Excellent! Thanks for the tips. I got it working, and it's better than ever.
I removed the two carburetors, unscrewed the jets and sprayed them with Orange Degreaser. Then I soaked them in stove fuel followed by gasoline and finally shot them with compressed air.
I thoroughly cleaned the bowls and blew out a ton of ancient calcified sludge and goo.
Then I reattached the carbs, connected all the hoses and fittings, primed the engine
and it started in less than a minute!
It was immediately obvious that the problem was solved
. I ran it up and down the ship canal and around lake union for a couple hours without so much as a hiccup!
Not too bad for a 1970s merc that I bought for a song and hadn't been run since 2003! (side note: the motor came with a junky 1957 Larson fiberglass
boat as part of the deal, which I couldn't resist refurbishing a year ago, minus the outboard. $250 for the lot!)
Thanks for the help, everyone!