yeh they do make an 81 but they are like $45 more than the 91?
I've got several planes with 91's in them and the fuel economy is extremely good.
I just discovered by googling the fs 70 ultimate that OS has been recalling and exchanging them for the 81's due to extremely poor runability/ tuning issues as well as issues with
the pump raising hell with the tank so I'll make contact with them monday and see what happens.
I really want to get this thing finished for spring and fly it , I've been puttin around with it workin on it a lil bit here and there for 2 years now, it's time to either finish it and fly it or get rid of it