[slight flight of fantasy]
For starters, the powerdynamo flywheel is much lighter than the stock RV90 unit
AND..but it seems to really fall off up top
If you can never get your power back with timing and jetting, you may have to either make the crank heavier or raise (or widen) the exhaust port.
First let me say that this is the last thing to think about, but it is a real effect, and is pretty cool to think about.
This can be an issue, especially if the engine has a rather high compression or is oversquare (has a short stroke and relatively big piston).
Huh? It seems that a 2 stroke engine changes RPM throughout each rotation.
The RPM slows down during compression, the RPM speeds up after combustion.
Allot of small racing engines have a race kit that includes a heavier flywheel or a heavier crank.
What happens is the engine speeds up while the piston is descending from combustion forces, right when you are needing blowdown. This robs you of blowdown time*area because the piston goes past the exhaust opening, to the transfer opening too fast, reducing the blow T*A, thus it will not support the same power at the same high revs when you lighten the rotating mass. (Actually I think it is the moment of inertia, IE... you gain angular momentum if the flywheel mass is further from the axis of rotation, or some shit like that, but you get the idea).
You Gotta be... YGBSM (look it up wild weasel)
Yes, a CR 250 changes RPM by something like 700 or 800 RPM at 8,000 RPM or something like that.
Many race bikes have heavy flywheels or cranks to smooth out the change in RPM over each cycle.
Anyway, just a possibility, maybe not a probability.
[/slight flight of fantasy]
Hope it is just timing