Is the starter tight? Do the starter pinion teeth look good on the 'front end' where they slide in between the ring gear teeth. Thats what helps the mesh. Also, on the foot-mash starters, the starter shouldn't spin before the gears mesh, so check the linkage. As others have mentioned, worn armature bushings, worn starter bendix, and bendix fork can also cause problems. 6v to 12 volt rewound starters can also exhibit the engagement problems and excessive brush wear.
Does the engine have a 12 volt ring gear? The 6 volt ring gears had 139 teeth, the 12 volts have 168 teeth. So if you try to use a 12 volt starter on a 6V ring gear, it'll grind like crazy. In those cases tho, you usually see tooth damage.
good luck
chip
