Small Engine & Motorcycle Maintenance: A Plain-English Guide

A push mower, a generator and a motorcycle look nothing alike, but the engines inside them live and die by the same handful of things. Keep the oil clean, the air filter clear, the fuel fresh and the spark strong, and most of them will start on the first or second pull for years. Skip those and you get the machine everybody has in the back of the garage: the one that ran fine last fall.

This guide covers the maintenance an owner can do with ordinary hand tools. It is written for four-stroke and two-stroke gasoline engines on lawn and garden equipment, generators and motorcycles.

What Every Engine Needs

A gasoline engine needs four things, and it needs them at the right moment: fuel, air, spark and compression. When an engine will not start or runs badly, one of the four is missing, weak or late. Nearly every job in this guide is about keeping one of them healthy, and the troubleshooting section near the end is nothing more than checking them in order.

Four-stroke and two-stroke

Most mowers, generators and street motorcycles are four-stroke. The piston makes four trips for each power pulse: it draws in fuel and air, squeezes it, fires it, then pushes out the exhaust. The oil sits in the crankcase. You check it, and you change it.

Chainsaws, string trimmers, leaf blowers and many older dirt bikes are two-stroke. They fire every time the crank comes around, and they have no oil sump. The oil is mixed into the gasoline, or fed in by a small pump on some motorcycles. Run a premix two-stroke on straight gasoline and it will seize, usually within minutes. Mix to the ratio in the manual, which is often printed on the fuel cap as well.

Before You Pick Up a Wrench

• Pull the spark plug wire on anything with a blade before your hand goes near it. Turning a mower blade by hand turns the engine over, and an engine that turns over can fire.
• Never run an engine in a closed garage or shed. Carbon monoxide has no smell and it does not take long.
• Treat gasoline like the fire hazard it is. No flames, no sparks, no smoking, plenty of fresh air, and an approved fuel can.
• Keep a fire extinguisher within reach whenever you are working with fuel or checking for spark.
• Let it cool. Exhaust pipes and hot oil leave burns. Warm oil drains well; hot oil is a trip to urgent care.
• Hold the motorcycle up properly. Use a stand made for the job and strap the bike before you pull a wheel.
• Battery work: take the negative cable off first and put it back on last.
• Wear eye protection, and catch used oil and coolant in a pan. Auto parts stores and county recycling sites take both.

A Basic Tool Kit

You do not need a full roll cabinet. This covers nearly everything below:

• Socket set and combination wrenches. Most motorcycles are metric; check what your equipment uses.
• Screwdrivers. Many Japanese motorcycles use JIS cross-head screws, which look like Phillips but are not. A Phillips driver tends to climb out and chew them up, so a set of JIS drivers is cheap insurance.
• Spark plug socket and a plug gap gauge.
• Tire pressure gauge you trust.
• Torque wrench for drain plugs, axle nuts and brake bolts.
• Drain pan, funnel and plenty of rags.
• Multimeter for battery and charging checks.
• Carburetor cleaner, chain cleaner and chain lube, and a bottle of fuel stabilizer.

Engine Oil

Oil does three jobs. It keeps metal parts from touching, it carries heat away, and it holds dirt and combustion grit until you drain it out. Small engines hold very little oil and many have no oil filter, so the oil they have works hard.

Checking the level

Check it on level ground, the way your manual describes. Some dipsticks are read screwed all the way in and others are read resting on the threads. A motorcycle sight glass is read with the bike held upright, not leaning on the side stand. Get that wrong and you will overfill or underfill a healthy engine.

Changing it

1. Run the engine for a few minutes so the oil is warm and the dirt is stirred up.
2. Shut it off, pull the plug wire on equipment with a blade, and set a pan underneath.
3. Remove the drain plug, or tip or pump the oil out the fill tube on engines with no drain. Let it finish dripping.
4. Replace the oil filter if the engine has one, with a light film of clean oil on the new gasket.
5. Refit the drain plug with a new sealing washer where one is used, and tighten it to the manual's torque. Aluminum cases strip easily.
6. Fill with the grade and amount in the manual. Run the engine briefly, let it settle, then recheck the level and look for leaks.

