Its sear does not set. Since the first portion of the upper's motion, when fired, is when the interface occurs, the recoil spring is less likely a culprit than the sear (trigger bar) spring. If it's loose, it would function as you describe, so install that spring and see how it works afterward. Should eliminate the popppity-pop-pop-pop...
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"... Liberty is the seed and soil, the air and light, the dew and rain of progress, love and joy."-- Robert Greene Ingersoll 1894
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