Unfortunately you are performing static tests which may not be indicative of actual "in air" performance.

Your .61 is rated at 1.7hp ( unrealistically! ) at the high end of the RPM scale.... 16000 RPM.

To spin a prop that fast you would have to use a small diameter prop, but you would loose almost all thrust.

The prop "unloads" in the air, letting it spin faster in flight.

You didn't say if the engine is new. If so you'll need to run it for a good while before it will develop full power... also what Nitro content are you using and at what altitude are you taking off from.

Both make a big difference.