Re: merc 150 lower unit removal
I have a '96 150, so I'm assuming that the process should be pretty similar.
Please note from the get-go that I am not a mechanic, have minimal mechanical skills, but was able to easily drop my lower unit, change out the entire water pump (base assembly, pump housing, face plate, gaskets, o-rings, impeller, etc.), and reassemble the motor.
First thing, put the shifter into forward, and make sure that no one touches it while you have the lower unit off.
Next, remove the prop, and then remove the trim fin anode that is just above the prop. There is a bolt that is up and under this fin anode, and so you need to be able to get to it. Make sure that you mark the angle of the fin if it was offset at all, so that you can put it back at the same angle when you put everything back together.
You should then have six bolts if I remember correctly (I did mine a few weeks ago, I believe six is the number). The first bolt is on the front edge of the lower unit, right by the speedometer tube. There are then two right behind and below this on each side of the motor. A close ended wrench is the best to use here. That's three total.
For four, this is the bolt that is under the trim fin anode. Five and six are also on the bottom side of the trim fins, just above the prop shaft.
Once you have all six bolts removed, as well as the speedometer tube disconnected, the lower unit will most likely still be solidly attached to the rest of the motor. Get yourself a mallet, and start tapping away to separate the two units. Make sure that you have something that will support the lower unit in case it comes apart quickly. Once you get yourself a gap of about 3/4" or so, you should be able to put the mallet away, and just use your hands to hold and pull on the lower unit. It should then slide out fairly easily. It weighs about 30 lbs or so (my estimate), but it is a bit cumbersome. You should be able to handle it by yourself, but having another person will be a big help.
Once you change the impeller and/or water pump, make sure that you grease up the drive-shaft splines, and then carefully fit the lower unit back into the powerhead. This gets a bit tricky, and having someone else with you is a major benefit. Check that that water tube connects, the shift linkage engages, and you should then have the lower unit up flush with the bottom of the motor. If there is a gap, try turing the prop shaft a bit to align the splines.
I would recommend that you use some type of anti-seize or gasket adhesive on the threads of the bolts. If you are in saltwater, don't use anything with copper in it, as corrossion could become a major problem with the dissimilar metals. I am using Quicksilver Perfect Seal gasket sealant, and it's been fine for me.
Tighten everything up to the torque specs if you have them (I don't have a a torque wrench, so I go to tight, and then about another 1/8 turn), and you should be good to go.
Test to make sure that you have the linkage correct. Do this by having someone put the shifter into neutral, and then you spin the prop. As the prop is spinning, have them put it in forward. The prop should then stop spinning as the gears are engaged. Do this again with reverse. If the prop stops both times, you're in business.
Remember, all of this is coming from a guy that is not mechanically inclined, but was able to do this the last two years with minimal trouble. I will note that I didn't check that the linkage engaged properly the first year, and I only figured this out after the boat was launched.
