ORIGINAL: HighPlains
The FP is a very nice handling engine, but it's no powerhouse. Mild timing (even by OS standards) and it's small carb inlet make it so easy to operate and tune, but severely limit it's power. I can't think of any engine with less power made in the last 40 years other than the LA series.
Ooh, you should have tried a Merco.
As you observed though, lovely to operate, quite economical on fuel.
Thinking back, I did use castor fuel at 20% for the first 5 years of its life. I use ML70 now which produces less mess.
The Irvine Sport 40 though just blew the socks off an OS40FP.