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Those old compressors ran quite slow, so a 1740 RPM motor with a small pulley driving quite a large pulley on the compressor would be normal. Two stage means that the air is first compressed in a fairly large cylinder, and then forced out of the large cylinder, usually through a cooler (typically a length of pipe with cooling fins), into the smaller second stage cylinder, where it's compressor further, to the output pressure, and routed to the tank, sometimes through another cooler. (And, of course, a check valve.) Each cylinder will have an inlet and outlet valve. Sometimes these are a flat disc held shut by a coiled spring made of light spring wire, and sometimes the valves are a flat steel reed. In any case, a chunk of dirt, or just built-up crud can hold a valve open, either type of valve spring can break, or the valve itself can crack or break. Some compressors have screw-in plugs over the valves, allowing easy access, and others require head removal for valve access. Check your's out, and remove the valve plugs, or the cylinder head, and check the valves for dirt, wear, or damage. If you have to remove the cylinder head, do it carefully, and you may be able to salvage the original gasket and re-use it. More rarely, poor compressor performance may be caused by bad piston rings, a damaged cylinder or piston, or a broken connecting rod. Before taking anything apart, make sure the tank is fully discharged of air pressure, so there's no chance of a surprise balst of compressed air in your face, or flying parts when they're loosened!
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