Re: 1968 mercury 650 65 hp/cooling issues
It isn't that hard of a job, and easier than the Johnrude in that there's no shift linkage connector to undo.
Instead, Merc uses a female-splined coupler at the lower end of the upper shift rod. This engages the male-splined end of the shift shaft in the lower unit. Rotary motion rather than up-and-down motion is used to shift gears.
So, when you pull the lower unit down, the shift shaft just "falls" out of the coupler.
The one caveat is, when the lower unit's off for service you don't want to move that shift shaft. And it's best to have shifted the control box to Neutral before you remove the L/U, and leave it there.
Most common error is to move the shift shaft, then when you put it all back together it won't shift out of Fwd gear. Since there are no flats on the shaft or coupler, they can be reassembled in any position. This won't happen if you leave things as is.
Aside from that, the only 'trick' thing is a hidden bolt under the trim tab. This is accessed from the top and could either be an allen socket bolt, 12-point head or hex. Merc used varied styles over the years. Pop the plastic plug (if it's still there) covering the hole and check out with a strong lite. If there's some rust/crud in there, dig it out and you'll likely find an allen socket.
One nut in the front, one on either side, two nuts underneath, and one bolt you'll see after the trim tab is removed.
Reassembly is very straightforward, use some waterproof grease on all the studs/bolts, also grease the end of the aluminum exhaust tube where it engages the rubber exhaust seal in the gearcase. There is a plastic 'guide tube' that sticks up out of the pump housing and helps 'catch' the water tube on the way up. Sometimes the guide tube works its way out of the pump and stays on the copper water supply tube; just pull it down and reinsert into the pump. Most pump repair kits will have a replacment anyway. A bit of grease on the end of the water tube will ease its way into the rubber pump grommet.
You can pick up parts here at the iBoats store, a few places where you'll find parts diagrams are
www.boats.net www.crowleymarine.com or
http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2
BTW if the local repair folks want to charge you much more than an hour or so for an 'easy' impeller replacement, think twice. Unless your motor is an Old Salty Dog it should come apart and go back together with little effort. If the gearcase has other issues while it's off (such as leaky seals etc), of course more time would be involved.
HTH............ed
p.s. do a compression check before you pour a lot of $$$ into it. Hopefully the impeller is worn from years of inactivity, but it also could be an indicator of a badly overheated/damaged powerhead.
p.s.s. if you keep this motor and want to properly maintain it, get a good service manual!