4 stroke engines give more horsepower with higher revs so they built engines like that ... hence the 14,500 rpm redline. When the engine is flat out it pumps out a lot of gas so it needs a big bore exhaust to do so.

But

A big bore pipe doesn't work at low revs which is where the torque usually is so most bikes have a compromise between the two.

Yamaha fitted a valve, the Exup, which is closed at low revs making the exhaust pipe smaller it then opens up increasing the volume capacity of the pipe until about 8,000 rpm when it's fully open.

This means in practicle terms you can go from idle to 8,000 more easily. Some race bikes blank off the exup or fit a non exup system because they keep the engine above 8,000 saving the weight of the exup, but this sacrifices the start.

hi everyone thanks again for all your help. lee thanks for the offer of the exup valve i will keep it in mind think iam going to contact the person who i brought the bike off to see if he has the exup system and get it back off him and try and refit it to the bike hopefully he will still have it if he has and i can get it fitted i will let eveyone know the outcome thanks again for the advice

ive got micron system fitted to mine from a 600 and i get a small flat spot just below 4000 rpm ive just learnt to ride around it