I have a 1969 evinrude 6hp fisherman. It starts and runs really well in neutral. I put the motor on my 11' flat bottom boat and when i put it in gear it does not want to rev up. it only goes just a little past idle even when i turn the throttle to full speed. i can tell that it wants to rev higher because it does for a split second and then drops back down. when i leave it a full throttle you can feel the motor trying to rev up but it never does. any ideas on what this could be?