I am having the same problem with my 125-11. After about 10 to 15 km it loses power making me coast to a stop, the engine does not cut-out and after a few minutes I can continue as normal for another 10m to 15km. The problem only happens if I have sustained continual speeds of above 40, around town I have no problems.
I am still trying to solve the problem but it definitely feels like fuel starvation to me.
Suggestions so far are.
Fuel tank building up a vacuum - there is a ball valve in the fuel cap and the rubber seal incorporating a breather channel. This might be blocked – I have cleaned mine but it has made no difference.
Fuel filter restricting flow. I have not checked this yet, but I am told that the paper inline filter does restricts fuel flow. I intend to try fitting an alternative gauze filter.
Float chamber adjustment - I will need to strip the carb for this. Idea is that the engine is empting the float chamber slightly faster than it is filling, hence the problem after a period of time at speed. Around town it gets a chance to fill-up at stops. When the bike loses power, the few minutes spent at the side of the road allows the chamber to refill. This might of course be caused by either the fuel cap or fuel filter.
The final suggestion that has come my way is that it could be carb freeze - I personally find this difficult to believe on a bike of this nature.
I will let you know how I get on with the above. If you solve the problem before me please let me know
Regards
Graham
Graham
March 2008