I have a GasGas TXT280 pro year 2004 and it's hard to start. The spark although good is clear sometimes yellowish in colour (no hint of blue). It will start after a lot of kicking and altering the air screw, once started it ticks over no problem.

But it only seems to start after altering the air screw, which then has to be set back to prevent the bike racing, is this normal???

Any ideas would be gratefully appreciated.

Clean your carb, Especially the choke circuit, Do a complete strip and remove choke plunger too, The airway is probably blocked.

 As above clean the carb. Make sure the pilot is not plugged. The Pro takes a certain kicking method to start. Slowly kick it through the stroke two or three times. (At this point you are not trying to start it.) Then slowly get the starter to top dead center and the top of the stroke. You have to feel the kick start gears are engaged before you kick or it will self destruct. Once all the above is right. Give it a good strong kick. But remember it is a good short stroke.