125-200cc mid range issues

Hi i have not long purchased a 2010 wr 125, my first husky and loving the bike. but it doesnt seem to have a very good mid range so ive put an fmf fatty and power core 2 muffler hopeing to help but it hasnt made much difference, ive also just purchased a jet kit for it, should i use the 4.0 or 5.0 slide for carby,if this doesnt help the midrange what else can i do ?? as it has an awesome bottom and top end. also it has been spitting black oil out the exhaust could this be the fuel mixture to rich as i run fully sinthetic oil at 30-1, this is my first 2 stroke always had 4 bangers

Sounds like it is too rich in the mid-range. Try lowering the needle (raise the clip position) and see if it gets better.Good luck.

I think it is just how it is . Cr. ignition will give more mid or bigger bore kit