You do have a thermostat but unless it's getting overly hot, or not warming up at all, that's not your problem.
Do the cylinder drop test and see if any cylinders are idling weak (if they are you won't notice much change when that plug wire is removed). If they all react equally then set your idle speed to 1100 rpm on the hose and go try again.
If one or two cylinders seem weak then you either need fresh spark plugs (gapped to .030), or to have the carbs overhauled and re-synced.
On rare occasion, leaking exhaust seals will cause rough unstable idle, but usually carbon will build up and "reseal" those leaks on its own.