the black plastic part has a needle in it that goes into the main jet, its called a slide, it should sit all the way down, when there is enough vacumn the slide moves up and pulls the needle out of the main jet some to let more fuel in to go along with the extra air thats coming in.
if you have a vacumn leak it will not get enough vacumn to operate properly or if its sticking. remove the top plate on the carb, remove the black diaphram, be careful not to damage it, clean the slide and check for scratches on the side
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