Thanks all. The exhaust riser blew out before the mixing elbow, so that explains part of the problem. Should have made sure (asked) the PO if he cleaned the elbow regularly. Water in the combustion chambers has come to mind.
Yo Doug Vinigar twice a year thru the engine will help beat down the build up.
1)Start the engine,
2) Put a gal on Vinigar in somthing you can fit your suction hose into.
3) take your suction side hose off the water strainer and let it draw the vinigar thru your engine.
4) when all the vinigar has been pulled thru the engine, hook your hose back to the raw-water strainer.
It should help some. You will still need to do a good exhaust riser cleaning once a year. Good luck. I have a 2GM 13 yanmar and Its a great little engine. A little maintenance and they will last a lifetime.