Clutch, stock plug caps often become porous and sometimes will leak to ground even when dry. New NGK caps are reasonably priced and exact replacements. on new projects I trim primary wires @ a 1/4" and spooge dielectric grease into cap rubber when installing new caps, and run a bead of silicon sealer around wires at coil mound. check wires for chaffing, and if suspect, sheath with heatshrink. luck g