Not one of Mecury's best designs, 2 carbs feeding 1 1/2 cylinders each, that plus a 1.41 hp per cubic inch will make for an engine with a less than great torque curve/power bandwidth.
Also, jogging my memory, the 50-60-70 of that series has a small lower unit gearcase swinging a 10" rather than 13" diameter prop that others in that HP range were using at the time. Takes torque to develop the thrust to swing a big wheel and obviously these engines were lacking. Really made for light, easily planing hulls.
The service manual for my last engine, 2002 Merc 90 2 stroke looper said that anything below 120 psi at 200 rpm cranking speed was questionable.
However, there may be light at the end of the tunnel: That series of engines were loopers (I know the later model 51.8 cu in 3 cyl, 3 carb engines, 1.64 gear ratio were) and loopers usually run 2 wedge shaped piston rings rather than the conventional 3, used on Cross Flow engine scavenging designs. The 2 rings have to be able to move in the piston skirt to expand under compression pressures seal off the combustion chamber. If your engine is gunked up with carbon they can't perform as designed, making for low compression.
Before you commit harey-carey, get some Sea Foam fuel treatment (any auto parts store) and get back on here and lookup "DeCarb"....or just plug that word into a Google search engine. Do that and with 2 oz per gal of SF added to your fuel/fuel mix, get the engine running and get some time on it. Then recheck your compression. May just squeak by and save an overhaul.
