I did a basic top end rebuild on my 2008 kx450f, cleaned the carburetor, and installed an hour meter last week. The bike runs, sounds and idles perfectly, but the head pipe began to glow during my first few startups. I was not alarmed because I know that the break-in requires a few heat cycles. I now have about an hour on the motor, but the coolant still boils over after about 5 minutes of mild riding. I recleaned the carburetor and realized that the plunger for the hot start was stuck (now fixed). I also made sure that the airscrew was set to the stock setting- 2 turns out. The plug indicated a lean mixture before, but I have not checked it since my recent 1/2 hour ride. What else could this be??? Or is this normal???