Deant23

Good afternoon, I have a 2011 beta evo 250 2t. Had it for a couple of years now. Having s issue where when I rev it, it can take ages to come back down. I have tried messing with mixture screw and doesnt seem to make any difference if 2 out or 3/4 out. I have had the card apart and clean all indivicdual components with carb clean. Again no difference. I have tried to spray brake clean at intake system to see if it revs it doesnt change. Am I right in thinking this could be the crank seal? Or am i barking up the wrong tree? Also its does idle fine. TIA