Wanted M25XP

I have a Universal 25XP with one weak cylinder, it doesn't puff until 2400 rpm then it puffs realy bad. There is blow-bi in the valve cover at all RPM's. What is the best way to figure out if it's rings, or a valve? I don't think its a head gasket as it doesn't overheat. Any sugestions would be apreciated.

Time for a ShortBlock

There is no such thing as one weak cylinder. This will lead to a multitude of rebuild problems. My advise is to remove engine / remove marine parts / purchase professional rebuilt ShortBlock / install Marine Parts / install Rebuilt Engine. Your Kubota Block is a commmon SkidStear(BobCat) Engine. Several ReBuilders can be found online. ReBuilding a Diesel Engine is beyond the capability and ToolChest of even the Best of us DockSide Mechanics !

Good Luck to You