Two things catch people. First, many motorcycles share one oil between the engine, the gearbox and a wet clutch. Use oil that meets the motorcycle specification in your manual, because some car oils contain friction modifiers that can make a wet clutch slip. Second, a new or freshly rebuilt small engine usually calls for an early first oil change. The manual will say when.

Air Filter

An engine breathes a huge volume of air, and on a mower that air is full of dust and grass. The filter is the only thing between that grit and the cylinder wall.

• Paper element: tap it gently to knock loose dirt out and hold it up to a light. If you cannot see light through it, replace it. Do not wash it, and do not blast it with shop air from the outside in.
• Foam element: clean it the way the manual says, usually in solvent or warm soapy water, and let it dry fully. Then work foam filter oil through it with your fingers until it is evenly coated. The oil does the filtering and the foam only carries it, so a dry foam filter does almost nothing. Squeeze it like a sponge; do not twist it. Foam that tears easily is finished.
• Two-stage: a foam pre-cleaner wrapped around a paper element. Service each part its own way.

A clogged filter makes the engine run rich. You will see hard starting, black smoke and a sooty plug. A torn or missing filter is worse, because it lets dirt straight in and the engine wears out fast. Never run one without the filter fitted, even for a minute.

Spark Plug

Blow or brush the dirt away from the plug before you unscrew it, so nothing drops into the cylinder. Then look at the firing end. It is the cheapest diagnostic tool you own.

Check the gap against the figure in the manual and replace the plug if the electrode is rounded off or the insulator is cracked. Use the plug type the manual lists. Start it by hand, a few full turns with your fingers, before a wrench goes on it. A cross-threaded plug in an aluminum head turns a five-minute job into an expensive one.

Fuel and the Carburetor

Stale fuel is the most common reason a small engine will not start after sitting. Gasoline does not keep. The light parts evaporate, ethanol blends pull in moisture, and what is left behind dries into a varnish that plugs the tiny passages in a carburetor.

• Buy fuel in amounts you will use up, and keep it in a sealed, approved can.
• Add stabilizer to anything that will sit, and run the engine a few minutes so treated fuel reaches the carburetor. Ethanol-free gasoline, where you can get it, stores better.
• Replace the fuel filter when it looks dirty, and replace fuel lines that have gone hard or cracked.
• Check the cap vent. An engine that starts, runs for a few minutes and then dies, and restarts after you loosen the fuel cap, has a blocked vent.

A gummed-up carburetor

If the engine only runs with the choke on, or surges up and down at steady throttle, the carburetor is likely partly blocked. Start with the easy step and drain the float bowl; a lot of trouble comes out with that first cup of old fuel. If that does not cure it, take the bowl off and clean the jets and passages with carburetor cleaner and compressed air.

Do not push steel wire or a drill bit through a jet. The hole is a measured size and it is soft brass. Open it up even a little and the engine will run rich forever after.

Fuel-injected motorcycles need less of this. Keep the tank clean, change the fuel filter when the schedule says, and leave injector and sensor diagnosis to someone with the right equipment.

Cooling

Air-cooled engines get rid of heat through their fins. Grass clippings, mud and mouse nests packed under the shroud will cook an engine that is otherwise perfect, so clear the fins and the blower housing every season.

Liquid-cooled engines need the coolant level checked at the overflow bottle with the engine cold. Never open a radiator cap on a hot engine. Look over the hoses and clamps for seepage, and change the coolant on the manual's schedule, because the corrosion protection in it wears out long before it looks bad.

Battery

Batteries die from sitting more than from use. A battery left discharged for a winter rarely comes back all the way.

• Keep the terminals clean and tight. A loose terminal acts exactly like a dead battery.
• Put a smart maintainer on any machine that sits for more than a few weeks. A maintainer is not the same thing as an old trickle charger, which can overcharge.
• On batteries with removable caps, keep the plates covered using distilled water only. Leave sealed batteries sealed.
• A multimeter across the terminals, engine off and then running, tells you whether the battery is holding charge and whether the charging system is working. Your manual gives the readings to expect.

Motorcycle Checks

A mower that quits leaves you with long grass. A motorcycle is different, because tires, brakes and controls are what keep you upright. These checks matter more than anything else on this page.

