Problem i found with a starter was connections where dirty..We put a new starter on my son's Merc and it did the same thing wouldnt turn fast enough to engage, so we pulled all the battery cables off and cleaned them with a wire brush on a drill and they had some kind of white oxidation on them. We put the cable back on and it worked great, so you may want to check all your grounds and positives on the motor......looking at them they didnt look dirty.....just my 2 cents...