Timing would be far down on my list. Cleaning and oiling do wonders for our old beasties. Yes, check to make sure you have the right needle and that it is in correctly. Have you pulled on the bobbin thread to make sure it isn't stuck? Using new thread and not stuff that has been there for decades? I don't know Hs but I know Sears had many suppliers. Are you sure of your threading? Tension ok for the top thread?