Ever since I got my 2011 250 Six Days bike, I've been fighting with the jetting.
Tried the JD kit, it was close but not suited to my riding style.
I have the Keihin 38mm Air Striker II carb.
I had some down time from riding so I decide to have my head modified to reduce the squish and have the full carb mods done.
I've had
RB designs machine work done on my previous bikes, so it was a no brainer.
Finally got to ride the bike with the head/carb mod this past weekend.
AMAZING!!!!!
I did not have to change any jets, just some minor fine tuning with the air screw. (sea level, 98F)
The roll on throttle response pulls like a freight train. It was totally lacking before.
I like to roll the throttle on at the apex of the turns, the bike just pulls away cleanly. Before, the power would not start building until the throttle was 1/2 way open.
I even found myself riding a gear higher. If I got sloppy and let the revs drop, a quick blip of the throttle and I'm back up to speed.
The JD jet kit NEVER ran this good.
My compression jumped up 15psi after the head mod. I'm pleased with that.
The bike would ping before-now it doesn't.
For the price, I would say the RB Designs head and carb mods are the best thing you can do to your Gas Gas.
http://www.rb-designs.com/