I HAVE A 2008 KTM 250 SXF , I HAVE REPLACED THE WATER PUMP KIT AND GIVEN THE BIKE A SERVICE, NEW SPARK PLUG, OIL AND OIL FILTER AND EVEN REPLACED THE RADIATOR HOSES AND THE T-PIECE THROUGH THE YOKE AS WELL. THE PROBLEM IS THAT THE BIKE NOW WILL NOT START AT ALL OR EVEN FIRE WHEN I KICK START IT.

SOURCE: ktm 250 sxf 2008 backfires

go on google check the air mixture and altar the air screw on the carb to wot u find on the net easy job thers only 2 screws on the carb

SOURCE: how much oil do you put in a 2006 ktm 250 sxf with

You can download an owner's manual for free here

http://www.ktm.co.uk/250-SX-F.28.19.html?&detailview=4&cHash=dcbf8829e1

SOURCE: 2008 KTM 530 EXC WON'T FIRE WITH STARTER

change engine valve

SOURCE: bike only starts with choke on and wont idle

If your bike ran good previously - and nothing has been changed that would require rejetting or a needle adjustment, I would look for problems elsewhere. Such as:
Has the bike been stored for more than a few months at a time? If so, the carburetor jets could be plugged with fuel varnish. Only being able to start with the choke applied - and an inability to idle are both classic symptoms of this. Remove and disassemble the carburetor. Clean it and the jets out with carb cleaner and compressed air, then reassemble. Also make sure the fuel in the tank is fresh and that the fuel strainer (usually with the fuel petcock) is clean.
There are three ingredients required for combustion: fuel, compression, and a spark (at the right time). So long as you've got each of these - it has got to run.

SOURCE: 2008 ktm sxf 250 when engine is under load there

you may want to relube your water pump. this usually happens if the water pump cannot put out what it gets from the inlet valve. what will happen is that coolant will get stuck on the negative reservoir, then will get out of the over flow tube. you may think that you have too much coolant, but yet you only are running half.

avoid further damage and overheating have it replaced.