If the Polaris 50 Scrambler only runs for about five seconds and shuts off, it could be a problem with the idle setting. This type of ATV has an adjustment screw on the carburetor for turning the idle up. This should fix the problem.
Low oil pressure at idle. Can be idling to low, weak oil pump, defective oil pressure sending unit, or filter is not compatible. Change to another brand of filter and see if you still have the problem.