I own a 1976 merc 200 20hp and since i have owned it, it has never cooled properly, i get very little water if any at all, coming out of the spout, I recently replaced the impeller and all gaskets included in the kit from old merc.com and still have the same problem. Could there be an obstruction in the actual power head? Any help would be great!! Thanks!
by the way, if you decide to take off the side cover to look into the water jacket, you had better be really careful because you could create a worse problem by breaking off the cover bolts in the head-- they are often corroded and seized in the head. try to hook up an air pump to the telltale tube and see how the air flows through the head-- it could also blow out an obstruction. hope this was helpful. would appreciate it if you let me know if you find the answer to your problem as well!