when i put the choke down the 4 wheeler dies it blows out black smoke from the exhaust i changed the spark plugs when i put the choke half way up it runs fine but when i put it down it starts missing and then it dies any help?

sounds like your jetting is off. seems like your running rich and thats why cutting off half the air will make it run right. but im no jetting expert nor 650 expert maybe somone else will chime in. but thats my best educated guess