Just got my self a 2005 kx250f was runing fine one day and now i cant start it changed air filter spark plug and cleand the carb it back fies alot

Does it have spark? Will it start if you spray fuel into the air cleaner?

Clean or change air filter. Clean, gap or change spark plug. Empty gas tank add fresh gas with Stabil. When spark plug is removed pull rope several times to blow out cylinder. Spray starter fluid in spark plug hole before replacing spark plug. Clean outside of carb so choke works freely. Spray starter fluid into carb breather hole with air filter removed and try to start. if running replace air filter and cover. If no start disassemble carb and clean. If no start replace carb.

clean your carb and change needle and seat also

You either have a air leak in a fuel line, carb jets are plugged, or carb seals went bad. They sell a rebuild kit for your carb, 10-15 dollars. I would start by removing carb spraying it with carb cleaner and blowing it out with a air gun, put it back on and see if this worked. If not I would replace fuel lines and rebuild carb.

try doing a idle drop procedure. this will help out alot.

If you are requiring more fuel and it isn't getting there, look for a clogged fuel filter. Also check air filter and carb float bowl for crud build up from gasolin.

If you have spark, try changing the fuel filter. If you don't have spark, get back to me.

Remove spark plug, clean and dry. Replace plug. Remove air filter. Spray WD-40 into carb opening; one second blast. Turn over engine, (pull handle). Engine should start briefly and run for 3 seconds.  If not, you are not getting spark to spark plug. If engine starts, you have a dirty carb and you can buy a rebuild kit from manufacture.