Small Engine Repair

Small engines power lawn mowers, snow blowers, chain saws, generators, motorcycles, boats, snowmobiles, and many other tools and toys. This Fix-It Guide on small engine repair tells how a small engine works, what often goes wrong, how to identify a small engine problem, and what parts and tools you will need to fix it. It then gives simple step-by-step instructions for how to maintain a small engine, how to replace a recoil starter rope, how to replace a recoil starter spring, how to clean a foam air filter, and how to service fuel lines and a fuel tank. Small engine repair may be the key to leaf blower repair, motorcycle repair, outboard motor repair, ATV repair, chain saw repair, wood chipper repair, dirt bike repair, and more.

How Does a Small Engine Work?

A small engine is an internal combustion gasoline engine that produces less than 40 horsepower. To produce power, the engine:

  1. Mixes fuel and air.
  2. Compresses the mixture.
  3. Adds a spark to ignite it.
  4. Exhausts the resulting fumes.
Small Engine Repair

Components of a typical small four-stroke engine used to power lawn mowers, tillers, and other equipment. A two-stroke engine has most of the same components.

These four steps make up the power cycle. A two-stroke or two-cycle engine mixes and compresses in one rotation, then ignites and exhausts during the second rotation or stroke. A four-stroke or four-cycle engine requires a full rotation for each of the four steps. Most small engines are two-stroke and larger ones, like that in your automobile, are four-stroke. Two-stroke engines aren’t as powerful, but they’re much cheaper to build. Small engines have one or maybe two cylinders or areas where the explosions occur.

Another important fact is how the engine is cooled. Car and other larger engines are cooled by circulating liquid through channels within them. Because small engines don’t develop as much heat, they typically are cooled by the surrounding air.

Fix-It Tip

Some two-stroke engines need special fuel that includes lubricating oil. Four-stroke engines use separate fuel and lubricant. Follow your owner’s manual suggestions for mixing fuel and lubricant for a two-stroke engine, or buy premixed fuel/lubricant.

What Can Go Wrong with a Small Engine?

Regular maintenance can keep repairs to a minimum. A starter rope may be broken or jammed. A recoil starter spring may be weak or broken. An electric starter battery may be low, connections may be loose or corroded, or a power cord may be damaged. Air filters get clogged. Fuel can become contaminated; a fuel cap breather hole may be blocked or a fuel filter clogged. A fuel line can be clogged or damaged. A choke may become sticky. A belt may slip.

How Can I Identify a Small Engine Problem?

  • If you want to keep repairs to a minimum, follow a regular maintenance program (see below). For further maintenance, consult your owner’s manual and/or a technician.
  • If the starter doesn’t make the engine turn over, check the starter rope (see below), then look for a weak or broken recoil starter spring (see below). Also check an electric starter’s battery and connections and the electrical cord.
  • If the engine does not run smoothly or smokes, clean a clogged air filter (see below).
  • If the engine turns over but won’t start, stalls, or dies during use, add fuel, clean a clogged fuel filter with solvent or replace it, clean a clogged fuel line (see below), and clean a sticky choke mechanism with solvent.
  • If the engine starts or stalls or stalls at idle speed, check your owner’s manual for adjustment to the idle speed.
  • If the engine dies during use, overheats, is very noisy, or runs unevenly, add oil to the oil tank if it is low and let the engine cool, then restart it.
  • If the engine runs, but the device doesn’t, look for a slipping belt (see other Fix-It Guides for specific machines).

What Do I Need for Small Engine Repair?

Replacement parts are available from the engine’s manufacturer and aftermarket suppliers as well as from small engine service shops and larger hardware stores. The tools you will need to fix a small engine may include these:

  • Screwdrivers
  • Stiff plastic-bristle brush
  • Lightweight oil, lithium grease, or silicone lubricant
  • Pliers
  • Engine oil
  • Gasoline
  • Carburetor-cleaning solvent
  • Petroleum solvent
  • Dish detergent
  • Soft cloth
  • Pipe cleaner
  • Needle or wire
  • Scissors

What Are the Steps to Small Engine Repair?

Maintenance is the key to keeping a small engine running smoothly.

Before each use:

  1. Remove debris from the air intake screen, muffler, oil filter, and governor linkage areas with a stiff plastic-bristle brush.
  2. Remove the air intake screen to clean the flywheel fins.
  3. If you are working on a four-stroke engine, check the oil level and add oil as needed.

Twice a year (depending on use and manufacturer’s recommendations):

  1. For a four-stroke engine, change the oil. Drain the old oil into a container through the drain plug or pour it out through the filler hole and refill with new oil. (Two-stroke engines don’t need this step because the oil is in the fuel.)
  2. Apply a small amount of lightweight oil, lithium grease, or silicone lubricant to all exposed control cable and pivot points (clutch and throttle controls).
  3. Clean or replace any air filter as needed.

Fix-It Tip

How can you safely get rid of dirty fuel left over from cleaning an engine? The best way is to buy an approved fuel container and mark it “Recycle” for dumping fuel, oil, and other petroleum waste products. Once full, take it to a hazardous waste station in your area (call your recycler for details). Make sure the container is sealed and kept away from other combustibles and heat sources.

Replace a recoil starter rope:

  1. Disconnect the spark plug cable for safety.
  2. Disengage the throttle cable from the housing and remove the housing as needed to access the starter.
  3. Remove the handle from the rope, which is often held in place by a knot or retainer pin.
  4. Unfasten the other end of the rope from the starter using pliers or a cutter as needed.
  5. Knot the end of the replacement rope.
  6. Tighten the pulley counterclockwise all the way, then back it off two turns and hold it firmly.
  7. Thread the un-knotted rope end through the pulley hole.
  8. Thread the rope through the housing hole and pull it taut.
  9. Slowly release the pulley, allowing the rope to wind around the pulley.
  10. Attach the handle and reassemble as needed.
  11. Reconnect the spark plug cable.

Replace a recoil starter spring:

  1. Disconnect the spark plug cable for safety.
  2. Remove the housing and the spring cover.
  3. Carefully remove the rope from the pulley to release spring tension.
  4. Remove and replace the spring as recommended by the manufacturer.
  5. Reinstall the pulley, rope, and housing following the instructions above.
  6. Reconnect the spark plug cable.

Clean a foam air filter:

  1. Remove the cover and lift out the foam filter.
  2. Clean dirt and grease from the housing interior with a soft cloth.
  3. Wash the filter with dish detergent and hot water and rinse with clear water.
  4. Allow the filter to fully dry before reinstalling.

Service fuel lines and tank:

  1. Disconnect the spark plug cable for safety.
  2. Use a needle or fine wire to unclog gas cap breather holes, then clean the cap with a carburetor-cleaning solvent.
  3. Remove and inspect the fuel. If the fuel has sediment in it or smells like mothballs, it is contaminated and should be drained and replaced.
  4. If there is a blockage in the fuel line(s), clear it with a pipe cleaner and petroleum solvent.
  5. Replace the fuel filter.
  6. As needed, flush the tank and fuel line with fresh fuel.
  7. As needed, fill the fuel tank.