2008 Polaris ATV Sportsman 300 400 H.O. Service Manual, Page 90Get this manual
ENGINE (300) TROUBLESHOOTING
Spark Plug Fouling troubleshooting
Restricted air filter (main or pre-cleaner), or air inlet ducts Choke cable adjustment or plungercable sticking Foreign material on choke plunger seat or plunger Incorrect spark plug heat range or gap Jet needle andor needle jet worn or improperly adjusted Excessive carburetor vibration (check all engine and carburetor mounts) Oil contaminated with fuel ETC switch mis-adjusted Low compression Spark plug cap loose or faulty Loose jets in carburetor or calibration incorrect for altitudetemperature Incorrect float level setting Carburetor inlet needle and seat worn CVT system calibrated incorrectly or components worn or mis-adjusted Fuel quality poor (old) Restricted exhaust Restricted crankcase breather system Weak ignition (loose coil ground, faulty coil, stator, engine to chassis ground, or ETC switch) Improperly assembled air intake system Engine Does Not Turn Over Dead battery Starter motor does not turn Engine seized, rusted, or mechanical failure Engine Runs But Will Not Idle Idle speed set too low Faulty ETC system Restricted carburetor pilot system Carburetor misadjusted Choke not adjusted properly or sticking plunger Low compression Crankcase breather restricted Air filter restriction Engine Idles But Will Not Rev Up Spark plug fouledweak spark Broken throttle cable Obstruction in air intake Air box removed (reinstall all intake components) Incorrect or restricted carburetor jetting ETC switch limiting speed Reverse speed limiter limiting speed Carburetor vacuum slide stickingdiaphragm damaged Incorrect ignition timing Restricted exhaust system Camshaft lobes worn Flywheel key sheared Engine Has Low Power Spark plug fouled Cylinder, piston, ring, or valve wear or damage (check compression) CVT not operating properly Restricted exhaust muffler Dirty carburetor Cam lobe worn