Ok, I am gambling on this one. Here is story I am told. Bought boat and motor for teenage son, he used it 5-6 years then put it in shed for aboutr 20 years. Son moved away and now dad wants space. So he checks out motor, puts in new water pump and changes oil in foot. startes her up, she runs but only a dribble comes out of water jet. He rechecks it all same story. He pulls head, chock full of white powder aluminum corrosion. Dad is over it, sells motor to me for good price. It is a yamaha 15hp. looks fine, prop is fine.

I am thinking of trying phosphoric acid diluted first to dissolve the corrosion. Then 30% sulfuric, heated up if the phos does not work. I know nitric acid would work but you cannot purchase it anymore. Any suggestions?

thermostat is also shot, $60 bucks for that little gizme and $20 for head gasket (I have not purchased yet)