Yes, like so many other things, there's a trick to removing the lower unit of a Mercury 150. The trouble is, no one tells you until you are already up to your ears in trouble with the lower unit. Fortunately, the "trick" is one that keeps the lower unit from hitting the ground while you search for the answer and the appropriate tool. There's a hidden bolt holding the lower unit to the rest of the motor; you just have to know where to look for it.
Tilt your motor upward until the lower unit is just above waist height. This pre-positioning puts the lower unit in place so you can remove it with minimum risk of injury.
Use a socket wrench and a 9/16-inch socket to remove the four equally spaced visible bolts that range around the front of the flanges between the lower unit and the leg of the motor.
Remove the bolt that holds the zinc anode (the "zinc") to the bottom of the anti-cavitation plate -- the flat, horizontal, rectangular piece that sits above the propeller. There's a hole at the rear of the zinc, that gives you access to the bolt holding the zinc in place. You can remove the zinc with the same 9/16-inch socket with which you removed the lower unit's forward mounting bolts, but you'll need an extender for the socket wrench.
Look inside shallow cavity that's exposed when the zinc drops away. Use the socket wrench, extender and 9/16-inch socket to remove the "hidden" bolt holding the lower unit in place. .
Pull the lower unit away from the rest of the motor twisting the lower unit, if necessary, to free the drive shaft and shifter.
- Use appropriate caution when removing the unit and wear safety shoes. As light as the lower unit is, it's still heavy enough to break a few bones if it falls on a foot.
- toy outboard motor image by pearlguy from Fotolia.com