Urban Tangleweed

Hi, I bought a shed of a DT50 (yes, off ebay...............) and have done lots to it, but it won't run right. It has had a rebore and piston, the 14mm carb was kaput so I got a brand new 16mm (yes, ebay again...............) Starting is great, tickover fantastic, it revs up a treat, but, when ridden, it won't go onto the main jet, you can bimble along on a part throttle all day long (hills are no good though) but open it up and she just dies, go back to part throttle and she picks up again. I've checked, and altered, the float height, it's running the correct 88 main jet, but just won't have it. Do any of you guys know what may be the cause ? Something else which may influence any suggestions, it's running pre-mix, the oil pump does not exist any longer, perhaps I need a much bigger main jet now the petrol has oil in it ? Hope someone out there can shed some light. Thanks, Roger.