Mike,

My local dealer (a marine yard) isn't sure that an alternator comes with the Nissan 6 hp four-stroke, and the Nissan Marine web page:
http://www.nissanmarine.com/products/6_4_specs.html

implies that it doesn't, because the 12V charging system space on the specification table on that page is left blank. I know that you have an alternator, so it would be interesting to know what year your motor was manufactured. How many amps does the alternator put out? The two-cylinder Yamaha 6 four-stroke puts out 6 amps.

And finally, does the issue Bill raised about the "upper exhaust port" apply to the Nissan 6hp four stroke? Does your motor choke if run in the lazarette with the top hatch closed?