I showed my mates how to do their 115 yammie a few months back i have a crap memory but i think there were rubber rings around the thermostats make sure these are in place because from memory i think that rubber ring sand wiches the thermostat cover to the block had to think back there could be gaskets but im not 100% sure i think there are only seals and u have two thermostats on this engine
You can always start the engine on the muffs with the cowling off hold one hand on each cylinder head towards the top and see if they heat up a simple 10min idle on the muffs should only get to about 30c if everything is working fine
My diagnosed my mates engine of having faulty thermostat about 2 years ago they ran it as is untill replacing it starboard was stuck open