Would a flywheel weight add

[kx5rider]

engine braking?
I run my kx500 in superrmoto and could use a little more engine braking, would a nice heavy flywheel help it out?

[Mick]

It'll do just the opposite.  The engine will have a tendancy to carry rpm well after you've cut the throttle.

[kx5rider]

Where I would need it to work would be motor at low rpm's, downshifting coming into a corner to start sliding so you would be coming from low r's to high r's as the clutch was released............. I was thinkin the extra weight wouldn't let it spin up as fast and would allow the rear to slide a bit better.

[BDI]

Mick is correct on that, you may try the powervalve bolt mod to keep the valve from over closing that may give you more engine brake.

[doordie]

Where I would need it to work would be motor at low rpm's, downshifting coming into a corner to start sliding so you would be coming from low r's to high r's as the clutch was released............. I was thinkin the extra weight wouldn't let it spin up as fast and would allow the rear to slide a bit better.

This is the downside with twostrokes at supermoto! 
You have to be more sensitive when braking so try to modyfie your rear brakepedal and caliper.