Yamaha blaster wont shut off!?!?!!?

ok i just bought a 2005 yamaha blaster and test drove it and all it had like no gas when i rode it so i couldnt really try it out but for what i had it semmed nice, i brought it home and put mixed gas in it becouse it has a bad oil pump (not hooked up) and i guess i put to much oil cuz it wouldnt fire up so iput more gas and it started then the wierdest thing happend i hit the clutch to cruse a little cuz i had to slow down and my rpms were going nuts so i quickly hit the killswitch and it didnt work i turned the key and it still didnt work i quickly let go of the clutch and killed it??? so then i tryed starting it again and it worked normal everything worked good?? sp i went for another spin and it happend again when i hit the clutch it would go nuts but when i let go it acted normal again like as i was driving it wouldnt go full rpm only if i hit the clutch it go nuts any ideas wtf is going on??