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Are you sure?  If there is no RPM drop with the switch in the "L" position, meaning the engine should be running on the left mag only, a zero RPM drop (compared to the "BOTH" switch position) indicates that it is the right mag, not the left, that is not being grounded.


The mag drops in the L and R positions sound reasonable assuming that the engine runs smoothly in both cases.  The engine continuing to run roughly with the switch in the OFF position suggests that one or both of the mags are not being well grounded.  First thing to do, as you suggest, is to check the grounding of the engine to the airframe.  But based on the "before" symptoms as you posted them it's possible that you sent off the wrong mag for bench testing.


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