I own a 2006 250 ninja. Just did the valve adjustment by the way.
You have to ask yourself what can you do to an engine that will rev to 14,000 rpms. That fact alone has to tell you Kawasaki is wringing max out of that motor.
I also own a bunch of Harley’s. Harley leaves a ton of power in the motor. Why? They want to sell you all sorts of aftermarket add-ones. My Fatboy has Rush slip-one ($400 dollars), Screaming Eagle tuner (the one Harley got a $15,000,000 Million dollar fine for selling, cost $450 dollars) cam (another $500 dollars) air cleaner($250 dollars). Oh yea I forgot Dyno time, another $1,000 dollars. Gain 20 HP and 25 foot pounds in torque. That is a big price to pay to knock off a second or two in the quarter mile. Did I mention I drive my Fatboy more than a 1/4 mile at a time. Harley gets folks to think they need to get that extra HP.
Don’t confuse your 250 pregen ninja with a Harley. Kawasaki already extracted the HP. Harley survives selling add-ones, leather goods, t-shirts and all manner of shinny stuff.
About all you can do is change the sprockets to push your ride from max acceleration at One end to top speed on the other.
My advice is leave it alone, it is not a Harley! Plus open pipes on a 250 ninja is never going to sound like a Harley.
