[Klondike1020]

Count the teeth on your sprockets and see what they are, thats probably the reason.
step two
IF you need to go faster than 80 you must live in germany and be driving on the autobahn.

in that case sell your ninja and buy a liter bike
finally
if you really need to make it go faster
Drop it off a cliff. it should reach 100 before it hits the ground.

make sure duck tape your phone to it so that the accelerometer can tell you how fast it was going before it stopped

also use a otter box. if the phone doesn't survive you will never know how fast it went