I have a Bultaco Astro, which is the same as the derbi senda, really. It also throws a lot of oil from the exhaust. My problem is different. The carb o the astro is 18mm, i think yours is 14mm. This gives less lower end but much more higher end. When mine is cold, it has NO lower end but nice high end, and when it heats up its great. The lower end problem of yours i think is because your carb or air filter is dirty. The oil probkem can be soved by reducing the oil going into the cylinder. The oil pump is to the right of the cylinder, inside the "Carter"(i'm nt sure if that is how its called). Anyway where all the tubes go in that area. You should take off the cover and turn the screw in a little. A common problem on Derbi is having a high engine oil level (the oil in the gearbox and clucth). To know if there is too much: when you put first gear (disengagind the engine), is there a slight ted to pull forward? This is because the oil level is too high.