I've been shaking nuts and bolts around inside with fresh gas. The sludge is so thick that you can't even hear the nuts hitting the metal.
Just tonight I decided to start flushing it with water and that seemed to help as now I can see progress and you can actually hear the nuts hitting the walls.
I think before I cut it, I'll try a pressure sprayer, and maybe something like a snake to get areas the nozzle won't reach.