Here's my $.02 worth:

The 20% Nitro will make the most difference at idle speeds. Your idle will be much more solid and reliable with 20% than it will be with less nitro content, even if it's only 5% less. Don't expect higher nitro content to cause a huge increase in RPM's at high speed, it won't happen. At least it doesn't with my YS engines.

Does it run smoothly at 7300 RPMs, or is it running rough? Does it smoke well, or do you have very little smoke output? Does the engine act like it's running hot? Does it have smoke coming off it after you shut it down?

I'll bet it's running lean at full throttle. Try richening the regulator and/or the HS needle some until the engine seems to be running rich, with lots of smoke and running sorta rough, and then lean it up to the maximun RPMs. That's the best way to get the maximun out of whatever fuel/prop combination you're running.

Also, you might try a little less pitch on that prop. A 16x12 might be bogging the engine some. I like a 16x8 on a 140 size engine.

David