What causes a starter to become slow to spin and so weak I can hold the shaft from turning when hooked to a strong battery. It got a lot of oil inside but I cleaned it out.
Check and clean (wire brush) all the connections, including the grounds, then try it again.
Mark is absolutely right.
I read your travails on your other thread. And since you have one starter that is "dead" and one that works, your question I believe, is associated to the starter specifically. You have mechanical and electrical possibilities associated with just the starter itself.
Mechanical: partially seized babbit bearing, worn bronze bearing, mechanical interference of armature to pole faces.
Electrical: Insulation breakdown of oil-soaked windings to the case, brush spring pressure too low, dirty connections, corroded brush holders (poor grounding to brush block), corroded connection between brush block and housing (riveted interface).
Rarely is it actually the armature...it's almost bullet proof (electrically) other than bending, and they are most frequently bent which can lead to slight or significant binding of the bendix to the babbit bushing in the snout/support on the starter. This babbit is often totally worn out from bendix on bent shaft wallowing it out, further straining the bronze bush in the housing...it's a down hill spiral from there.
Lots of stuff to look for. Every one of these items is remedied with a proper rebuild.
It is an interesting exercise for one learning about their car, to check these things, but takes hours to do. A rebuilder on the other hand, does not bother to trouble shoot any one of these items and simply devotes the same hours to fixing every single thing on this list with the full and justified expectation that it will provide the proper torque when finished and can be warranted for service.
I am mentioning this last thing, because the local starter repair guy in town is not likely to have run across many of these and is definitely not tooled up for some of the machine work...if they discuss troubleshooting it or patching it, politely walk away and save yourself some $$ and lots of aggravation. That way, you'll remain friends and in the future when you move on to generators and want a generator armature tested on a growler, he'll be happy to see you and maybe test it for free.
If you cleaned out a lot of oil inside, chances are that it won't take too long to get more fresh stuff in there. It takes a lot of solvent to clean out accumulated oil and goo from inside a starter. If you don't feel comfortable having the local auto shop guys check it out, look for a place that repairs farm equipment. Frequently those folks know a little more about 6 volt electrical systems because lots of older tractors used that style of system. I had the same problem of oil contamination in the starter. Cleaned it out with good solvent, had the armature checked on a growler, then installed new brushes and bearings with an oil seal front bearing. Also checked every connection and wire going back to the battery and checked for a good ground. Also don't assume your battery is good. Check it for the correct resting voltage.
Or, send it to Ron Patterson for rebuilding and never have to worry about it again.
Is the drain hole in bottom front of the starter clogged? Wondering if you cleaned up the graphite contacts and reseated them .. and brightened up the armature with some sand paper ... and tightened all contacts you might find it running better. Thatís all it took on the starter that was on my Ď23 and I know it hadnít run for about fifty years or so.
Is the cable stud loose inside and needing to be resoldered?
Don't forget the cables. I cant tell you how many cars with weak starters i have fixed by replacing the 12 volt cables with the heavy 6 volt cables.
Never use 12 volt cables, they have too much voltage drop..
Fortunately we have a "real" auto parts in our town. They can make cables at a reasonable cost. Making cables using 1/0 wire can make a real difference. We also make sure all contact points are clean of rust or paint and use copper washers. In some cases it has made a remarkable improvement. Doing this will not overcome a bad starter but it works often.
My cables are correct and new. I bought cable from a welding supply house and ends at the NAPA store that I worked at before my accident on July 18. I soldered the ends on and used heat shrink where needed.
Today I disassembled my original starter, again. I plan to take the housing with coils to work tomorrow and clean it in the "dishwasher". I have cleaned everything else, especially the wire ends and connections.Tomorrow, I will reassemble it and see what happens.
you will be doing yourself no favors if you soak the housing and windings with any kind of solvent whether aqueous or other. If the coils are original, every wrap of each coil is insulated with a paper insulator and each assembly is wrapped with a cotton tape. A thorough soaking will compromise any remaining isolation that currently exists within them.
I know you have a working starter in the car now and suspect you're doing this for the knowledge and experience. If that is the case, you may wish to swap end housings to see if the success/failure is related to the brush block or windings.
Myself, I wouldn't do any of this, but I suspect you are interested in finding out, so thus the suggestion.