burning oil like petrol

i think i might have a valve stem problem as it burns oil like fuel when its reved it smokes white /grey smoke could this be vale seals

White smoke sounds more like water than oil to me, have you had the plugs out to look for a "washed clean" one?

some are ok and some are black they have only been in a week

If the smoke is whitish then you are burning water. Data's suggestion should help identify that. Oil smoke will have more of a blue tinge to it. If you are having to top up with alot of oil then you are either buring it or losing it to a leak. If not the latter then check the breather hoses on both the rocker covers and make sure they are clear. If they are then the most likely culprit will be worn oil control rings. This isnt so easy to check but a clue can be found by doing a compression test and then squirting a bit of engine oil into the cylinder and redoing the compression test. If there is a difference in the reading then the piston rings are worn - you can therefore assume the oil control rings are warn also. £75 will buy the new rings but you will burn throught a lot of other things just to get at them (head gasket kit, big end bearings etc.)

A white clean rag on tail pipe whist ideling and warmed up should produce a black dry soot if bad oil burner you wil find soot too be wet and oily