But at WOT and 3/4-max rpm I assume the power valve will be fully open and will have been fully open for quite a long time? (does any-one know rpm's the port opens as std?)

What pipe and silencer are you running?

The power valve (or exhaust valve) opens up proportionally with the engine's RPM.

I haven't seen the mechanism properly but seen photos of it. I'm pretty sure you'll be able to add some washers or spacers to limit the travel of the mechanism, thus preventing the exhaust valve opening fully and in turn softening the power at higher RPM.

You could also experiment with disconnecting the linkage and locking the exhaust valve in it's closed position. In effect this lowers the exhaust port giving more power at lower RPM and less at higher RPM.

These are a bit expensive, but might be exactly what you're looking for...

http://www.enduropuro.it/shop/en/gas...lve-cover.html

Good luck, Dave.

That scenario sounds like it could be solved with some clutch.
Other than that you got lots of suggestions, seems like you will need to do a bunch of experimentation to find the right trade off.

Few variables. I know on the 250 (which has more pronounced surge as it comes onto song in the mid to upper rpm) that I tend to clutch it onto the pipe all the time. I couple it up with a quick pull throttle cam which means it takes very little effort for me to have poor 'throttle control', and very easy to twist it wide open. The clutch is where the control comes from and it modulates how much power gets to the ground.

Again, you face issues with clutch control as fatigue builds throughout the duration of a race, but imo that becomes more to do with conditioning (and gassing our from poor technique/line selection) than with the engine and setup itself.

Last couple hours I've been riding with my stock expansion chamber kinked at the header, coupled with a nice big dint on the backside where it's been pushed into the frame. It's definitely moved the power down and where it used to pull like a train in the upper rpm you can easily feel the power drop off as the revs climb on. For 90% of my riding it hasn't slowed me. The point being I wouldn't discount a change of pipe either.

Port/deck height changes can move it around too.

And in relation to the original jetting side. Richer. You can go rich till she starts to blubber. It might soften it up some, probably suck some more juice too.