These figures were obtained this past weekend when we tached a US 41 with an upgrade from RCign (straighten and true up hub) and the addition of a WA144 Walbro carb (same carb that is on a Quadra 52) Proped with an 18 X 10 Pro Zinger, the US 41 turned 7700 rpms static on the ground.
I've seen a U. S. engine so equipped power an American Eagle 85" span P-51 at 24 pounds and a Yellow Aircraft Spitfire, I don't know how much it weighed, very adequately. I was actually very pleasantly surprised at the level of performance obtained with this little engine upgraded as described above. One engine was running a straight header pipe while the other was using a B&B Muffler.
For the price, with the upgrade, they are hard to beat for reliability and performance in an engine that size.