Think 2 cycle…the crankcase has to be tightly sealed, the reeds have to be free of rust for the charge to be crammed into the cylinder. The rings have to be free in their grooves and rings, piston and cylinder need to be in reasonably good shape.
40# dry is low. I have torn down a few KE4/Mark 7 First one was a learning experience. Unless need can be shown, not a good idea to go looking for trouble that is not there. Best to diagnose the patient before the operation. Do the tissue test in front of carburetor opening to see if reeds are sealing, dirt and oil under magneto plate may indicate the upper crankcase seal needs replacement,. Oil added thru spark plug openings may raise compression indicating worn rings or cylinder or pistons.
If rings are stuck in grooves, I would pour in some Marvel Mystery Oil and see if rings will unstick. Running motor may also cure problem.
If worse comes to worse and you need to tear down powerhead, it will be a learning experience. You will need to learn the Mercury Way of doing things, techniques and make a few simple tools. The tools are well worth time and effort to make.
My 0.0002 cents worth
Louis
Photos show ready to reassemble powerhead and new upper crankshaft seal (7415) installed. Crankcase with bad crankshaft was picked up at swap meet. Needed to source cylinders, but, rest was on shelf…left overs from parts motors and what all. Sort of like the Cadillac that Johnny Cash sang about.
