I have a 04 Honda trx 400ex. Recently the engine was rebuild and when I'm riding straight its normal but when I'm going down hill and up hill oil starts running Into air box and I don't know what to do are the piston rings bad or the engine isn't correctly build

 

Are you sure you have oil level in engine correct? If so you must have a problem with breather pipes or piston rings. 400cc single cylinder engines blow 400cc of air into airbox on every downstroke, but suck 400cc of air back into crankcase every upstroke, so are "heavy breathers" but oil fumes should be sucked back in as well as blown out, most breather systems have enough baffles and pipe length so that crankcase air gets as far as the baffles and breather pipe(s), only clean(ish) breather pipe air gets blown into airbox. Piston blow by and the expansion due to engine heating up causes a bit more air to be shoved out than is drawn back in though, so some fumes do get through into airbox.