1961 Mercury 700 FGS

I have an old 1961 Mercury 700 FGS (6 cyl.) that I have been working on. I got the boat/motor from Dad, and when I started messing with it I did a compression test. I couldn't test the top and bottom cylinders since the adapter i have is too long (I could take the top cowl off to test the top cylinder but haven't). Cylinders 2-5 all read pretty low compression (~60 psi), but they are all even. The motor runs, although the idle is pretty rough at ~800 rpm. I was thinking about using Seafoam to decarb the motor. I found a post about decarbing the motor that I was going to follow: 16oz. Seafoam to 3 gals fuel/oil mix, run for 15 mins, then let it sit for 15 mins, repeat. I'm not sure how much it will help, but I'm not sure how much it would hurt either.
If anyone has any other advice or tips, I'd appreciate it.

Re: 1961 Mercury 700 FGS

1 can of seafoam and one gallon of premixed gas ( i say 24:1 others say 40:1 or 50:1 your choice)

mix together.

prime gas start motor and warm up at about 1200 rpms

kill the motor by putting seafoam in it by using deep creep (air can with seafoam) or a spray bottle. try to kill with it in all carbs.

take all plugs out and spray each into each cylinder reinstall plug and boot and wait

15 sounds about right guess.

restart. might take a few tries. beware it will smoke a lot at first then come down but will still smoke. white smoke then lots of blue.

repeat for as long as you want

after is suggested to take for a high speed run

assuming you have everything else checked out?

replace impeller pumping good water. no overheat

changed gear oil

clean carbs

good fuel pump

spark.

Re: 1961 Mercury 700 FGS

Yeah, the first two things I did when I brought it home was to change the impeller and gear oil in the lower unit. The boat hasn't been in the water for 10 years before I got it, but the carbs still look pretty good as far as I can tell. Dad had them rebuilt back when he bought it. The tell-tale was only dripping when I first had it running, so I replaced the tube (it had a kink) and now it's peeing fine. It gets warm after idling for a bit, but not hot.
I've also replaced a handful of wires that were rotting and had insulation falling off. I don't have a spark tester (the guy at the auto store never heard of it even after I described it - time to look elsewhere). I only used a timing light to check for spark on all cylinders.
Not sure how to test the fuel pump(s). I've had a few leaks between the tank and the motor; the fuel lines in the motor show some age (cracks) but don't appear to be leaking. I added a fuel filter and a couple hose clamps and the bulb seems firm when I squeeze it. I've only ran the throttle up once and it seemed to accelerate well, but I didn't let it run very long.