I don’t think the spring loaded shock absorber on the drive shaft below the water pump has been mentioned here yet. If the prop has hit something hard, the spring will release and allow the shock absorber to expand, dragging on the bottom of the bearing carrier located below the water pump. The motor will run in this condition but the idle quality will be compromised. Can the motor be turned over easily by hand with the spark plugs removed?
If you are going to service the impeller, it would be a perfect time to remove the carrier and inspect the bottom of it. Any contact between the two surfaces will be obvious. If a sprung shock absorber is detected, it can be pressed or pounded down to its original 6 11/16″ length. If the shock absorber is removed from the housing, the gearcase will more than likely have to be opened up to get the pinion gear back onto the shock absorber correctly. Meanwhile, let’s see what that head gasket looks like.
Here’s a shock absorber and a housing with severe signs of rubbing.
