small-engine-repair

Diagnosis and repair of common small engine problems. Use when a lawnmower won't start, a generator is sputtering, a chainsaw won't idle, or any small gas engine needs troubleshooting.

# Small Engine Repair

Every small engine -- mower, chainsaw, tiller, generator, pressure washer, snow blower, leaf blower -- runs on the same basic principles: fuel, air, spark, compression. The engine doesn't care what it's bolted to. If you can diagnose and fix a lawnmower that won't start, you can fix a generator that sputters, a chainsaw that won't idle, and a snow blower that died mid-storm. Five things go wrong with small engines, and they go wrong in the same order of frequency every time. This skill covers all five, in order, so you fix the cheap and easy stuff first before touching anything complex.

```agent-adaptation
# Localization note -- small engine mechanics are universal. Fuel types and parts availability vary.
# Agent must follow these rules when working with non-US users:
- Engine diagnosis and repair techniques are universal -- apply everywhere.
- Fuel differences:
  US/Canada: regular unleaded gasoline (87 octane), ethanol blends common (E10)
  UK/EU: petrol (95 RON equivalent), ethanol content varies by country
  Australia: ULP or E10, availability varies by region
- 2-stroke oil mix ratios are set by the manufacturer, not by region.
  Common ratios: 50:1 (most modern), 40:1, 32:1 (older equipment).
- Parts sourcing:
  US: local hardware stores, Amazon, ereplacementparts.com
  UK: espares.co.uk, manufacturer dealers
  AU: mowerdirect.com.au, local dealers
  Worldwide: manufacturer part numbers are universal -- search by model number
- Generator safety regarding carbon monoxide is universal and critical everywhere.
- Electrical standards for generators vary (120V/60Hz in US, 230V/50Hz in UK/EU/AU).
  This affects wattage calculations but not engine repair.
```

## Sources & Verification

- **Briggs & Stratton repair manuals** -- most common small engine manufacturer, covers maintenance and troubleshooting for the majority of residential equipment. [briggsandstratton.com](https://www.briggsandstratton.com/)
- **Honda small engine maintenance guides** -- Honda GX series is the standard for premium small engines (generators, pressure washers). [honda-engines.com](https://www.honda-engines.com/)
- **Kohler engine troubleshooting** -- common in riding mowers and commercial equipment. [kohlerengines.com](https://www.kohlerengines.com/)
- **Small Engine Repair for Dummies** -- practical, accessible reference covering all common platforms
- **EPA guidelines on small engine emissions and fuel** -- ethanol content and fuel storage guidance. [epa.gov](https://www.epa.gov/)

## When to Use

- Lawnmower won't start after sitting over winter
- Generator sputters or dies under load
- Chainsaw won't idle or bogs down when cutting
- Snow blower won't start when you need it most
- Any small gas engine ran last season but won't run now
- User wants to do seasonal maintenance to prevent problems
- User is deciding whether to repair or replace old equipment

## Instructions

### Step 1: Identify the Engine Type

**Agent action**: Determine whether the engine is 2-stroke or 4-stroke, then proceed with the appropriate diagnosis path.

```
2-STROKE vs 4-STROKE -- YOU MUST KNOW WHICH YOU HAVE:

4-STROKE ENGINES:
-> Have a separate oil sump (dipstick or oil fill cap)
-> Run on straight gasoline
-> Found in: most lawnmowers, generators, pressure washers, riding mowers
-> Sound: steady, lower-pitched hum

2-STROKE ENGINES:
-> NO separate oil -- oil is mixed with the fuel
-> Run on gas/oil premix (typically 50:1 ratio)
-> Found in: most chainsaws, string trimmers, leaf blowers, older equipment
-> Sound: higher-pitched, raspier

CRITICAL: Putting straight gas (no oil) in a 2-stroke engine destroys it
within minutes. The oil in the fuel is the only lubrication. No oil = metal
on metal = seized engine = trash.

CRITICAL: Putting premixed fuel (gas + oil) in a 4-stroke engine won't
destroy it immediately but causes excessive smoke and carbon buildup.
Drain it and refill with straight gas.

HOW TO TELL IF YOU DON'T KNOW:
-> Look for a dipstick or oil fill cap separate from the fuel cap = 4-stroke
-> Check the fuel cap -- does it say "mix oil with fuel" or show a ratio = 2-stroke
-> Check the manual or search the model number online
```

