I do not know what happened but I noticed (or I am imagining) in the last few days of riding my YZF-R125 that it is as though it has lost some power. It's hard to explain but I can just feel it. The bike has started to make more noise than power. The revs go high but the acceleration is less than what it used to be. Nothing too dramatic of course but I can definitely feel it...and its bugging me!

So my question is, should I change the air filter or could there be another underlying problem? Thanks

Does the engine rev and this is not matched by the acceleration?

Have you checked the exhaust to see if you have any leaks or loose bolts?

Doubt if it is the air filter.

could it be a slipping clutch, it would be worse in higher gears uphill

Does the engine rev and this is not matched by the acceleration?

Have you checked the exhaust to see if you have any leaks or loose bolts?

Doubt if it is the air filter.

Yes and yes, I have checked and there doesn't seem to be any.

could it be a slipping clutch, it would be worse in higher gears uphill

Is there any way I could check that visually or whilst riding? My bike seems to make less noise on higher gears. The issue is mainly with moving off from stationary position. I'm worried I might kill the bike by accelerating harder at traffic lights.

I think its just your imagination-and a touch of Noobism ?, its a common ailment, sometimes shows itself with 'compulsive chain tension checking' (mainly in males though)