The 150 is a 5 cylinder with 3 carbs and 1 of the carbs feeds only 1 cylinder. Complete oddball made for around 2 yrs. The 55 hp uses the lower unit off of the smaller engines a known problem with those.
Does'nt that motor have points? And you might want to make sure that is not a phasemaker ignition. I dont remember when they stopped making it but I believe it carried on in the smaller engines for quite a few yrs. somebody will correct me if I am wrong.
Mercury/Mariner are the same motor after the late 80s. Only difference between them is the tag that says what color to paint it and what decal package it gets. Buy the Mercury manual for that serial # and fix your motor.