📘 Guides › 💡 Heat, light & power · 🆘 Emergency · 📇 Index A–Z

🔧 Engines, Motors and Generators

How to keep small gas engines, chainsaws and generators running, how to test and rebuild electric motors, and how to turn motors and car alternators into generators for charging batteries. The full shop manuals are in Engines-Motors\ (Small-Engines, Chainsaws-and-Saws, Electric-Motors, Motors-as-Generators). Creek hydro and solar are in Off-Grid-Power\; electricity basics are in the Navy NEETS modules there.

Fuel itself (storing, testing, getting it out of tanks) is in the Fuel guide.

⚠️ Rules that prevent the worst accidents

⛽ Small gas engines: why it won't start

Every engine needs three things: compression, spark and fuel. Find which one is missing. (MAVCC Small Engine Repair, p. 590; how it works: the four strokes drawn, Four-stroke engine, Two-stroke engine)

The four strokes of a gasoline engine: intake with the intake valve open and the piston moving down, compression with both valves closed, power when the spark ignites the mixture, and exhaust with the exhaust valve open Intake, compression, power, exhaust: fuel, compression and a spark at the right moment (Navy Construction Mechanic, p. 23).

  1. Fuel: pull the plug, cover the hole with a finger and pull the starter 2–3 times. A wet finger means fuel is arriving. If dry, check: fuel in the tank, shutoff valve open, cap vent not blocked, carburetor bowl (drain it — water or gum?), fuel filter and line. (MAVCC, pp. 595–598; drawings)
  2. Spark: put a spare plug on the plug wire, rest it against the metal head and crank. A bright blue spark is good. Yellow, weak or none = clean or replace the plug, check the kill-switch wire, then the coil/magneto. (MAVCC, pp. 599–600; spark-tester drawing)
  3. Compression: spin the flywheel backward against compression — a sharp rebound is good. Weak = loose plug or head, stuck valve, worn rings. With a compression gauge, crank at least six times before you read it. (MAVCC, pp. 602–603; flywheel check and gauge drawings)

A spark tester plug on the spark plug wire, clipped with an alligator clamp to the engine's cylinder head A tester plug on the plug wire, grounded to the cylinder head: crank and look for a bright blue spark (MAVCC, p. 599).

Quick troubleshooting chart (MAVCC, p. 592):

What it does Look at these first
Won't start, plug dry Empty tank, fuel valve shut, plugged tank vent, stuck or clogged float valve, blocked filter or line, choke not closing
Won't start, plug wet Choke stuck or overused, stale fuel, water in fuel, flooded (mixture too rich), too much oil in 2-stroke mix, fuel-soaked air filter
Won't start, no spark Kill button or wire grounded, bad plug, bad plug wire, dirty or wrongly gapped points (older engines), bad coil
Won't start, weak compression Loose plug or head bolts, blown head gasket, valve stuck open, worn rings or bore
Starts but lacks power Dirty air filter, carburetor out of adjustment, clogged muffler or exhaust port, low compression, governor fault
Overheats Cooling fins or blower blocked, engine overloaded, clogged muffler, late ignition timing
Smokes or uses oil Wrong oil, oil diluted by a rich mixture, worn rings, clogged crankcase breather, wrong 2-stroke mix
Kicks back when you pull it Ignition timing off, sheared flywheel key (drawing), loose blade

Old fuel is the #1 problem. Gas goes stale in about 60 days without stabilizer (sooner with ethanol). For longer storage, drain the tank and run the carburetor dry, or add stabilizer and run it a few minutes so it reaches the carburetor. Keep cans sealed — water in ethanol gas separates and ruins engines. (Oklahoma Extension BAE-1746, p. 2)

Cleaning a carburetor (AAVIM Small Engines Vol 2; float-type carburetors, floats, needle valve and float settings):

Two-stroke fuel (chainsaws, trimmers): mix oil at the ratio printed on the engine or in its manual (MAVCC, p. 186). Never run straight gas — it seizes the engine in minutes. Rule of thumb: most modern saws are 50:1 (about 2.6 oz of 2-stroke oil per gallon); older gear is often 16:1–32:1. Don't mix mineral and synthetic 2-stroke oils. (Two-stroke oil)

Routine care: check oil every use (4-strokes), clean the air filter often (a choked filter makes an engine run rich and weak), keep cooling fins clear, replace the plug yearly. (Air filter)

Replacing a starter rope: for a wrap-around (non-recoil) starter, use 3/16-inch braided nylon rope, usually 4–5 feet long — enough to wrap the pulley 3–5 times plus about 6 inches. Singe the cut ends with a match so they don't fray, and tie a figure-eight or double overhand knot in each end. (AAVIM, pp. 14–15; knots drawn step by step: figure-eight, double overhand) Recoil starters take thinner rope — most often 5/32 inch — that fills the pulley groove without binding. (AAVIM, p. 20; parts of a recoil starter)

📷 What the parts look like: Carburetor, Spark plug, Magneto, Flywheel, Governor.

