I HAVE 1968 LARSON 166 WITH A 4CYLINDER CHEV 2.5L MOTOR: MY PROBLEM IS I CAN GIVE THE MOTOR THROTTLE AND IT RUNS FINE AT FIRST, UP TO 4000RPM, AND THEN IT STARTS SLOWING DOWN, THE RPM DROPS TO BETWEEN 2500 AND 3000 AND THE BOAT SEEMS TO BE GOING ABOUT 15 TO 20 MILES AN HOUR. ONCE YOU LET IS SIT...