94 yamaha xj 600 want rev over 6000 rpm

Hi m8,,1st,, would u no if the bike has been resricted ,,to 33bhp,,if it hasent2nd,, have a look at the spark plugs,, start her up rev until it coffs or wont rev then hit the kill switch,,see wot colour the plugs are ,,they should a biscuit brown,,if not clean them up or change them,, 3rd test,start her up again throttle it a few times ,,untill shez warm,, then ,this sounds not nice, but either spit on your hand or dip hand in sum water, then quickly touch each exhaust down pipe, and make sure there isnt a pipe cooler than the other 3,, if u do find there is that would narrow the test down to that cylinder thats playin up,,try that 1st m8,, then come back if any probs still, and il take u threw next steps,,