DT125R losing power.

First think safety w speed..ur on twowheels. Jetting a carb is touchy..leave it to xperiencd pros. Too lean an engine w fry it

your clutch disc might be bad.

Not much to go on here. Power loss in a high gear would likely be attributed to carburetor. No bike, no year, no info, little tough to help you. The throttle would likely be fairly wide open in 6th gear where the bike would have the least power and would be calling for gas and probably not getting enough- or possible to little/too much air. Either way if runs in lower gears it isn't likely to be plugs or ignition, timing, valves or other obvious mechanical stuff.

Most probably you need to check the clutch

If all other gears are doing okay then I would check with your local dealers parts department and ask the number of teeth on the counter sprocket and on the rear sprocket on a stock dtr 250. The prior owner may have mounted a larger counter sprocket and / or a smaller rear sprocket on the bike. The drive ratio may be out of range such that the engine is trying to make for a really high top speed but just doesn't have the strength to do it.

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Suspect worn needle valve & seat assembly !---Quick test , with care>Whilst riding on open road &holding throttle at set position, switch off fuel supply & note IF engine speeds up, IMMEDIATELY Back off throttle, watch rear vision mirrors for traffic behind & switch on fuel supply!---If that small test indicates Carburettor area of further examination, UNLESS YOU ARE QUALIFIED the needle valves need to be examined byTrained eye!-(Especially wear on sides of needle valve & ALSO, check "floats"Stay Upright Des