My bike is a 06 yz250f its getting very hot .

does the bottom hose not get hot? start by replacing the coolant thermostat, the bottom hose must get hot

could be your head gasket , blown hose and overheating point that way

The thermostat is stuck closed replace it and the problem is fixed. I would recommend replacing the hose as it has been weakened from the swelling and overheating.

a few things to check before we condemn it as a leaking head gasket(pressurising water out).
First check the radiator cap seal,no cracks and that it takes some pressure to push the spring loaded part down.
Does both the top and lower radiator hose get hot as temp gets to normal(where thermostat should be open).if not,the thermostat may be a faulty one or has slipped out of seat as the cover was bolted back down onto it.(temperature will differ either side of it).I have had some of these come in with part of the waterpump impellor rotted away.same symptom of one hot and one cold hose,and water doesnt flow inside radiator with cap off(ensure this is not removed if engine is hot)and temp where the thermo should be open.
Possibly the radiator needs sending out for the top or side tank to be removed and each tube cleaned out.this is a lot more effective than trying to reverse flush with water only.
But if temp is normal these areas should be fine.
Do the hoses swell with pressure and become hard to squeeze in a short time?this could reveal a leaking head gasket and should be tested with a teekay type tester for the presence of cylinder gases in cooling system.
Do the fan(s) come on if sitting idling at normal temp for 5-10mins?water can be pushed out if they dont come on or their cut in/out temp is incorrect.hope this helps.
let me know some times re when it pushes water out,do you hear a boiling sound from inside engine or radiator when this happens,any other info that may help diagnose.

Check for possible overheating prob.waterpump,radiator, thermosensor.

First check to see if you fan is coming on when the van gets hot then check the top and bottom hoses on radiator when the van is hot USE GLOVES if the hoses are real soft replace them they could be collapsing usually its that bottom hose that has this problem

Assuming you dont have a blown head gasket the most likely thing is a clogged radiator. Lee