Newbie with motor question

Hello all, new to the site. I found it this weekend while trying to find info on a boat/motor my father in law gave to me. And may I say a great site it is.
Heres my situation. 1999 xpress h-51, 1999 90hp mercury 3cyl 2 stroke. Father in law brought it in to a local dealer to have a spring service done. They told him it had a "knock", and would require $2500.00 to repair it. They had TOTALLY disassembled the motor, foot and all. He had them box it up, and he brought it to my brother in law (ex brother in law now) who is an outboard mechanic, who put it back together. It ran, but poorly, wouldnt develope enough RPM's to plane off the boat. He got mad and gave the whole rig to me. So, ive done a compression check, 1 and 2 105 pounds, 3 100 pounds. Cleaned the gas tank, added stabil and seafoam, and refilled it. Cleaned the plugs, and adjusted throttle linkage. Took it for a run this weekend, it would plane off but only get to about 3500 RPM's. Then at one point it took off like a scalded ape got up to 4500 RPM's. I turned around to go back to the launch and it would not go above 3500 RPM's again. I plan on taking the carbs off and soaking them them trying it all over. Anything else I should check that could be a cause? Thank in advance for any and all help. John Hodges

I would put new plugs in it. My motorcycle was not running right this year, I thought it was bad gas or carbs, I tried new fuel filter and cleaned plugs, they "looked" good. Mechanic told me to change the plugs, that fixed the problem. Cost was about $8 for 4 plugs and ten minutes work.
Tim

Those compression #'s seem low for a Merc.

I had issues with my Merc and it was sucking air at the quick disconnect at the motor. My other issue was the plastic tank with the 360 degree swivel outlet that would not seal properly. I went back to a metal tank and the old style quick connectors,

Thanks for the input guys. Plugs and a new fuel line comin up