2010 kx250f WON'T START

Look I understand that no body wants to hear this about their bike, but...
Valves can have the correct clearance, but still be leaking. That would explain why it will bump start, but not kick start.
How old are the valves?
When was the last top end done?

Paw Paw

Ensure the timing is correct. At TDC, the exhaust cam shaft lobes should point at at "10 o clock" and the intake cam lobes at 2 o clock. At the same time, check that the dots on the camshaft gear are in their correct positions at TDC. There should be 12 pins on the cam chain in between the top dot on exhaust cam and the top dot on the intake cam. I've seen the cam gear slip on the cam shaft many times and it can throw off the timing slightly. This will make it hard to start. The circular hole in cam shaft gears should line up with the lobes of the camshaft.

If timing is correct, there is probably a problem with the surface of your valve seats. After long hard hours, the KX250F is known to have the valve seat crumble or crack. This crumbling seat inhibits the valve to seal and therefore compression is compromised. A leak down test will tell the tale if you don't want to remove the head and valves to check.

You can try getting a new pilot jet (stock is 42, I liked the 45 better) if you think it is a fuel issue. But considering you say it idles good, I don't think this is your problem.

Did you replace the cam chain?
Also check the neutral safety switch down by the shifter. If it is bad it will not allow the engine to start.

Paw Paw

If youre sure its in time, try running race gas in it. My KX450 all the sudden quit starting last july or so. Fresh top end, valves, all that. The only thing it will run on is race gas. I get CAM2 over at speedway. Also known as Sunoco 110. Give that a try and see if it wakes up. For some reason they just do not like pump gas.