What was found
ABOVE 2,100 RPM, ENGINE WOULD HAVE ROUGHNESS. LEFT MAG WOULD CHECK GOOD, BUT RIGHT WOULD RUN ROUGH AND CAUSE AFTERFIRING, SOMETIMES VERY LOUDLY. BELOW 2,100 RPM, MAG CHECKS WERE NORMAL. PROBLEM TRACED TO POOR CONTACT ASSY GROUND CAUSED BY CORROSION IN MAGNESIUM CASE THREADS WHERE DIST BLOCK RETAINING SCREWS (PN 10-35937-20) GROUND CONTACTORS. THREADS WERE BLACK IN COLOR, BUT RESTORED TO BRIGHT METAL BY RUNNING A TAP IN. OHMMETER TESTS RESULTED IN 3 TO 10 OHMS RESISTANCE BEFORETHREADS WERE CLEANED. (X)