Sounds like to me your engine is getting an air leak or bubbles in the line my Irvine 46 done the same and it was down to carburation as for the black stuff I had tones of it come out off my Irvine when it had its bearings replace all I done with my engine was wash it out after I used it and oil it up after a while it stopped but if you don’t wash the engine the little metal bits could keep wearing metal away mines had about a gallon of fuel through it before my engine really settled down