Small Engines (Lawn Mowers, etc.): i have an older 5hp b&s engine..., excessive fuel, black hope

Question
i have an older 5hp b&s engine off a tiller on a go kart. if i give it gas when there is noone on it will sometimes rev up but also die sometimes and when there is someone on it it will die most of the time and if it gets going if will only rev half way and die out after a while. everytime i take the spark plug out it is completely black with carbon. i completely cleaned the carb and tuned the jet on the carb but it still does the same thing

Answer
It still sounds like there is an issue with the carb.  If the spark plug is completely black, there is an excessive fuel problem.  Because you say it sometimes won't rev up from an idle, I suspect the low speed circuit in the carb may have a blockage (lean) which you are compensating for by enrichening the high speed circuit but this is causing excessive fuel at higher RPMs and that's why the plug is coming out black.  Hope this helps.