How a Small Engine Works (30-Second Version)
Four-stroke small engines work in a repeating cycle: air and fuel enter the cylinder, a spark ignites the mixture, the expanding gases push the piston down (making power), and exhaust gases exit. Oil lubricates the moving parts but doesn't enter the combustion chamber.
Three things are required for an engine to run: fuel in the right mixture, spark at the right time, and compression to make ignition work. Every small engine problem traces back to a failure in one of these three.
Diagnostic Sequence: Engine Won't Start
The Carburetor
The carburetor mixes air and fuel in the right ratio for combustion. It's also the most common failure point in stored small engines.
How It Works
The carburetor uses the venturi effect — air moving through a narrowed passage creates low pressure that draws fuel up from the float bowl. Jets (tiny precisely-drilled holes) meter the fuel at different throttle positions. The float maintains a constant fuel level in the bowl.
Why Carburetors Fail After Storage
Ethanol in modern gasoline (E10, E15) is hygroscopic — it absorbs water from the air. When fuel sits in a carburetor for 30-90 days or more, the ethanol separates from the gasoline, water corrosion begins, and the lighter gasoline fractions evaporate. What remains is a sticky varnish that clogs the jets.
The main jet — typically .028-.040 inches in diameter — requires only a partial clog to prevent the engine from running.
Carburetor Cleaning Procedure
Supplies needed: Carburetor cleaner (aerosol), small wire or thin strand of wire, compressed air if available, carb rebuild kit (optional but recommended for a thorough job, $5-15).
When to Replace Instead of Clean
A corroded float, cracked bowl gasket, worn needle or seat, or severely varnished passages may not clean well. Carburetor replacement is often cheaper than extended repair: a replacement carb for a common Briggs & Stratton engine costs $15-30 and drops on in 20 minutes.
Ignition System
The ignition system converts mechanical motion (flywheel spinning) into a timed high-voltage spark.
Spark Plug
The spark plug fires the air-fuel mixture. It's the first ignition component to check — inexpensive and fast.
Spark plug reading:
- Tan/gray with slight deposits: Normal operation
- Black sooty deposits: Rich mixture or oil fouling
- White/light gray, no deposits: Lean mixture (may indicate air leak) or correct
- Heavy black oil deposits: Oil is entering the combustion chamber (worn rings or valve seals)
- Cracked porcelain, eroded electrode: Replace immediately
Replacement: Match the heat range and thread size exactly. The number on the plug (Champion RJ19LM, NGK BPR6ES, etc.) is the spec. Don't substitute a different heat range without knowing what you're doing — wrong heat range causes pre-ignition or fouling.
Gap: Most small engine plugs require .030 inch gap. Check with a feeler gauge and bend the ground electrode to adjust. Many replacement plugs come pre-gapped — verify anyway.
Ignition Coil / Armature
If spark plug is good but you have no spark, the coil or armature may be faulty. The coil gap (distance between the coil and flywheel magnets) can also affect spark quality.
Testing coil gap: Use a feeler gauge or business card as a spacer. Most small engines specify .010-.014 inch gap. Loosen the coil mounting, allow the flywheel magnets to pull the coil in, then insert the spacer, tighten the mounting, and remove the spacer.
Testing the coil: Without a coil tester, substitution is the most practical test. If a known-good spark plug produces no spark, the coil is suspect.
Kill Switch
The kill switch works by grounding the ignition coil — connecting the hot side to ground, which prevents spark. A faulty kill switch can stick in the "kill" position. Disconnect the kill switch wire temporarily to test: if the engine now starts, the kill switch is shorted.
Fuel System
Fuel Line
Rubber fuel lines harden, crack, and collapse with age. Fuel that gets through a collapsed line intermittently causes hard starting and stalling. Replace fuel lines that show cracking, hardness, or any visible deterioration.
Fuel line is a standard part — sold by inside diameter at any small engine shop. Cut to length.
Fuel Filter
Most small engines have an inline fuel filter — a small plastic housing with a paper filter element inside. Replace annually or if you see debris. A clogged filter restricts fuel flow, causing lean running and stalling under load.
Primer Bulb
The rubber primer bulb on many small engines draws fuel into the carburetor for cold starts. A cracked or hard primer bulb either draws air or doesn't create suction. Press it: it should spring back cleanly. Hard, cracked, or deformed primer bulbs should be replaced — they're inexpensive and easy.
Oil Service
Four-stroke engines require oil changes. Running an engine on low, dirty, or broken-down oil causes accelerated wear and eventual seizure.
Specifications
- Check the engine manufacturer's specification for oil viscosity (SAE 30, 10W-30, or synthetic equivalent are common in small engines)
- Generator engines run at constant RPM under load — they run hot and consume oil faster than intermittent-use engines
- Use a quality conventional or synthetic motor oil meeting the engine spec
Change Interval
- Lawn mowers and similar seasonal equipment: Change oil every season or every 50 hours of use, whichever comes first
- Generators: Change oil after the first 8 hours of break-in, then every 100 hours of use or annually
- New engine break-in: Change oil after the first 2-5 hours of operation — the initial run sheds metal particles that you want out of the oil quickly
Low-Oil Shutoff
Most modern small engines include a low-oil sensor that prevents starting or shuts down the engine when oil is critically low. If an engine cranks but won't start and you've ruled out fuel and spark — check the oil level. This sensor fails in both directions (won't start even with correct oil, or doesn't protect when oil is low) but a cold engine that cranks without starting is worth a 5-second oil level check.
Two-Stroke Engine Specifics
Chainsaws, string trimmers, and small hand-held power equipment typically use two-stroke engines. Key differences:
- Oil mixed in the fuel: Most small two-strokes require 50:1 gasoline-to-oil ratio (2.6 oz oil per 1 gallon gas). Using the wrong ratio or straight gasoline will destroy the engine quickly.
- No separate oil system: There is no oil to change — the oil in the fuel lubricates the engine
- Simpler design: Fewer moving parts, but more sensitive to fuel quality and oil ratio
- Carbon deposits: Two-stroke engines carbon up more readily. The spark plug, exhaust port, and muffler accumulate carbon with age and require periodic cleaning.
Common two-stroke failure: Running with incorrect oil mix or stale fuel. Two-stroke engines are less tolerant of old fuel than four-stroke. Use fresh fuel with fresh pre-mixed oil every season.
Sources
Frequently Asked Questions
Why won't my generator start after sitting in storage for a year?
Almost certainly the carburetor. Gasoline with ethanol (E10) absorbs water and degrades over time. After 30-90 days, the lighter fractions evaporate, leaving a sticky varnish that clogs the carburetor's tiny jets and passages. The engine cranks but won't run because no fuel is reaching the combustion chamber. Clean or replace the carburetor.
How do I know if it's the spark plug or the carburetor?
Remove the spark plug. Spray a small amount of carburetor cleaner or starter fluid into the cylinder. Reinstall the plug and try to start. If it fires briefly and then dies, fuel is the problem (carburetor). If it doesn't fire at all, ignition is the problem (spark plug, coil, or kill switch). If it fires and runs normally, the issue was fuel starvation.
What is the difference between a two-stroke and four-stroke small engine?
A four-stroke engine has separate intake, compression, power, and exhaust strokes — it runs on straight gasoline and has separate oil lubrication. Most generators, mowers, and stationary engines are four-stroke. A two-stroke engine completes the cycle in one revolution — it requires oil mixed into the fuel (typically 50:1 ratio) and has no separate oil system. Chainsaws, string trimmers, and many small handheld tools are two-stroke.