40 hp evinrude ob help please

she runs awsome no issues there but after like 1/4 to 1/2 throttle in forward it revs and freewheels out of geare like going in neutral? then when i back down the throttle it reengages in forward gear and runs fine under 1/2 oe 1/4 throttle any help? im thinking maybe need to reshim the pinion gear? its all original 1989 bass tracker w tracker/evinrude 40hp 2 smoker power tilt?
thanks tim

Pinion gear would have zero to do with it. It actually engages both forward and reverse gear teeth at the same time. What you probably have is a spun hub in the propeller. With the engine not running, in gear, and with the key switch off and safety lanyard pulled, try to turn the propeller by hand and see if the flywheel turns with it. It should turn the flywheel no problem, You can also mark the hub and the propeller body and run it and see if the marks move. If still in doubt remove the prop and visually look at the hub for signs of slippage and then smell it. If it smells like burned rubber take it to a dealer and have them send it out to a prop shop and have it rehubbed. Usually it will come back the same week if dropped off on a Monday.