Bought a brand new welsh pit bike 125 ran the bike in slowly and never over reved it and bike has had regular oil change bike is only 2 mounth old but its starting to blow blue smoke form exsauhst

is the bike a2 stroke or a four stroke? regards Ray.

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ok so if its BRAND NEW take it back to the place you bought it mate its engine is dead there is no compression either that or your sparkplug is not in tight or at all (lol)

For any 2 stoke motocross bike you can get specific two stroke gear oils from bike shop or simular just make sure it is motor bike specific as your clutch depends on it... as for brand and weights not so important just change regularly and use motorbike specific two stroke gear oil and will be just fine. I always use Bellray oils but its just personal preference and my local bike shop just happens to stock bellray products cheers mate

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how mutch engine oil do u pit in a derbi crosscity 125 my bikes 4 stroke

Check your standard factory specs for spark plugs, it sounds like ur bike getting OVERHEAT!

Did you happen to check the over head valve camshaft? Ussually that bike , w/ age happen to be more tha 10 yrs old, cam shaft, valve guide are worn out and out of clearance! Check it please!

Hope that make your bike be reliable again!

regular is fine but synthetic or semi-synthetic is even better because it makes your bike run a lot better... once you use it though you can't go back to regular...it is a little more exspensive

Are you mixing regular motor oil in the fuel? Doesn't this bike have a reservoir for 2-stroke oil? You must use 2-stroke oil in that reservoir. Buy it online if you can't find it locally.

If it continues to rev, it is probably not a carb problem.
I assume is has a wet clutch:ie the clutch runs in the engine oil.

Try an oil change, use a four stroke motorcycle engine oil,
Car oils have friction modifiers in them that can cause clutch slip.
and sometimes the expensive synthetic oils are just too slippery.
So try for a mineral oil, the bike shop will help you choose.

If the engine is running correctly and the bike has a good clutch then the causes for the lack of forward motion are potentialy: 1) brakes are not releasing fully, 2) a shear pin has sheared on the clutch shaft or on the crankshaft output gear, 3) the output sproket shear pin is sheared, 4) the chain is badly rusted and lacking oil. I would check number 1, then number 4, then have a mechanic check #2 and #3 unless you want to tackle it yourself.