Chain, belt or shaft

Clean and lubricate a chain regularly, and sooner after rain or dusty roads. Check slack midway between the sprockets at the chain's tightest spot, with the bike sitting the way the manual specifies. Adjust both sides evenly using the marks on the swingarm, then sight down the chain to confirm the wheel is straight.

A chain that is a little loose is safer than one that is too tight. A tight chain loads the bearings and gets tighter still as the suspension moves. Sprocket teeth that are hooked or pointed like shark fins are worn out, and a chain and both sprockets should be replaced together. On a sealed O-ring chain, use cleaner and lube marked safe for O-rings and keep the wire brush away from it.

Belt drives want an inspection for cracks and missing teeth and a tension check. Shaft drives want their final-drive oil changed on schedule.

Tires

Check pressure cold, before you ride, against the figures on the bike's placard or in the manual. The number molded into the sidewall is the tire's maximum, not your setting. Look for tread down to the wear bars, cracks in the sidewall, nails and flat spots. Tires also age out: the four-digit code on the sidewall gives the week and year it was made, and an old tire with plenty of tread can still be hard and slick.

Brakes

• Pads: look into the caliper at the friction material. Most pads have a wear groove or indicator. Replace them before metal touches the disc.
• Fluid level: it should sit between the marks in the reservoir window with the bike upright.
• Fluid condition: brake fluid absorbs moisture from the air and darkens with age. Change it on the manual's schedule even if the level is fine.
• Fluid type: use what is printed on the reservoir cap. Silicone DOT 5 does not mix with the glycol fluids, DOT 3, DOT 4 and DOT 5.1.
• Lever feel: a spongy lever that comes back to the bar means air in the line. The system needs bleeding before the bike is ridden.

Brake fluid strips paint. Cover the tank, and rinse any spill with water straight away.

Controls, lights and fasteners

• The throttle must snap shut by itself from any handlebar position.
• Clutch and brake levers need the free play the manual calls for. Lubricate cables and pivots.
• Turn the bars lock to lock with the front wheel off the ground. Any notch or tight spot points to steering head bearings.
• Check headlight, tail light, turn signals, horn, and the brake light from both the lever and the pedal.
• Look at the fork seals for oil, and look for anything loose or weeping: axle nuts, pinch bolts, caliper bolts, engine mounts, exhaust clamps. Torque the critical ones to specification.

Before every ride, take half a minute to walk around the bike: tires, controls, lights, oil, chain, side stand. The Motorcycle Safety Foundation's T-CLOCS checklist covers the same ground in more detail and is worth learning.

Mowers, Generators and Two-Strokes

Mowers

• With the plug wire off, check the blade. Sharpen and balance it, and replace it if it is bent or cracked. A blade that is out of balance shakes the whole machine.
• When you tip a mower, check the manual for which way. The usual rule is air filter and carburetor up, so oil and fuel do not run into them.
• Scrape the packed grass from under the deck. It holds moisture and rots the deck.
• The handle bail that stops the engine when you let go is a safety device. Do not tie it down.

Generators

• Run it under a real load every so often. Generators fail from sitting, and the day you need one is a bad day to find a varnished carburetor.
• Check the oil every time. Many generators have a low-oil shutdown and will not start, or will stop, when the level drops.
• Before storage, shut the fuel valve and let the engine run the carburetor dry if your manual allows it.
• Run it outdoors, well away from doors and windows. Never in a garage, even with the door open.
• Never plug a generator into a wall outlet to power a house. Feeding a building needs a transfer switch installed by an electrician, or it can electrocute a lineworker down the road.

Two-stroke handhelds

• Mix fresh fuel at the right ratio, in a clean can, and mix only what you will use soon. Premix goes stale as well.
• Clean the air filter often. Chainsaws and trimmers live in their own dust.
• A clogged spark arrestor screen in the muffler makes a saw or trimmer lose power and bog. Pull it and clean off the carbon.
• On a chainsaw, keep the chain sharp and correctly tensioned and the bar oil tank filled. A dull chain is hard on the engine and on you.

Tillers, snow blowers and pressure washers

Check gearbox oil where there is a fill plug, and look over the belts. Replace shear pins with the correct pins, never an ordinary bolt, because the pin is meant to break before the gearbox does. Never run a pressure washer pump without water flowing through it.

