Reducing weight on a track bike
Reducing Curb Weight
It's actually fairly easy to cut weight off the Ninja 250, as long as it's only going to be used for track days or racing.
The first place to look is in the mirror. If you're not in good shape, consider that most professional racers are athletes. They do a lot of cycling and gym work to deal with the amount of effort needed to get a motorcycle around a track.
As for reducing bike weight, make sure you check with the track day provider or race organization to make sure of what you are allowed to remove and what you must keep. The bike dissected for this project is going racing in the WMRRA Ninja 250 Cheapskate Cup, which has decidedly different rules than other race series.
It's not uncommon for racers to spend large amounts of money to lose less than a pound off a race bike (think replacing lots of steel fasteners with titanium). It's cool to know that you can drop over 20 lbs on a Ninja 250 just by taking stuff off.
These are the things that were removed from this stock 2005 EX250F and their respective weights:
Weight savings so far: 21.5 lbs
The stock airbox weighs about 2.9 lbs. A set of Uni 4" filters (installation of which is detailed here) and a little crankcase ventilation filter weigh about .5 lbs. It's only a 2.4 lb savings, but pulling that much off the bike is not to be scoffed at, particularly in competitive riding. As with installing a Muzzy below, changing the intake means you must adjust your jetting.
Removing the stock exhaust and replacing it with a Muzzy system will save you some weight. The stock system (as removed from a 2005) weighs about 18.75 lbs. A used Muzzy (which may be missing some packing, and is therefore probably a bit lighter than a new system) weighs 7.25 lbs. This is a savings of 11.5 lbs.
Naturally, swapping in a Muzzy will require rejetting. The stock jetting is almost too lean to start with.
Other aftermarket exhaust systems (all of which use two cannisters, leaving the stock headers intact) may or may not reduce weight. Check with the manufacturer. If their cans are lighter than 6.75 lbs (the weight of one stock exhaust can), you'll see an improvement in curb weight.
Chances are you'll be running some kind of fairings on your track bike. Most aftermarket race fairings, though, are made of fiberglass and are lighter than OEM. Check with the manufacturer. Here's what the stock ones weigh: