I don't know of any company selling heavy flywheel for 520....I don't think the 520 as abrupt power,just a LOT of it ...Your problem is probably one of adaptation to the bike ...don't ask me how i know ....I bet it was in nasty slow stuff you crashed,i had a tendency to be too tense on the bike at first and it did bite me from time to time !! Now it feel OK, it's so smooth next to a 500 2 stroke or my XR 654 that going back to these seems unthinkable now...It help if you install a 50 or 52 t sprocket at the back,it will allow you to use second to load the engine so it won't rev so quickly...Have fun !!
I think it's possible the 520 is a bit much in the tight stuff, but this depends on the style of the rider. My crash was simply a fluke, came to a mud hole, moment of indecision, twitched the front end, hand slipped on the throttle, bike lunged, shot me like an arrow into the trees, hit a stump, bike endo'd tossing me over the bars. I had been riding it in some fairly tight stuff and it felt okay. If I had reacted quicker, like a 2-stroke guy is accustomed, with a finger on the clutch lever, possibly could have avoided this. But being used to the forgiving habits of my former DR350, I was careless, and the KTM bit hard.
Thank you for the feed back, hope everythings is healing up well. In thinking I would contact Steahly or Moose for flywheel weights. It is interesting that in '00 model year the 400 exc, 520 sx and 520 exc all shared the same flywheel. the 400 sx had a different one according the parts books I have.
The 01 400 SX flywheel is a lighter one for quicker throttle response...fun on a motocross track but not in the woods :D .The 520 is a little much in the tigth stuff ,but better than most big bore ,the finger on the clutch is mandatory in tight stuff ,i have one anytime now ..always usefull to pop a wheely or coasting on a slippery patch of soil or disengaging the engine when thing get a little out of shape (lessen the probability of those shot like an arrow incident ) ...heal quickly quadrunner...and roost on !