I have a 1986 140hp mercruiser and boat was working fine but now it will shift into gear and push the boat at idle but when you throttle up the engine revs but the boat goes no faster. I checked to see if the propeller is spun It does turn on the hub but it takes a lot of pressure and in gear the spline that the propeller goes on does not turn so I am at a loss. Also the drive shaft does move faster when the engine revs and the gears in the upper unit look good. And the propeller moves faster when out of the water on ear muffs. Any help would be greatly appreciated! Thanks!