i have a 2000 plate dt125r all running sweet apart from when i get up to about 7500 - 8k i get a sort of clicking noise. could the fuel mixture be leaning out?
Fit a new spark plug, take the bike for a spin at full revs and check the plug colour , if it is looking lean i.e. very light grey colour then up the main jet 2 sizes. If you are running standard jets and have messed with the bike like removing snorkel or de-restricting the exhaust the chances are you will need to richen the mixture with a larger jet . It is always better to run a bit rich at high revs to avoid engine seizing or holing a piston .
