Been doing the same here. On fourth starter. Have four in pieces on the bench taking what looks like the best to make one. Have found frayed wires. wires loose in the brush's themselves,Bad brush springs, bad bearings,broken planetary gears. First brush should be insulated if in contact with the case will cause low voltage. there is a connection from the field coils to the brush's that breaks and cooks, this needs to be clean clean clean. I have three to repair the fourth is currently in use and is starting my ride but not well. I too have gone through the grounds and cleaned. A light electrical grease between the starter and ground wire will help It is used to keep aluminum wires from corroding in the fittings on the entrance to your house buss bars. I took the start solenoid apart and cleaned it as well, and do recall it gave me the most improvement. The wires themselves can be offering resistance instead continuity and should be checked with an analog volt meter. Have been told a digital will not always be able to provide you with accurate readings. I have no meter at this time and a friend came and attempted to read with a digital, gave up and took the starter to his work bench. Brought it back without telling me the voltage readings and only two more starts. I pulled the starter again to see his craftsmanship and he had assembled the starter out of alignment. This two is important to get right and can also be a reason for a bad ground. also note the washers on the commutator need to be present. If you shake the starter or rock it back and forth and feel a clunking or movement. the washers are missing misaligning the brush's on the commutator. I am saving for another starter but have to make what I have do for now. Hope this helps.
