if it is the old gas turned to tar it's a tough job. Best thing I found short of costic chemicals is to fill it with gas or kerosene and let it sit for awhile to dissolve it. Agitate it from time to time. This is how I clean that gunk out of my 55 Studebaker that sat in our barn from 1964 to 1992 when I pulled it out.It had gas in it all those years and it turned into a solid.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!