2941 Pretty Branch Dr West • Mobile AL 36618 terryplatt@yahoo.com 251-583-5955

Big Daddy’s Fix-It Shop

Lawn Mower Repair Shop Mobile Alabama

Lawnmower Troubleshooting

LawnMower Troubleshooting

This category page lists a variety of common problems and nearly all possible causes. Diagnostic procedures will then be needed to determine which actually apply. The possible causes are listed in approximate order of likelihood. Most of these problems are covered in more detail elsewhere on the website.

While this page lists many problems, it is does not cover everything that can go wrong. However, it can be a starting point for guiding your thinking in the proper direction. Even if not listed here, your particular problem may still be dealt with somewhere else on the site.

(Portions of the following from: Chilton, Small Engine Repair 2-12 HP, (1).)

Problem: Engine will not start or is hard to start

Possible causes:

    1. Fuel tank is empty or shutoff valve is closed, or fuel line or fuel tank cap vent is clogged.
    2. There is water in the fuel.
    3. Carburetor is overchoked.
    4. Carburetor is improperly adjusted or needs service.
    5. Ignition system or its wiring is defective or ignition switch is off, or timing way off (e.g., broken flywheel key).
    6. Deadman or other cutoff switch is open or defective.
    7. Spark plug is fouled, improperly gapped, or damaged.
    8. Engine compression is poor.
    9. Operator needs to read user manual. :)

Problem: Engine starts easily but dies after a few seconds

Possible causes:

    1. Fuel tank is empty or shutoff valve is closed, or fuel line or fuel tank cap vent is clogged.
    2. Carburetor is overchoked.
    3. Carburetor is improperly adjusted or needs service.

Problem: Engine idles roughly, unevenly, or surges

Possible causes:

    1. Carburetor is dirty.
    2. Air leak in carburetor or intake manifold (e.g., bad O-ring, gasket, primer).
    3. Carburetor is improperly adjusted or needs service.

Problem: Engine misses under load

Possible causes:

    1. Spark plug is fouled, improperly gapped, or damaged.
    2. Breaker points are pitted or improperly gapped, breaker arm is sluggish, or condenser is bad.
    3. Carburetor needs adjustment or service.
    4. Fuel line, fuel filter, or fuel tank cap vent is clogged, or fuel shutoff valve partially closed.
    5. Valves not adjusted properly or valve springs weak.
    6. Exhaust ports blocked (2 stroke).

Problem: Engine knocks

Possible causes:

    1. Magneto is not timed properly.
    2. Carburetor is set too lean.
    3. Engine has overheated.
    4. Carbon buildup in combustion chamber.
    5. Flywheel is loose.
    6. Connecting rod is loose or worn.
    7. Cylinder is excessively worn.

Problem: Engine vibrates excessively

Possible causes:

    1. Engine is not mounted securely.
    2. Blade or other driven equipment is unbalanced.
    3. Crankshaft is bent.
    4. Counterbalance shaft is not timed correctly.

Lawnmower Problem: Engine lacks power (possibly after warmup)

Possible causes:

    1. Old gas, bad spark plug, very thick/dirty oil.
    2. Choke is partially closed.
    3. Carburetor needs adjustment or service.
    4. Ignition not timed correctly.
    5. Air filter is clogged.
    6. There is a lack of lubrication.
    7. Valves are not sealing properly.
    8. Piston rings are not sealing properly.
    9. Head loose or head gasket blown or damaged.
    10. Exhaust ports blocked (2 stroke).

Problem: Engine operates erratically, surges, and runs unevenly

Possible causes:

    1. Fuel line or fuel tank cap vent is clogged.
    2. There is water in the fuel.
    3. Fuel pump is defective.
    4. Governor is not set properly, sticking, or binding.
    5. Carburetor needs adjustment or service.
    6. Loose carburetor/intake pipe resulting in vacuum leak.
  • Problem: Engine overheats.
    Possible causes:

    1. Magneto is not timed properly.
    2. Carburetor set too lean.
    3. Air intake or cooling fins are clogged.
    4. Shroud or blower housing missing.
    5. Excessive load.
    6. Insufficient or excessive oil.
    7. Improper oil viscosity (4 stroke) or mixture (2 stroke)
    8. Valve clearance is too small.
    9. Excessive carbon buildup in combustion chamber.

Problem: Crankcase breather passing oil

Possible causes:

    1. Too much oil in crankcase.
    2. Engine speed is excessive.
    3. Oil fill cap or gasket is damaged or missing.
    4. Breather mechanism is dirty or defective.
    5. Piston ring gaps are aligned.
    6. Piston rings are worn.

Problem: Engine backfires

Possible causes:

    1. Carburetor set too lean.
    2. Magneto is not timed correctly.
    3. Valves are sticking.

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