Mercury - Quicksilver 816761Q4 Spark Plug Wire Kit V8 GM MerCruiser (LH) Engines (305 & 350 CID) Thunderbolt IV Ignition 816761Q4. Overview. When your boat starts instantly and runs smooth, that's good spark at work. The Quicksilver 816761Q4 Spark Plug Wire Kit keeps that spark steady in V8 left-hand rotation MerCruiser engines (305 and 350 CID) equipped with Thunderbolt IV ignition systems (1981-1992).. Each wire in this OEM kit is built to exact Mercury specs - the right length, perfect resist