try some other perhaps geared setups...... I did a USA star this last winter that was less than 10 lbs, and it had adequate power.
used a Maxcim motor and 20 round cells....not lithiums...I'm not convinced that Lithium is the answer for E powered models yet, especially when I can get 10 min flights fairly easily with Nicad or Nimh cells at a LOT less expense, and not really any problem with performance. I say this after having used lithium cells in some of my larger models, and coming to the conclusion that there wasn't really enough advantage to justify the cost. That being said, I have to say that for the smaller type ( park) models, lithium cells are a God send. Several years ago a friend of mine did a 72" Dream that used nicads, and it flew well with really good power, and would still KE after 15 min . larger motor/lithium battery setups don't necessarily give better service for 95% of us. He used a Maxcim Y wind...4 to 1 reduction, 24 cells and a 14X10 APC E prop. Same goes for the argument that a certain motor is 5 or 7 % more efficient than another..... 99% of us would never tell the difference between them if we flew identical models with the two different motors.
7% makes a difference of less than 1 minute every 12 ( theoretically) and a smaller power train doesn't necessarily translate into less power./performance..........................just my opinion from experience.