2008 Polaris ATV Sportsman 300 400 H.O. Service Manual, Page 91Get this manual
ENGINE (300) Piston FailureScoring Lack of lubrication Dirt entering engine through cracks in air filter or ducts Engine oil dirty or contaminated Excessive Smoke and Carbon Buildup Oil level too high Excessive piston-to-cylinder clearance Worn rings, piston, or cylinder Worn valves, guides or seals Restricted crankcase vent Air filter dirty or contaminated Low Compression Decompressor stuck Cylinder head gasket leak No valve clearance or incorrectly adjusted Cylinder or piston worn Piston rings worn, leaking, broken, or sticking Bent valve or stuck valve Valve spring broken or weak Valve not seating properly (bent or carbon accumulated on valve area) Rocker arm sticking Backfiring ETC or speed limiter system malfunction Fouled spark plug or incorrect plug or plug gap Carburetion faultylean condition IntakeExhaust system air leaks Ignition system faulty: Spark plug cap crackedbroken Ignition coil faulty Ignition or kill switch circuit faulty Ignition timing incorrect Sheared flywheel key Poor connections in ignition system System wiring wet Cam lobe worn or Valve sticking Lean condition