Try that leak down test with the air cleaner off and throttle plates open and listen at the carb and the exhaust.I guess you could maybe have a bad valve guide or more letting air into the valve cover,but you may have something up with the piston,crack,broken ring,but I would check everything else first.Maybe when you remove the head for the valve work,you may see something,but do all the checking first,once the head's off,the checking is over.