Lots of suggestions to consider. I appreciate the responses. There were several things I had not previously considered. I pulled the boat last weekend, so my ability to diagnose some of these problems is somewhat cramped now. I'm thinking of pulling the belt on the seawater pump and trying a few things. There are a couple of requests to elaborate on the sound it makes. It occurs every few seconds I'd guess I'd call it a weak backfire. You have to consider that it is occuring at least 10 feet up a water filled pipe and is much attenuated by that. It is not associated with a shudder, a cough or a change in rpm by the engine. You could live with it if it didn't seem to point to a more serious problem. The thing I found most interesting about it is the fact that it appears as soon as the engine starts, no matter how cold. This seems to point to a weird ignition problem to me. I think I will change the distributor cap as a simple precaution. By the way, I want to change the plugs on both engines after 3 years. What does everyone recommend. I took the spark plug numbers listed in the CC 427 manual to my local Auto Zone and got completely blank stares from them. Serves me right. They are not into 40 year old boat engines.
Thanks,
Eric