📚 Go deeper: MAVCC Small Engine Repair — full troubleshooting unit and job sheets; AAVIM Small Engines Vol 2 — carburetor, starter and valve repair step by step; TM 9-2805-262-14 6 HP engine — Army maintenance checks for a small engine; Oklahoma BAE-1746 — ethanol fuel in small engines.

🚜 Diesel engines

Diesels have no spark plugs — squeezing the air makes it hot enough to light the fuel. So a diesel needs compression and clean fuel with no air in the lines. (Practical Action Diesel Engine Repair, p. 5; gasoline and diesel strokes side by side; Diesel engine) Diesels usually need less maintenance than gasoline engines because they have no ignition system. (FM 5-424, p. 114)

Won't start or keeps dying: suspect air in the fuel lines (an "air lock"), an empty tank or a clogged filter. (Practical Action, p. 12) Bleed the air out (p. 16):

  1. Fill the tank with clean diesel and open the tank valve.
  2. Loosen the bleed screw on the fuel filter (and on the injection pump) until fuel runs out in a steady stream with no bubbles. Tighten it.
  3. Loosen the injection line nut at the injector and crank the engine until fuel comes out without foam.
  4. Tighten the nut and check every joint for leaks.

Bleeding a diesel: open the tank valve, loosen the bleed screw on the fuel filter and then on the injection pump until fuel runs without bubbles, then loosen the line nut at the injector and crank until fuel comes without foam Bleed in order, from the tank toward the injector (a diagram drawn for this guide).

What the smoke and symptoms tell you (Practical Action, pp. 11–14):

Symptom Common causes
Lots of black smoke Faulty injector (p. 14)
Overheating Low coolant, loose or torn fan belt, blocked water jacket, bad water pump, low oil, overload, long running at high speed
Burns too much fuel Leaks in tank or lines, clogged air cleaner, wrong valve timing, overload, lots of stopping and starting, idling when not needed
Burns oil Worn or wrongly fitted piston rings, worn cylinder, damaged seals or head gasket
Knocking or noise Poor-quality fuel, loose connecting rod, worn or badly adjusted parts

Cleaning an injector nozzle: soak the nozzle valve and body in clean diesel, scrub the soot off with a copper (not steel) wire brush, and check the needle slides freely. (Practical Action, p. 15; injector valve, taken apart; cleaning the spray holes; Fuel injection)

Don't run a diesel generator lightly loaded for long. It runs too cool, soot builds up in the engine and exhaust ("wet stacking"), and over time this can permanently damage it. (FEMA P-1019, p. 150) Give it a real load when you run it.

Diesel fuel catches fire less easily than gasoline, but once burning a diesel fire is fierce (Practical Action, p. 6). Cold-weather gelling, water and fuel substitutes are in the Fuel guide.

📚 Go deeper: Practical Action Diesel Engine Repair and Maintenance — short, plain-language diesel fault-finding; Navy Construction Mechanic — diesel fuel systems, governors and bleeding; TM 9-8000 Principles of Automotive Vehicles — how every engine and vehicle system works.

🪚 Chainsaws, crosscut saws and axes

Filing a saw chain, side view of one cutter: the round file sits with one-fifth of its width above the cutter's top plate; a depth-gauge tool laid on the chain shows how much of the depth gauge to file off, never lower than the tool The file's height on the cutter, and the depth gauge filed only down to the tool (a diagram drawn for this guide).

Cleaning a saw (USFS, p. 31):

Things that make kickback more likely (USFS, p. 32; the kickback zone on the bar nose): a loose or dull chain, wrong filing angles, depth gauges filed too low or left square, loose rivets or cracked links, and anti-kickback parts removed.