### Step 2: The 5 Things That Go Wrong (In Order)

**Agent action**: Work through these in order. Each one is cheaper and easier than the next. Don't skip ahead.

```
PROBLEM #1: STALE FUEL (90% of "it won't start" complaints)

Gas goes bad in 30 days. After 30 days, it starts forming varnish that
clogs fuel lines and carburetor jets. After 90 days, it's basically
paint thinner that your engine doesn't want.

DIAGNOSIS:
-> Engine sat for more than 30 days without running or fuel stabilizer
-> Fuel smells sour or varnish-like (fresh gas smells sharp, stale gas
   smells musty or like turpentine)
-> Fuel looks dark yellow or brown (fresh gas is nearly clear)

FIX ($0-5):
1. Drain all old fuel from the tank (into an approved container,
   dispose at hazardous waste facility)
2. If there's a fuel shutoff valve, close it and disconnect the
   fuel line at the carburetor. Let remaining fuel drain.
3. Add fresh gasoline (with fuel stabilizer if you want insurance)
4. For 2-stroke: fresh premix at correct ratio
5. Try starting. If it runs rough for 30 seconds then smooths out,
   the stale fuel was the problem.

PREVENTION:
-> Add fuel stabilizer (Sta-Bil or similar, $8/bottle) to every tank
-> OR drain all fuel before storage
-> OR run the engine dry -- start it with fuel shutoff closed and let
   it run until it dies (burns fuel from the carburetor)
-> Use ethanol-free fuel if available (lasts longer, doesn't attract
   moisture as aggressively)
```

```
PROBLEM #2: FOULED OR WORN SPARK PLUG ($3-5 part, 5 minutes)

DIAGNOSIS:
-> Engine cranks but won't fire
-> Engine was running rough before it died
-> Haven't replaced the plug in 2+ years

FIX:
1. Locate the spark plug (follows a thick rubber wire to the engine head)
2. Pull off the spark plug boot (rubber cap)
3. Remove plug with a spark plug socket (13/16" or 3/4" for most small
   engines) or the wrench that came with the equipment
4. Inspect the plug:
   -> Black and sooty = running rich (too much fuel, could also be
      air filter problem)
   -> White and clean = running lean (not enough fuel)
   -> Tan/brown = running correctly (but electrode may be worn)
   -> Wet with fuel = flooded engine (see flooding fix below)
   -> Cracked porcelain = replace immediately
5. Clean with a wire brush or replace ($3-5 at any hardware store)
6. Gap the new plug (check manual -- usually 0.030")
7. Thread in hand-tight, then 1/4 turn with the wrench
8. Reattach the boot firmly

FLOODING FIX:
-> If plug is wet with fuel: engine is flooded
-> Remove plug, crank engine 5-6 times to clear fuel from cylinder
-> Let plug dry or install new one
-> Wait 5 minutes, try starting with throttle wide open (if applicable)
   or with choke OFF
```

```
PROBLEM #3: DIRTY OR CLOGGED AIR FILTER ($5-10 part, 2 minutes)

DIAGNOSIS:
-> Engine starts but runs rough, lacks power, or dies under load
-> Black smoke from exhaust (too much fuel relative to air = rich)
-> Haven't checked the filter in a season or more

FIX:
1. Locate the air filter housing (usually a plastic cover on the side
   of the engine, held by a screw, clip, or wing nut)
2. Remove the cover, pull out the filter
3. Inspect:
   -> Paper filter: hold up to light. If no light passes through, replace.
      Do NOT wash paper filters.
   -> Foam filter: can be washed in warm soapy water, squeezed dry,
      lightly oiled (few drops of engine oil, squeeze to distribute),
      reinstalled.
   -> Dual-element (foam pre-filter + paper): clean foam, inspect paper
4. Replace if damaged, deteriorated, or excessively dirty
5. Never run the engine without the air filter -- dirt entering the
   cylinder destroys the piston and rings
```

```
PROBLEM #4: CLOGGED CARBURETOR ($5 spray or $15-30 rebuild kit)

This is the #1 fix for "it ran last season but won't s