what year is it first ? the head mod for the j /k is machining a blend from the squish to the combustion chamber , 8mm wide , i,ve never done it on any of mine ,besides they only ping when u back the throttle off ,done in a lathe using a mandril that fits the plug hole , use the highest octane u can get ,98 is good 33 to one oil tts is good ,for j or k ,ign timing is std at 2mm btdc, i use #50 pilot jet , (richer than std#45) std nozzle and needle q8 , 6f16 clip in #3 , get a new set if it,s got any wear on it , std main is 440 , but with the leaner oil mix u might have to go smaller , boisen reeds work well but roll them over regular and/or replace them to keep the lip down on the block , fiber reeds will probably knock the main down a size or two , dg pipe or similar is a must reeds and pipe and any porting may affect jetting , the 490k i bought off 090 had a main 5 sizes down from std ,it,s no good doing anything to a worn engine , check cylinder for clearance and blow by scorching ,check big end and mains , it,ll ping less if it,s worn , gearing , start from std 15/50 for a std bike , 15/49 for an open track , 15/48 , for a fast bike and fast track , drop the front end as far as u can for a tight course , adjustment to the twin link shoes is vital for them to work , use the sintered shoes with groves , use the linkage between the brake cams to adjust the slop out of the system , idea is to get both shoes expanding together , with new shoes and the brake plate in your hand adjust the linkage so the cam on the second shoe contacts it at the same time as the first shoe , give it a run then check wear pattern and tweak if required ,unless u r riding full on mx soften up the forks , if it,s a j i,m not sure how to get the rear suspension to behave , just make sure it,s all in good nick , or pissit all off and fit a 465 swing arm on it and soften the shock , if it,s a k their good in the rear , if it,s an L -lucky u the 490,s aren,t the best in the class but are easy to maintain and get parts for ,