Running out of fuel isn't the end of cutting. A sharp crosscut saw and ax still fell and buck trees. Raker depth for a crosscut: 0.008 in for hard, dry wood up to 0.030 in for soft, springy wood; start at about 0.012 in. (USFS Crosscut Saw Manual, p. 16, with drawings of good and "whiskered" shavings; how crosscut teeth and rakers cut) Set the teeth only enough to stop binding — about 0.010 in is a good first try; up to 0.030 in for soft, punky wood. (Crosscut Saw Manual, p. 21) More in Saws That Sing and An Ax to Grind.

Chainsaw, Crosscut saw. More pictures: Axe, and the parts of an ax.

📚 Go deeper: USFS Chain Saw and Crosscut Saw Training Course — maintenance, felling and bucking; MAVCC Chain Saw Repair — engine, clutch and oiler repair; USFS Crosscut Saw Manual — filing and setting a crosscut; USFS Saws That Sing — using crosscut saws safely.

🔋 Generators

Where to put it (CPSC, p. 1):

Maintenance (rule of thumb — the generator's own manual comes first): oil change every 50–100 hours (sooner when new), clean air filter, fresh or stabilized fuel, run it under load for 20–30 minutes once a month. Machines that sit idle for long periods should be "exercised" on a schedule to keep oil moving and moisture out. (TM 5-683, p. 41)

Load: add up the running watts of what you'll plug in, plus the start surge of motors (fridges, pumps, power tools can need 2–3× their running watts for a second — rule of thumb). Keep steady load under about 80% of the generator's rating.

Grounding: follow the generator's own manual. The Army 3 kW set procedure (TM 5-6115-271-14, pp. 60–61; drawings of rod, plate and pipe grounds, ways to ground a generator and driving a ground rod; Ground):

No output but the engine runs? Check the breaker on the generator, then the brushes and voltage regulator (most small generators). A generator that sat unused can lose its residual magnetism — see "flashing the field" below.

📚 Go deeper: CPSC Portable Generator Hazards — CO, shock and fire rules on one page; TM 5-6115-271-14 3 kW generator — grounding, troubleshooting tables and field flashing; FM 5-424 Electrical Systems — picking and loading a generator, wiring and grounding; FEMA P-1019 — generator sizing and fuel planning.

⚙️ Electric motors: testing and repair

When a motor won't run, check the simple things first, in this order (Illinois Electric Motors for Farm Use, pp. 24–25; know the parts first: cutaway of an induction motor, main parts laid out, Induction motor):

  1. Power: is the right voltage reaching the motor terminals? Check fuses, breakers and the switch.
  2. Load: take the belt off. If the motor now runs, the machine it drives is stuck.
  3. Bearings: power off, turn the shaft by hand. If it won't turn freely, a bearing is dry, worn or stuck.
  4. Thermal protector: many motors have a red reset button; others reset themselves once cool. A tripped protector means something else is wrong — find it.
  5. Starting switch: a motor that hums but won't start, yet runs if you spin the shaft by hand, has a fault in its starting circuit (see below).
  6. Windings: a burned winding has a strong burned smell, and you may see charred insulation.

Single-phase motor hums but won't start (pumps, compressors, shop tools): suspect the start capacitor or the centrifugal start switch, which should click open at about ¾ speed (Illinois, p. 28; capacitor-start motor; the two parts of a centrifugal switch; Motor capacitor). Discharge the capacitor and check it; if it's open or shorted, replace it (TM 5-683, p. 56). A bulged or leaking one is dead; a meter with a capacitance setting tells for sure. Replace with the same microfarads and same or higher voltage (rule of thumb). Switch contacts can be dirty, burned or pitted, or a wire can burn in two. (Illinois, p. 24)

Basic winding checks with a meter (Navy Electrician's Mate):

Bearings: a rumbling or hot bearing is the most common motor failure. Replace with the same number stamped on it. Fit new bearings by heating them to no more than about 200 °F (oil bath or oven, not a flame). (Navy EM, p. 225, with bearing-puller drawings) Never wash sealed bearings in solvent — factory-sealed bearings shouldn't be disturbed. Open bearings get repacked half to two-thirds full of motor bearing grease; too much grease makes heat. (Illinois, p. 23; TM 5-683, p. 58; greasing a bearing step by step; Ball bearing)