A Maintenance Rhythm

Your manual's hours or mileage always win. Where you do not have a manual to hand, this rhythm keeps most machines out of trouble.

Putting It Away for the Season

1. Deal with the fuel. Either fill with fresh, stabilized fuel and run the engine a few minutes, or drain the tank and carburetor. Follow the manual for your machine. A steel motorcycle tank is better stored full, which leaves less bare metal to rust.
2. Change the oil first. Used oil carries acids and moisture you do not want sitting in the engine all winter.
3. For a long lay-up, protect the cylinder. Put a small amount of engine oil in each spark plug hole, turn the engine over a few times with the plug leads grounded, and refit the plugs.
4. Look after the battery. Take it out or connect a maintainer.
5. Clean and dry the machine, lubricate the chain and wipe a light film of oil on bare metal.
6. Set tire pressures, and take the weight off the tires on stands if you can.
7. Keep mice out. Plug the exhaust and the air intake, and hang a tag on the handlebar or pull cord so you remember to take the plugs out.
8. Use a cover that breathes. A plastic tarp traps damp against the machine.

In spring, go through the every-use list, put in fresh fuel, and check tire pressures and brakes before the first ride.

When It Won't Start

Work through it in this order. Each step takes a minute or two, and the order runs from the most likely cause to the least.

1. The obvious. Fuel in the tank, fuel valve on, choke set, plug wire attached. On a motorcycle: key on, kill switch on RUN, in neutral or clutch pulled in, side stand up. On a mower: bail held against the handle. On a generator: enough oil to satisfy the low-oil shutdown.
2. Fuel. Is it fresh? Loosen the float bowl drain; fuel should run out. If nothing comes, look at the filter, the fuel line, the valve and the cap vent. If the machine sat for months with fuel in it, suspect a gummed carburetor.
3. Spark. An inline spark tester between the plug and its lead is the safe way to check: it flashes when the ignition is working. No spark? Try a new plug, then check the kill switch, the wiring to it and the plug cap.
4. Air. Pull the air filter and look. A choked filter or a nest in the intake will stop an engine cold.
5. Compression. If the pull cord comes out with almost no resistance, or the starter spins the engine too freely, compression is low. A compression gauge reading against the manual's figure confirms it. The causes are inside the engine: valves, rings or a head gasket.

Unknown history, or sat for years? Turn the order around. Test compression with a gauge first, then spark, and leave the carburetor for last. Dan's Free Motorcycle Repair Course makes this point from decades at the bench: owners clean a carburetor over and over on an engine that never had the compression to run.

Flooded? If you smell gasoline and the plug is wet, turn the choke off, hold the throttle open and crank the engine to clear it. Dry or replace the plug.

Running, but badly

• Surging or hunting: running lean. Partly blocked carburetor jets, stale fuel or an air leak.
• Black smoke: too much fuel. Choke stuck on or a clogged air filter.
• Blue smoke: burning oil. Overfilled, tipped the wrong way, or wear.
• White smoke with a sweet smell on a liquid-cooled engine: coolant is getting into the cylinder. Stop running it.

When to Hand It to a Shop

Doing your own maintenance saves money and teaches you the machine. It also has limits. Take it to someone qualified for internal engine work, for valve adjustments you are not comfortable making, for fuel injection and electrical faults beyond the basics, and for any brake, tire or steering job you are not sure you did right. If you finish a brake job with any doubt at all, get it checked before you ride.

This guide is general. It is not a service manual for any particular machine and it does not replace one.

Where to Learn More

This guide covers upkeep. When you are ready to go deeper, into carburetor circuits, ignition timing, valve adjustment and full engine rebuilds, we recommend Dan's Free Motorcycle Repair Course at dansmc.com. Dan has been a motorcycle mechanic since 1967 and has kept the course online, free, since 1999. It runs to well over a hundred lessons.

A few of his lessons pick up where this page stops: Trouble Shooting a Bike that Will Not Start, Winter Storage, Changing Your Oil, Carburetor Theory and Tuning, Rear Chains & Sprockets, and Engine Compression Testing.

This page is TCB's own write-up. It is not a reprint of Dan's course.