Recently ive been having problems with my throttle cable sticking (Which it seems it will unless, as the manual says, you oil it).
The wrong throttle cable tension will also cause it to 'bottom out' when going full throttle. Try going two thirds throttle.
It's much more likely to be your fuel mix if the plug is black - either that or its been flooding OR you left it sitting around without closing off the fuel supply.
In any case, grab a small file or something coarse (I actually used a brick haha..) and a toothbrush and, using a little bit of petrol work it until its clean and shiny. Try to really get it in there as to make sure there isnt a conductive film on the white electrode casing in the middle of the plug.
Another thing you could check is if theres any moisture or anything inside your magneto cover - a bad magneto can also cause very sluggish performace.
You really shouldn't need to adjust your carb slide from stock just to get it running unless you're in a weird climate perhaps.
