Tom, did you bother to collect a little of the "bad" gas in a glass mayo jar, spin it gently and check out whether there was water or solid crud at the bottom of the vertex? As was mentioned, the idea is a process of elimination without going too fast. Before I went tearing into the tank, I'd add a little gas, let it flush through the tank, drain and spin and check it for particles of crud. If none, move on..no sense in messing with the tank if it doesn't indicate solid contaminates.....run a point file through the existing points to knock off any oxidation (you've already checked the gap, but they could be oxidized) and then crank it up, burn out any cruddy fuel from the carb pot and pump/carb line. If it doesn't improve or there are crud particles present in the fresh fuel, then I would pop off the air horn, check the float level and make sure the little sliding power valve was not stuck. If the carb was crudded up it would be time for a good disassembly and soaking in carb cleaner, hi pressure blowout and finally a kit.
As Tiny pointed out, bad things CAN happen during a restart on an extended shutdown engine. I'd pop off the valve cover to verify it was oiling up on top (you've already done this) and then run it at the lowest idle possible until you either have it smooth out with the fresh fuel or elect to clean up the carb if there was crud present in the float bowl. Can't speak to Sea Foam as a fix.....I'd focus on getting new, clean fuel to the carb and then moving forward from there.
Dave
