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400ex a ton of black carbon on valves why


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My 2003 400ex has been smoking forever so I broke down and bought a new wiesco 416 piston and stage 2 hot cam,but when I tore it down the bottom of the head where the valves are was covered with black really hard carbon and I mean a lot also my spark plug was one big ball of the same stuff my head pipe looks the same inside is it valve seals or just bad rings Im going to replace the valves any way with the rebuild but I m just wondering if it could be anything else?

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