Brushes and commutators (drills, saws, mixers, car starters, DC motors): replace brushes that stick, are chipped, or are worn to about half their original length (or to within ⅛ inch of a riveted clip). (Navy EM, p. 228; brush holder and brush; Brush) Seat new brushes by pulling fine sandpaper (grit toward the brush) under them in the direction the motor turns, lifting the brushes on the return stroke, then blow out the carbon dust. (Navy EM, p. 229; sanding a brush to fit) A blackened, grooved commutator can be cleaned with a commutator stone or fine sandpaper — never emery cloth; emery dust conducts electricity and causes shorts (Illinois, p. 24; commutator stone) — or turned lightly on a lathe (no more than 0.010 in per cut). (Navy EM, p. 233; turning a commutator)

Seating a motor brush: a strip of sandpaper, rough side up against the brush, wrapped over the commutator under the brush, seen from the end and the side Sandpaper under the brush, rough side toward it, pulled the way the motor turns (Navy Electrician's Mate, p. 229).

Keeping spare motors (salvage is your parts store): wipe them clean, lubricate the bearings, grease the bare shaft against rust, wrap them in heavy paper, and store them dry. (Illinois, pp. 23–24)

Rewinding a burned motor (Braymer & Roe Rewinding Small Motors; DOE Repair Specifications):

Commutator, Centrifugal switch, Megger.

📚 Go deeper: Illinois Electric Motors for Farm Use — plain-English motor types, care and fault-finding; Navy Electrician's Mate — bearings, brushes, armature and stator tests; TM 5-683 — insulation tests and full troubleshooting tables; Braymer & Roe Rewinding Small Motors — rewinding step by step.

🔁 Motors as generators

What works best, easiest first:

  1. Permanent-magnet DC motors (treadmill motors, wheelchair, scooter, some cordless tool motors): spin them and they make DC directly. Output voltage rises with speed, so choose one that makes at least 14–15 V at the speed you can actually turn it to charge a 12 V battery. Add a blocking diode so the battery can't run it as a motor. (Rule of thumb) (Brushed DC motor, Diode)
  2. Car alternators: built-in rectifier and regulator, but they need high speed (rule of thumb: about 1,500+ rpm — step-up pulleys or belts for wind, water or pedals) and a battery connected to power their field — they are not self-starting. The field draws about 3 A, which wastes a lot at low power. (Alternator and Regulator Servicing, p. 11; how an alternator makes current; testing the rotor)
  3. Induction motors (ordinary AC motors) as generators: drive it about 1–5% faster than its synchronous speed — e.g. a motor rated 1725 rpm (1800 synchronous) is driven at about 1820–1890 rpm. Output rises with the extra speed. (DOE Microhydropower Handbook Vol 1 Pt 2, p. 54) On its own (not tied to the grid) it needs capacitors across the windings to excite it. Sources disagree here: the DOE handbook (same page) and the European hydro guide say an induction generator can't run cut off from the grid (ESHA Layman's Guidebook Pt 2, p. 30), while village hydro schemes run them off-grid with a capacitor bank and controller (Practical Action Micro-Hydro O&M, p. 23). Expect some trial and error.
    • Rule of thumb: start with run capacitors whose current at rated voltage roughly matches the motor's no-load current, then trim until voltage and frequency are right. Use AC motor-run (film) capacitors, never electrolytic start capacitors.
    • Expect about 70–80% of the motor's rating as output (rule of thumb). Needs a ballast (dump) load or controller so voltage doesn't swing as loads switch. Aging capacitors make frequency creep up; a capacitor running hotter than the others is failing — feel them once a month. Too many capacitors overheat the generator. (Practical Action, pp. 23–26)
  4. Build a permanent-magnet alternator from magnets and magnet wire for a wind turbine or slow water wheel — full plans in Hugh Piggott's Axial Flux Alternator and PMG Construction Manual. His PMG makes about 180 W (15 A at 12 V) at only 420 rpm — slow enough to drive directly from blades. (Piggott PMG Construction Manual, p. 4; exploded drawing of the PMG; parts of a small wind generator; Small wind turbine)

Rules for any homemade generator:

Induction generator, Car alternator. More pictures: Micro hydro, Rectifier.

📚 Go deeper: Piggott PMG Construction Manual — build a slow-speed permanent-magnet generator; DFID-ITDG Small Wind Systems — controllers, dump loads and batteries; Practical Action Micro-Hydro O&M — running and fixing a small hydro plant; North Dakota Homemade Six-Volt Wind-Electric Plants — 1939 car-generator wind charger.

🛠️ Reusing a generator for water or wind power

When the gasoline and diesel are gone, a portable generator still holds two valuable machines: an engine and a generator head (the part that makes electricity). There are two ways to keep using them.

