Get a thermometer and put it in the water and see how long and to what temp
it is warming up. If the Mikunis are OK when warm then it probably is a temp
issue although making the carbs a bit richer would very likely help a lot.
(Two or 3 flats richer on HS6s will eliminate need to use the choke here in Oklahoma
in warm weather.)
Thermostats are pretty much necessary but they are kind of a weak link both in
failing to close and failing to open. I think a new design would be a great
thing but would undoubtedly cost more! Be sure and get a new rubber ring if/when
you change thermostats and don't torque the water outlet fitting down any more
than it takes to pull it down snugly.
--
George Downs Bartlesville, Heart of the USA!