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Small Engine Diagnostic Tool

A 30-year mechanic in your pocket. Tell us what you've got and what it's doing, and we'll walk you through diagnosis the way a shop veteran would - one question at a time, with real specs and the pro-tips most people miss.

Safety: Disconnect the spark plug wire before any mechanical work. Drain fuel before tipping equipment. Wear eye + hand protection. If you're past your comfort level, stop and call a small-engine shop - this tool tells you what's wrong, not whether you should fix it yourself.
Step 1 / 5 Pick your equipment

How this tool works

This isn't a generic troubleshooter. The diagnostic tree was built from the way an actual 30-year small-engine mechanic works a problem: cheapest checks first, most-commonly-missed causes flagged early, real torque specs and gap dimensions baked into each step.

Five-level depth on every path: Equipment → Symptom → System → Component → Cause + repair walkthrough. Most paths end with a full procedure including tools, specs, torque values, and a repair-vs-replace gut check if the fix is expensive.

Universal pre-flight checklist

Run this before the tree. ~60% of "broken" small engines get fixed at this step:

  • 1. Fresh fuel? If gas is > 30 days old (especially E10), drain and replace before anything else. Old fuel causes 70% of "won't start" calls in spring.
  • 2. Oil at the full line? Overfull = smoking + hard start. Under = oil sensor shutdown on Honda / many newer engines.
  • 3. Spark plug clean + gapped? Most small engines: 0.030". Stihl/Husq 2-stroke: 0.020". Replace if > 2 years old.
  • 4. Air filter not gray/oily? A collapsed paper filter is the #1 cause of "no power" complaints.

FAQ

Is this accurate for my exact model? +
The specs are pulled from manufacturer service manuals for the most common engine families (Briggs L-head + OHV, Honda GX, Kohler Command/Courage, Kawasaki FX/FS/FR, Tecumseh, Stihl, Husqvarna, Echo). For unusual engines, verify torque + clearance specs against your manual before final torque-down.
What if I can't find my symptom? +
Pick the closest match and follow the tree - most starting and running problems share root causes across equipment types. If you've genuinely got something exotic (vapor lock on a propane log splitter, etc.), the diagnostic logic still applies; just substitute your fuel system in your head.
Can I save a diagnosis to text my customer? +
Yes - hit Copy share link at the end. The full diagnostic state is encoded in the URL so anyone who opens it lands on the same diagnosis page. Print Shop Ticket gives a clean PDF-ready printout with parts, tools, and torque specs.
What's the most-missed diagnosis? +
Three of them: (1) sheared flywheel key on a Briggs, (2) clogged spark arrestor screen on 2-stroke handhelds, (3) loose seat safety switch on riding mowers. All three are in the tree.