My friend Fred is breaking in his Magnum XL .70 RFS right now, and one issue he ran into was that the spring tension on the high speed needle wasn't tight at all. Basically, when Fred would try to dial in the high end needle, the engine would rev up toward peak rpm and sound great, but as soon as Fred let go the high speed needle would start turning on it's own.

The vibration from the engine would cause the needle to start spinning on its own back toward counter-clockwise. The engine would richen up and the rpms would drop until the vibration wasn't enough to turn the needle anymore.

Your .70 RFS may seem way down on power due to this same problem.