tension

So i got myself a new hyper bicycle with a new 66/80cc motor. After awhile of riding I felt that my chain had fully stretched so i tried to tighten it up again. While tightening I found that the slack of the chain would change to loose and then tight when I would spin the wheel so I figured...

What should I do? I have my chain tensioner centered almost perfectly so that the chain isn't on one side more than the other and come off part of the rear sprocket, and also, I have the tensioner wheel set a little above so that the wheel spins, but since I don't think my rear sprocket is...

What should I do? loosen the nut? I have it set down to the lowest height anyways....