A bit more testing today. I cut 3/8" off the header and got a fair increase.

Temp: 67°
Powermaster GMA 5/22
K&B 1L plug
APC 11x7
14,297
1.87hp
9.68lbs. static thrust

The engine is continuing to break in, so part of the increase is due to that, and part due to the shorter header. This is getting close to the maximum rated hp. I will cut off one more 1/4" and test again. If I get close to 14,500 I'll stop there and be happy. My goal is not to get the absolute maximum performance; this is an old engine. What's interesting is that you can hear it stepping up on the pipe well before the throttle is wide open. I don't have a lot of experience with pipes, so I don't know if this is typical. It's very loud, on the verge of being uncomfortable.

David