[drolston]

That is a complicated distributor, with both mechanical and vacuum advance, plus an array of springs and bushings to tailor the advance curve. It needs to be set up by an expert on a machine, with some knowledge of the cam.

That said, stumble at steady high rpm does not sound like distributor advance or timing. unless it is a week off. Could be coil, plugs, or ignition cables, but more likely a carburetor/fuel problem.