noses over at top end

[jonyz]

I have not long got hold of a 00 125 yz and love it. It has a pro circuit system and seems to have very good low and mid power but seems to nose over at top end.
To get any real speed and power out of it I have to hold it flat and short shift, then it goes like a rocket.
Could this just be as simple as jetting or do I start somewhere else.
Please note I have only had 4 strokes up till now so not a great deal of experiance in this stuff.
Any ideas would be great, and jetting specs if pos, thanks.

 

[kx_guy86]

I don't know much about yamaha's, but with my KX 125 I had the same problem and the reason was that the power valve was broken in the shut position which resulted in good bottom-mid but then didn't rev. That's where I would look....

 

[jonyz]

I'll have a look at that, thanks.
If there is anyone with a good stert on jetting that would be good too.
I am at sea level, temp range around 30 deg C most of the year.