Re: Mercury 150 Xl Overheating
On the lower (starboard) right side of the block, there is a cover (looks like an 8) the with 4 small bolts heads. That is the cover for the poppet valve. With your new info, then I would think seriouly about taking a look at the the poppet. If you are careful, you may be able to take the cover off w/o damaging the 2 gaskets. The poppet is spring loaded but will not jump out at you, just work your way around the 4 bolts a little at a time, be especially careful not to strip out the threads, it you have been using the engine in saltwater, soak w/WD-40 or the likes for a while if the feel tight.
Look at the Merc Express site and you get an idea of how the poppet is put together so you won't get mixed up on the sequence of gaskets, grommet, washer and the poppet itself.
Easy access can be gained to the poppet cover by dropping the lower cowling half, just remove the 4 small screws/bolts and tap the cowling lightly and it will drop down, coming loose from the rubber round things.. Nothing to disconnect.