Solved 90% of the issues with a Shimano compatible 8 speed derailer
I'll admit a sun race m50 rear derailer is cheap. But for the first 4000km it served me very well. Recently it started to ghost shift between 7 and 8 and require a lot of force to get it to shift. After much head scratching I got the limit screw and cable tension right. This fixed most of the issues, but it still shifted under load. Then I noticed the derailer hanger bolt was loose (duh!) that solved the ghost shifting and some of the tension problems.
Still it takes twice as much force to get it move as my old 7 speed shimano unit (and that thing is 6 years old!) I'm sure Sun Race is pretty low of the food chain, but is there anyway I can get it to shift with less force on the controls? I can't see and reason for it. I've lubed the crap out of both the bar and wheel end, I've cleaned out any crap that might have been in there. The cable looks clean, but I may change that as well if need be...
The cable might be clean, but the housing can deform and/or crimp at the ferrules. If you pull on the exposed bit of cable on the chainstay (preferrably while pedaling or with the rear wheel off) does the derailer move easier than if you pull on the exposed cable on the downtube? If you have slotted housing stops, can you relax the cable and slide the housings easily?
How is the alignment of the RD cage to the plane of the cogs?
It was the cable. Rusted right where it emerged at the bottom of the frame. Now they shift 4 at a time. I also took the opportunity to get some twice used Shimano deore disc brakes fitted, and repacked and regreased the rear wheel hub. All I can say is "Holy faster bike batman!" That and "Don't grab the brake like its a v brakeeee.....". Those diores have some serious stopping power.