden it twice without any problems. This past Saturday, it ran great in the morning, but that afternoon, it got to the point that it would not kick start. I drift started it and once started, it still ran great, but when the motor was shut off, you could forget about kicking it. I changed the spark plug but that didn't help. Any ideas about what this could be. Thanks.