Motorbike losing power, why?

It's only done 6,500 miles, its a 125 Honda cg, it's only a spare run about bike of mine but it shouldn't be losing power! Got enough clean oil and full tank too (not like petrol should have anything to do with it) Breaks aren't touching ect why is this?

There's a list of things that need maintenance on a regular basis, have they been done?
Air-filter clean?
Valves been adjusted?
Spark plug been changed, properly gapped?
Oil changed with a new filter?

Like Chris said - it might need a tune up.

Water in the exhaust or hard as a stone coal buildup close to the header. Exhausts can be blocked on the rear end sometimes When there's dirt round the opening poke with a stick. Are the spark plug cables properly connected to the caps? A sort of clear coat can have formed on the inside in the fuel tank when you did not ride the bike and little pieces will clog the fuel filter. Condense can gather in the carburettor so you will have to drain the gas in a pot to see. Also the float may have a small leak and gas inside. Search and you will find.

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