Polaris Sportsman 400 Runs Lean


What causes a Polaris Sportsman to Run lean? From my understanding a lean condition is when the vaporized fuel mixture has a high proportion of air.

My 4-wheeler engine runs hot, idles high and the idle is a little erratic. It seems to have some other issues while running. What are symptoms of a Polaris Sportsman engine that receives too much air and too little fuel?

My all-terrain vehicle is a 2005 Polaris Sportsman 400.

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John Fox 3 years 2020-12-02T17:04:58+00:00 2 Answers 559 views Beginner 1

Answers ( 2 )


    A lean-running engine or fuel starvation can have a host of symptoms and possible causes. Difficulty starting or no start, engine runs hot, high idle, surging or hesitation, backfire or popping through the intake/exhaust are all symptoms of fuel starvation or lean mixture.

    Detonation, which is the premature combustion of gasoline in an internal combustion engine, causing knocking, is also a sign of lean mixture. Low power is a sign that the engine runs too lean.

    This is a serious condition. If a motor runs too lean it will generate excessive heat. It can cause spark plug erosion and in some cases it can burn a hole through the piston. There are a number of components you should check, this includes:

    • Incorrect pilot screw adjustment.
    • Incorrect jet needle position.
    • Clogged fuel filter.
    • Pinched or restricted fuel line.
    • No gas in fuel tank.
    • Restriction in carburetor vent line(s).
    • Jets or inlet passages blocked with an accumulation of debris.
    • Obstructed or restricted inlet needle or seat screen.
    • Float level too low.
    • Fuel pump malfunction.
    • Intake air leak.
    • Kinked or pinched impulse line.
    • Impulse line air leak.
    • Float stuck, inlet needle closed or stuck.
    • Vacuum slide stuck closed or sticky.
    • Vacuum slide diaphragm ruptured.

    A main jet that is too small can cause a lean operation condition. If you know the vehicle is running lean, and it is backfiring, the carburetor might be faulty. If the engine overheats and you suspect a lean mixture, check for restricted jets, vents, fuel pump or fuel valve.

    In the meantime, do not operate the Sportsman 400 until the problem has been fixed. A lean condition can cause serious engine damage.

    Best answer

    My ATV was running rough and I thought it was getting too much air and not enough fuel. The first thing I checked was spark but it had, what looked like, decent spark.

    Then a mechanic told me it was probably a bad carburetor so I replaced it, it still ran rough. Turns out that in the end it was the spark, it just had a weak spark. I put a new spark plug in and it ran perfect.

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