That would be the head.. more specifically the squish band, terrible design. Once you get that fixed they are twice as reliable and they actually run right. Also like you said a carb thats jetted right makes a big difference too!paulie has a yz400 motor... the yz 490 CAN be a good motor BUT you need to get a diff carb and jet it rigth, also i think somehting with the ports needs to be changed.. i know they were lean from the factory
itll rattle ur fuking teeth out but would be sick
yeah it sounds like alot of work. i think i will just save up for a 240 big bore kit and gear it better. and new tires, my suck in the snow lol