i have the same motor set up your talking about. i have the ninja pipe on the motor also. my motor did the same thing sounded like it had an air leak or something. after break in of about 12 tanks of fuel getting closer and closer to a race tune. i changed the con rod, output bearing and sealed up the motor with a light coat of o2 sensor safe rtv seal. carb,back plate,fuel needle inlet, everything my problem went away. like it was stated before the pinch is hella tight on the ninja motor, you should change it to be safe to get the total performance out of the motor. that motor tuned well can run up to "270-290" no problem. normaly i like to do a motor tear down after a break in anyways.