crf250?

no its not going to be the timing its set alread in the cdi unit and the valve timing should be ok providing the cam chain hasnt slipped you need to make sure its on top dead centre on the firng stroke to check the valve clearances and if you cant get the feeler guage in the gaps with the right sizes out then the valve clearance is too small and no doubt causing you a problem.if the bike doesnt have adjustable tappets then you will need to take the cam out to change the shim/s but this can be awkward as you need to now the size of shim u need and if you have no clearance/gap at all then u dont know ho much smaller the shim u need is going to be,check to see if as i sais its on the right stroke first there are two up and 2 down strokes if the exhaust valve is being pressed down when u turn the engine over in the right direction and just after it the inlet valve is going down then u are on the wrong up stroke ,both valves should be closed and the last valve to have opened before the piston gets to the top would be the inlet valve check either by putting something i.e a screwdiver in the spark plug hole to see when the piston is at the top or lookon the edge of the flywheel /magneto which will have a mark or two whaich will line up with a mark on the crankcase i expect the fltwheel will have a t mark and a f mark the t mark is for top dead centre and the f is the firing mark(when the bike sparks)well hope this is easy to understand for you let me know if you need more help on the matter ok ,adam oh p.s the chuffing noise you can hear from the exhaust could well be the burning petrol air mixture leaking past a tight valve !!!so good chance they are tight

adam bike doctor
March 2006