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When the points are closed there should be no voltage to them because they are supposed to be grounded. The set screw helps with that ground. Somehow you are getting voltage to the stationary point block when you shouldn't. Voltage is either traveling thru the the primary lead [ short to distributor body from a bad cable or being screwed in too far, etc] or there is no path for a ground from distributor body, upper plate, to cylinder head. The set screw and jam nut are part of a ground.