1. Keep the whole set and change the fuel. FEMA's wood-gas plans say stationary engines on wood gas "run electric generators, pumps, and industrial equipment" (FEMA Wood Gas Generator, p. 19). A generator sits still, so it needs a gas cooler and a second filter that a moving vehicle wouldn't, and hot gas costs power: about 1% for every 10 °F above 70 °F (p. 28). Most portable generators have engines of 15 hp or less; for those FEMA says to use a 6-inch fire tube with a narrowed throat (p. 67). The rest (building it, and the carbon monoxide danger) is in the Fuel guide.

2. Take the generator head off and turn it with water or wind. The rest of this section is about that.

What's inside: two kinds of portable generator

Standard generators (the open-frame kind) have a generator head bolted to the engine:

Inverter generators (the quiet, closed-case kind). There is no manual for one in the library; this is how they are generally built (rule of thumb):

Which parts are worth saving

Part Best use Watch out for
Magnet alternator (from an inverter generator) Charging batteries from a slow water wheel or wind rotor: rectify it to DC, then a diversion controller Test it first (below). Keep its magnets away from iron and don't drop it
Wound-field generator head (standard generator) A fast turbine, like a Pelton wheel on a good drop, belted up to its speed Wants 3,600 rpm; voltage holds but frequency swings with the load
Engine Wood gas (above), or turning a pump, mill or saw directly Same care as any engine (top of this guide)
Voltage regulator, capacitors Stay with the generator head Discharge capacitors before touching
Inverter board Rarely reusable without its engine; keep for parts Can hold a deadly charge
Bearings, pulleys, frame, outlets, breaker, wire Your hydro or wind build Check bearings (below)

Taking it apart safely

  1. Drain the fuel, unplug the spark plug wire, and work outdoors or with the doors open (rule of thumb).
  2. Discharge every capacitor (inverter board, voltage regulator, capacitor-excited heads): the charge left in capacitors "is, in many cases, sufficient to cause a lethal shock." Use a shorting probe with its ground clip on first, touch each terminal several times, and keep your hands off the metal (NEETS 21, pp. 32–33; shorting probe). Short a stored capacitor's terminals with a wire; "CHARGED CAPACITORS CAN KILL" (p. 34).
  3. Photograph and tag every wire, and punch-mark the frame and end bells so they go back the same way (Navy Construction Electrician, p. 255; punch marks).
  4. Turn the shaft by hand before opening anything: binding means a bent shaft, bad bearings or debris (same page).
  5. Taking a magnet flywheel off: hold it with a holding tool (the nut's thread direction depends on which way the engine turns), then use a puller, not a hammer: "Too much hammering will jar the magnetism out" (AAVIM Small Engines Vol 2, pp. 58–59; flywheel pullers). Don't drop it, store it away from iron, and pick metal chips off the magnets (p. 60; MAVCC, p. 374).
  6. Pulling a rotor out of a stator: lift the brushes first, support the rotor only by its shaft, and slide thin cardboard into the gap to protect the windings (Navy Construction Electrician, p. 256; Kuphaldt Vol 6, p. 174).
  7. Bearings: leave good ones on the shaft; pull with the puller on the inner race (see "Electric motors: testing and repair" above; Navy Electrician's Mate, p. 224).

Testing what you salvaged

Matching it to a water wheel or wind rotor

A big pulley on a turbine turning 1,060 rpm drives, by V-belt, a small pulley on a generator head at 3,600 rpm; the turbine's pulley is about 3.4 times the diameter of the generator's Pulley diameter × rpm is the same on both shafts (a diagram drawn for this guide).

Hooking it up

Wiring for charging a battery from a salvaged magnet alternator: three wires into a six-diode rectifier, DC positive and negative to the battery with a fuse right at its positive post, and a diversion controller across the battery that sends spare power to a water-heater element dump load Rectify, fuse the battery at its + post, and let a diversion controller send spare power to a dump load (a diagram drawn for this guide).

🖼️ Pictures & diagrams

Look before you try it. 🖼️ = drawings in the books on this drive; 📷 = photos in the offline Wikipedia.

Small gas engines

Diesel engines

Saws and axes

Generators

Electric motors

Motors as generators

Reusing a generator for water or wind

Measurements and settings in old manuals are fine for methods; for your specific engine, motor or generator, its own nameplate and manual come first.

↑ Contents