put the stock tensioner on,use a bolt behind it and 2 hose clamps on the bolt: this will keep the tensioner from going into the wheel.
adjust tensioner,but not too tight,walk bike and watch chain,listen for clicking and see how chain comes off,adjust chain tensioner in or out to line of chain with rear sprocket.
walk bike again to test it out. check motor mounts ,maybe engine is moving causing chain to come off,dont over tighten mount bolts or they will break,just snug will do and double nuts.
also is your wheel bent or not on straight,check that too.