What's the RPM reading with the 11X7 prop?
I think, based on personal experience with my Dash 5/O.S. .61 RF, that the RF Long Stroke motors like bigger props, like 12X8, even 12X9.

Even with pumps, it is my experience that with the OS engines, you want to find peak RPMs and back out about 400 to 600 rpms
I was flying mine with Byron 15% Nitro 18% oil fuel and my engine was spinning an APC 12X8 at well over 11,000 rpms on the ground.
I put over 150 flights with that set up with great performance.

Here is mine built from scratch with a friend's set of plans. This picture is after the upgrade to the O.S. Hanno.