If you are suspicious of the engine mounts - Replace them. They are not expensive and you can get into a world of trouble if they fail on you. Remember, the propeller
does not push your boat, it pushes the shaft that pushes the transmission
that pushes the engine itself that then pushes the Engine Mounts
and finally pushes the boat.
- - One check is to mount a gauge at the prop shaft flange and set it to an edge of the flange. Then put the boat in forward and watch as the whole engine moves forward in the engine mounts as the boat starts to move forward. Then stop and put the boat into reverse and see how far the engine moves aft as the boat starts to move. There should be very little movement of the engine forward and aft. Such movement is caused by the rubber inside the engine mounts wearing away.