RE: Rossi 53 - dud??
Sparing the tutorial on published HP info, can you identify where is the engine advertised as producing 2.3 HP ? Also, please note the rpm that might be suggested at which it develops that power?
you are on the right track - the "45 size" rossi mini-pipe is rather short-tuned. Needs to be propped to turn over 16,500 to function worth a hoot. Also, the o-ring style mini-pipe tends to leak at the o-ring if you to not properly support the tail of the muffler. Gas leakage past the o-ring can cause problems and inconsistant running.
The 9x8 info you have there is about in line with where it will want to run. In the air it probably comes on the pipe just about right. Something more along the lines of a 10x6 should turn up there too.
However, the 53 is bit lower timed (IIRC) than the .40 and .45 engines. More intended for the quiet euopean market. More along the lines of the Jett .46 and OS .46 timing. The displacement gives it some extra oommph to turn a bit more prop. So it can and does run well at lower rpms with "quiet" props, when combined with a properly tunned full-side pipe. It will be much better off with a full sized pipe, or a broader-band tuned muffler.