I appear to be losing top end power on my bike. Admittedly not much top end anyway beng a CB250... However, went on the organised ride on May 1st to Yarra Junction. At a couple of points bike was struggling to maintain 80kmph when wound out to throttle stops. I changed the spark plugs prior to this ride (exactly the same replacement) and thought this could be the problem but the bike was not misfiring at any point. When we got up to the top of the hill at Neerim Junction and started to go back down the other side, the bike would not speed up. I backed off the trottle and then wound it out again but still no improvement. Bike is due for 24000km service, so I will take it in as soon as I am able. Anyone got any ideas in the meantime?