Exhaust valve problem?

[Newbone]

Hi,
We have a KTM 125 EXC that after renovated cylinder behaves strange. It runs as expected pn lower rpm but on higher rpm it looses power, most of the times at least , a cople of test runs i has been running just fine. I'm suspecting the exhaust valve but when inspecticting it it seems to move correctly. How would a bike run if the valve is not operating correctly? I don want to take the cylinder off is there could be some other cause... ( have tries with a new spark plug).
Cheers

[IR8D8R]

Assuming you are asking about the power valve? This might help:
https://www.youtube.com/watch?v=VAfi_zy2hbc.
The comments on this video were also enlightening. Bear in mind that I know nothing about KTM 125 EXC's. Looks like a neat little bike.


IR8D8R