Just got a mid 70's 20 hp merc looks clean hooked the water hose to it started after a few pulls then shut it off started again 1st pull . operated the gear shift worked fine . went ahead and bought it . put on the back of my jon boat took it to the water , it started 1st pull reversed away from ramp put it in forward idled in gear fine eased into the throttle started picking up a little speed then r's shot up and i did'nt go any faster , top speed was just off idle . then it would start slipping or cavitating it has a 2 bladed stock prop on it . any body have any ideas ?