Harbor Freight tumbler: modifications

Vibratory tumblers won't remove as much material in the coarse grind, so the stones retain much of their original contour through the whole process. That's why some cutters finish polishing cabs in them. It's also why a nummber of the members of this board like to do their rough grind in a rotary tumbler. I found that using a dedicated bowl for polishing and being patient paid off for me. It takes me at least 3 days per stage to get a good ore-polish and polish in my 5 lb Raytech. vibratory tumbler. You didn't say exactly what sort of problem you're having with the coarser grinding stages, but I assume you're being left with polished but knubby stones. As for the polish, Ian experienced friend taught me that the stones are ready for polishing when a bit of polish on a damp cloth, used over a fingertip to rub a sample stone, leaves a near or fully-polished surface.
The tumbler itself is probably OK as it is. Before I messed with the mechanism, I would try using a dedicated polishing bowl, and lots of pellets in the polishing stage. Run the tumbler near full for most efficient operation. Be aware that modifications may cause decreaserd efficiency, or create enough vibration to damage some softer materials. Good luck!
Mark H.