Hey hooty!
Go to the YS Support forum here in RCU and ask Dave Shadel about the Ultra Thrust.

Try a smaller diameter / higher pitch prop too...that 12 X 4 isn't enough prop for that engine.

Why doncha get a few different props to experiment with...if that .61FX is still "healthy" (good compression, runs well ) I would try an 11 X 7 APC and a 10 X 8 and 10 X 9 just for the heck of it.

The fuel is O.K. but personally, I would add a few oz. of castor oil to it (to a full gallon )

For now, if you are running the stock muffler, take the baffle out of it and drill out the "tailpipe" 1/16" larger. (do that in 2 x 1/32" steps because the alum. is hard and the drill will want to catch and bite )