If you can't find any leaks around the manifold, check the brake booster. I've seen advice here to pinch the line shut - don't know if that's a good idea with such a thick-walled tube, but I'll let others comment. If you do this and there's no vacuum change, the problem isn't there.

If you have no leakage evidence, all spark plugs are gapped correctly and look normal, and ignition timing is OK, I'll suggest a long shot: did you recently have the timing belt off and replace it? If it went back on with the cam sprocket one tooth off, ignition timing will be unchanged (on 240's with block-mounted distributor), but the cam will be off by 38 degrees, which would make the engine run VERY poorly, no power, low vacuum, etc.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar 77 MGB, and a few old motorcycles)