My Polaris is STALLING!!

My 2009 Polaris sportsman 300 is carburetor driven and it usually runs fine but lately if I keep my thumb on the throttle it will run fine but if I let it go completely off the throttle it will stall out… I put a new spark plug but it did not help… I have lines running off it so it can breath under water but idk why it keeps on stalling please help