00 aprilia sr 50 finding it hard to start, when it does start its not right ,have to keep it half throttle full throttle it cuts out, it revs high then dies down then revs high again and so on, i,v noticed drops of black oil from exhaust and load of white smoke on high rev, it has new battery and plug have cleaned carb and blew out jets, have tried different auto choke, fresh fuel, still the same, it took ages to get it to turn over had to use half can of easy start , any help please
