ok firstly a powerband is a peak power dleivery between a set of rpm for instance a motorcross bike will typically produce its peak power from 11000 rpm to 1300 rpm so thats its powerband ,it is not an actual part of an engine just a description of peak power delivery.there are lots of reasons why an engine may not be producing peak power but the most common ones would be low compression,blocked air filter or exhaust pipe or on a powervalve engine like most 2 stroke motorcrossers have the powervalve/s may not be operating properly and not allowing max revs due to restriction of ports sizes or the amount of ports.for instance a kawasaki kx125 has a k.i.p.s system on it and that has 2 extra exhasut ports that come into play over 7000 rpm approx.and a yamaha tzr has a exhaust port that is made higher and the duration of opening is longer after 7000rpm .hope that clears it up for you a bit ?
adam bike doctor(of milton keynes motorcycle centre), January 2011
