Re: KX250F: Header pipe RED HOT
If the bike has started leaning out, it could literally be anything from cruft on the petcock filter to leaks around the exhaust pipes. That includes valves (worn, damaged, or out of adjustment), cylinders (lower compression causes lower vacuum), carb mounts, intake boots, cruft in the idle jet(s), etc.
Check the plugs - those should give you a good clue.
If the bike is running lean, the plugs will tell you. After that, if you tell us, we might be able to say something other than "could be almost anything."