Well...maybe you are already getting about as much as you're gonna get outa that engine? I have a .61FX and have had it in 2 different planes...a Top Flite Corsair and a scratch built original design...
I have had as much as a 13 X 6 APC on it, and believe it or not, it worked on the Corsair. Try an APC 13 X 6 (same design as the 12 X 4 with thin tips so it will spin and throttle up better than a 12 X 6 )
Either that, or hit it with a bigger hammer...(.90-.91 size) like you were thinking all along, aye?!
( I just thought maybe the .61FX wasn't quite optimized, but you have it close, so...?)
edit: I have dial-up...(I know
) so watching the video is a !*%#@ pain...couldn't get it to work...
How heavy and how big is this air boat anyway? (just outa curiosity )