If your take off runs are getting longer and longer, it's time to mow the grass. Judging from the photos of your prop, your prop is having to slice it's way through grass. That really drags down an engine, plus the build up on the leading edge of a prop really screws up the airfoil making it work poorly.

But I agree with Jester on prop selection. A 10-5 will often wake up a 40 sized engine by getting it up into it's power band.