Re: 1966 mercury 650 outboard/How much is it worth????
The old 650s are workhorses and are generally fairly easy to work on for basic maintenance stuff. If compression is good on yours, you're halfway home as it is.
Value is a tricky subject with old outboards. TX touched on it, as it is very subjective. The older 1950s era Mercs have a strong following, but the 1960s stuff is a much grayer area. They're not quite old enough to be antiques, but not quite new enough to be service motors for the general public. You pretty much HAVE to be a wrench to keep one as paying a marina to do anything more than basic maintenance will quickly tally up more in bills than the motor is worth, enough so that many places won't even bother with it.
That said, a mid-60s Merc 65hp will fit a broad range of vintage boats and is fairly sought after by a similar cadre of enthusiasts. Downside is there are more motors available than there are enthusiasts who are actively buying them, so the prices tend to stay down. I would say you got a good deal on yours, as a clean motor, with controls, that checks out as good on compression, and can be shown to run, would probably be worth several hundred. Keep in mind, it's still possible to put several hundred more into it, between fuel pump kits, carb kits, waterpump, going through the magneto, etc. Also, that several hundred you put into it likely will NOT be returned on your investment should you go to sell it later. It will still be a $400-$600 motor.
Weak point to keep an eye on - the driveshaft, particularly below the waterpump housing, is prone to rust. Mercury was late coming to the stainless steel game and as such their shafts were a carbon steel that was allergic to water. The pitting at that area is right at the upper gearcase seal and can allow water to enter the case. SST Speedi-sleeves can rectify the issue if the shaft isn't compromised from a strength standpoint.
Enjoy!