xr650r will not hit rev limiter in 5th

Hi All

Please could anyone help, I have a 2001 supermoto xr650r with a full power core 4 pipe, 175 main jet, 68s jet, B53E needle on 3rd clip, k&n filter, air box mod but not vented the side panel. Gearing is 14 front and 42 rear. Mixture is 2 turns out.

In every gear but 5th it will rev out then in 5th it will only rev to 6500 rpm, why does it need more air?

Thanks in advance for any advice.

You might have reached the peak power of the engine in 5th, not every engine will rev out in all gears, there is a point where your motor cannot provide enough power to make the bike go any faster and as such she won't rev out....bit like stick a 15 front and a megga small rear the gearing will mean it's impossible for the bike to ever achieve max rev's with that set-up.....if that makes sense. Easy way to prove the point is to reduce the gearing, try a smaller front or larger rear and see if the bike rev's higher....I am sure it will.

I agree with Mauser, put a 48 on the rear and that thing will hit the limiter in 5th for sure. You may have issues keeping the front on the ground on your way to 5th though.

How much time on the motor?
Compression readings?

Not gone as far as compression readings. Not sure what you mean by time. Bike top speed is 91 mph, yet friends hitting about 105 mph with same gearing as mine. Would it make much difference if I drill the side panel.

Cheers guys.

Remove sparkplug.
Attach compression tester.
Get a cold and warm engine compression reading.

Open up you airbox side panel!

How many hours are on your motor?
Are you the first, second or third owner?

Did you adjust the valves?

common possible issues -
loss of compression
incorrectly jetted carburettor
dirty airfilter
incorrectly adjusted valves
damaged/faulty spark plug - unlikely .

My side panel was not drilled, opened it up and fook me the beast is alive!