Re: 1968 650 merc issues
Well number 1 and dont get me wrong but your not going to be breaking any speed records.From what I understand in your post the boat was running a 13 pitch prop which would have given you approx. 5200rpm excellent.By dropping your prop down 2 pitch sizes you increased your rpm approx.400 rpm. 1 increase or decrease in pitch = 200rpm.Your spec's from mercury on your motor recommend if I am not mistaken 4800 to 5300 rpm.So you are over by 300 rpm (not good).For towing and weight wise I would maybe have selected a 13 pitch 4 bladed prop.As for 120 psi not bad if all cylinders are within 10% of each other.Selecting the correct prop size is a pretty tricky job every boat,horsepower,displacement,weight are all factors that will affect performance and speed.There are folks here on this forum that will get you the best advise possible,they got me on the right track.
Happy boating and good luck.......