Pop wrote:
It sounds to me like the carb might need a good degunking.
Probably plugged or partially plugged jet/s. Try screwing the
needle valves in (gently!) and back out a few time,s then put
back to where they were and see if it helps. If not, perhaps
something's plugged.
You did check for a waterlogged/plugged fuel filter, right?
Fuel's getting TO the carb?

HTH,

PopS

I'm thinking too that it's gunked somewhere. Fuel _IS_ getting to the
carb (if you pull the starter rope gently with the air filter removed
you are met by a mist of fuel around the choke valve). Haven't played
with any fuel filters. Is there supposed to be some sort of linkage to
open the choke plate? It's just that I can't see how it opens on it's
own (vacuum effect???) and that would explain richness symptoms on
the(new) spark plug and exhaust output popping.