Symptoms
- Engine suddenly loses significant power
- Engine backfires or runs very roughly
- Happened right after striking a rock, curb, or hard object
- Engine may still start but run poorly
Likely causes
- CommonSheared flywheel keyThis small key is designed to shear and protect the crankshaft when the cutting blade or attachment hits something hard, but it also throws off ignition timing.
- PossibleBent crankshaftA harder impact than the key can absorb may bend the shaft itself.
- Less commonDamaged flywheelThe flywheel itself can crack or be damaged in a severe impact.
What to do
- Stop using the equipment immediately after a hard impact to avoid further damage.
- Disconnect the spark plug wire before any inspection.
- Check the attached blade or cutting tool for damage, which often indicates the impact severity.
- If accessible, this repair typically requires removing the flywheel to inspect and replace the key, which is a job best done by someone comfortable with small engine repair.
- Spin the blade or attachment by hand slowly to feel for a bent shaft, which shows as a wobble.
- Do not attempt to run the engine extensively if a bent crankshaft is suspected, as this can cause further damage.
- A small-engine shop should replace the flywheel key and check the crankshaft for straightness.
Tools and parts
- Flywheel puller (for professional or experienced DIY use)
Flywheel keySearch: “small engine flywheel key replacement”
When to call a pro
This repair often requires special tools to remove the flywheel safely; a small-engine shop can replace the key and check for a bent crankshaft.
Safety
Use caution
Always disconnect the spark plug wire before inspecting or working near the blade, flywheel, or crankshaft.