violent head shake with fs650

I had the same problem with mine a few years back. 1st thing is check your tyre pressures, I run 24psi front and back on Michelin Pilot sports, then set your static and race sag. If no different try lowering the front end 5mm, in other words raise the forks through the yolks. This steadied mine considerably, you will still get a shake if you hit a cats eye or a ripple in the road if your going real quick but it soon settles. If that doesn't cure things then maybe try a little more compression damping on the rear shock.
Don't try all these at once or you will end up in a worse situation, do them one at a time and test ride after each alteration.

Got the same problem with mine. Gonna try a few things that Freaky said and see where i get cause dam!! this thing ***** me up every time i hit a bump or cat eye at speed. 8O

hello,
first adjust the spring preload in both front and rear. i guess you are in the softer side.
then set both compression and rebound on both front and rear to the oem setting.
check how much are your triple clamps lowered (if they are anyway). return to minimum lift (if you ride on the public roads that is).
check the offset in your triple clamps, if they are set in 18mm , set them to 16mm which sets the trail longer and as a result is a steadier front.

try to set the handle bar closer to you, this will cause your arms not be streched .

if non of the above is the case then surely something is wrong with your front tire.

as for me i have headshaking now over 120km/h but :

1. the bike is set for supermoto races in tight tracks (max 5th gear).
offset is 18mm (quicker turning)
triple clamps are lowered 20mm
static sag is 0mm both front and rear.
all compression and rebound settings is more or like +5 clicks harder from oem.
2. never under any circumstance headshake becomes a really serius issue, learn how to live with it!

3. i use michelin slicks.

4. i so stunned from the tremendus turning that my fs650 now has that i happily sacrifice any instability in the big straights , this is something that is easy to get used to it

I'm sure you have checked this already, but if not have a good look at your headset bearings. Especially if you throw the front wheel a lot. Even if they seem to be well adjusted you can still have a flat group of bearings right where you normally ride and the steering will dither over this area.