Tightening the screw on what "back cover"? The cam cover or the rewind assy.? You should have a plug or two around for any and all your equipment, just in case... Using the EDTA usually doesn't harm the plug unless you don't drain it all out of the cylinder and it gets the plug wet. It is possible that after using the EDTA that you may "carbon foul" a plug... Which means there is a small piece of carbon on the plug electrode and is grounding the plug out... Either remove the carbon that is fouling the plug or replace the plug.