Buddy of mine has a Kingquad, 2000 modle, it has had a realy rough time, Got stolen, taken apart...ect well he got it back and has been trying to get it running strong again, heres his most current problem,

When he has the airbox in the quad will sit there and crank its battery dead, With the airbox out it will start right up and run.
He has checked the filter and made sure it was clean, he has made sure there is nothing blocking air from flowing out of the airbox.. anyone have this idea or know what possibly could be wrong?