Engine problems can happen slowly or suddenly. Either way, unsuspected problems can be frustrating and catch you off guard. We've put together a few steps to help you diagnose the issue, or issues, at hand.

First we need to start by determining which system in the engine is malfunctioning. After we determine which system(s) are at fault we can narrow down our diagnosis and make a repair.

We'll start by listing the most common symptoms and their possible issues. From there, we break it down into repairs and your options.

Common issues

  • Failure to start
  • Starts and then stalls
  • Loss of power
  • Overheating
  • Electrical issues
  • Backfiring

Part 1 of 6: Failure to start

Failure to start: A small engine’s inability to start may be caused by a variety of issues:

  • Bad gasoline - Dirty, old, and improperly stored fuel will not ignite or function normally. Change out the old gasoline or 50/50 mix.

  • Dirty carburetor - A dirty or clogged carburetor will cause the engine to not receive the fuel air mixture properly. Secondly, a bad or weak mix of air and fuel will not ignite properly.

Clean the carburetor either while on the engine, or remove and then clean the unit. If the carburetor is damaged beyond repair, you may need to replace the unit all together.

  • Bad spark - A weak spark from its lead, damaged or improperly gapped plug, dirty plug, or old plug will all prevent the engine from firing when asked to start.

Check the spark plug condition. Look for signs of damage, improper plug gap, soiled spark plugs, or a broken connection to the spark plug lead (aka wire). Replace the spark plug(s) if needed.

  • Compression - compression is a huge factor in engine performance. Without proper compression, the combustion is weak or does not happen at all.

There are fairly simple tests that can be done with a specific compression tester. You may want to have this done by a professional, if you're not properly equipped.

Part 2 of 6: Starts and then stalls

Starts and then stalls The most common causes of a stalling engine pertain to the carburetor. If it’s not directly the carburetor’s fault, it is likely a gas issue.

  • Bad fuel - Fuel left in the gas tank over longer durations of time can turn into a weaker, thicker substance that will not combust under normal conditions.

Completely drain the fuel tank. Rinsing with clean fuel afterwards is advised. You will very likely have to clean the carburetor at this point as well.

  • Clogged or damaged carburetor - Being the most likely issue, a clogged carburetor will not allow the amount of fuel and air mix to be available for combustion. Many types of damages to the carburetor will also cause the engine to start and stall out.

A typical carb cleaner usually helps a lot. If the carburetor is clogged to the point carb cleaner does not fix the issue entirely, you will need to replace the unit.

If the issue is being caused by damages to the carburetor, you will need to replace the entire unit.

  • Fuel cap - Fuel caps need to be able to vent as the fuel is being used from the tank. Without this regulation in pressure, the carburetor will dump less fuel causing the engine to stall.

If you are able to unclog the vent in the gas cap, you can refrain from replacing it. If the vent is damaged or clogged beyond repair, a new cap will need to be installed in its place.

  • Spark plug or wiring defect - There are several circumstances that can lead to spark plugs performing intermittently. Most commonly are dirty plugs, damaged plugs, or an electrical short with the spark plug lead.

Replace the spark plug. If it is a gap or dirt issue, you can attempt to reset the gap or clean the plug. More than not, it is cheaper and more effective to replace the plug and lead.

Part 3 of 6: Loss of power

Loss of power: A small engine that runs poorly is typically due to failure to properly maintain the engine while not in use. Most general tune-ups will solve these running issues.

  • Bad fuel - Just like bad fuel will cause issues with starting, it will cause issues when running as well.

Remove all old fuel or mix or fuel and oil. Refill with brand new gas or 50/50 mix.

  • Warning: Do not confuse the two. Your engine will take one or the other.

  • Bad oil - Despite using a 50/50 mix, or separate fuel and oil, bad oil can cause strain on the engine and failure to perform as desired.

Remove all old oil or 50/50 mix and replace with fresh clean oil.

  • Air flow - A dirty or clogged filter, or clogged air intake can cause performance issues with the engine.

Examine the air filter. Check for damages, clogs, or debris which may reduce airflow. Replace air filter if needed or desired.

Part 4 of 6: Overheating

Overheating: Overheating can be the result of oil, fuel, fan, or gas cap failure. Overheating can also be caused by a mixture of all of these systems. Performing more than one of the steps towards repairing the issue may be necessary.

  • Low oil levels - Oil acts as a coolant helping to take the heat and disperse it away from the combustion chamber. Without the fluid to absorb the heat, the transfer will not occur.

Change or add oil to specified level needed for the engine.

  • Dirty engine - A dirty engine will not allow the hot air to vent from the engine. Use an engine grime cleaner to remove dirt from the surface of the engine. Be careful not to saturate parts of the engine you are not looking to access.

  • Broken fan - The fans on most small engines are subject to being moved around, dropped, jostled, and bumped. If a broken fan, or fan blade is causing a lack of ventilation, an overheating issue is likely the result.

The extend in which you can access and repair the fan is going to depend on the make and model of your small engine. Many parts are serviceable, and further investigation of the owner’s manual can help determine what you can fix.

  • Fuel issues - A lean fuel mix can cause the engine to need to work harder than its intended. If the filter screen is dirty or damaged, it will cause a lean mix. If your fuel tank vent is clogged, the amount of fuel dumped will decrease. Carburetors can also cause a lean air-fuel mix.

Replace the fuel tank cap, or the filter.

Replace the carburetor.

Part 5 of 6: Electrical issues

Electrical issues: Electrical issues surface in many forms. Battery issues, starting issues, clutch issues, and running issues can all become incredibly frustrating.

  • Battery will not charge - Batteries that will not charge are usually not repairable. Replace battery unit and charging system. Use a new charge when possible with new battery.

  • Battery loses charge - A battery that loses its charge quickly can be due to a faulty battery, or a bad battery ground.

Replace the battery if all of the ground connections appear to be fine.

  • Engine will not start, or starts when switch is engaged to off- Starting issues, even the odd ones, are typically simple. The ground for the ignition is usually at fault.

Check the ground at the ignition and at the battery.

If the ground terminals are intact and correctly connected, you can test or replace the ignition switch.

  • Clutch will not engage or disengage - When the clutch is not working properly, your small engine will be useless.

Verify ground wires and terminals are clean and correctly installed.

Check the regulator-rectifier for proper installation and correct type.

  • Fuses blowing- Fuses blowing consistently will be due to some kind of short in the electrical system.

Check for a properly functioning regulator-rectifier, and insure it is grounded properly.

Look for a short somewhere using a multimeter.

Check alternator stator if equipped.

Part 6 of 6: Backfiring

Backfiring**: Backfiring can be caused by a number of issues. Most commonly are related fuel contamination or delivery issues.

  • Water in fuel tank - water in the fuel will cause a backfire.

Drain the fuel tank, and rinse with fuel before storing dry.

  • Low on fuel - The easiest fix for backfiring is low fuel.

Refill fuel tank.

  • Choke set incorrectly - Adjusting the choke is becoming more and more obsolete, however if yours is adjustable, and you are experiencing backfiring, it something you to look at.

Use your owner’s manual or repair guide to readjust the choke.

  • Dirty carburetor - A dirty carburetor will often dislodge part of the clog causing a backfire.

Use a standard carburetor cleaner to remove all debris from the unit.

Troublesome small engine problems can be headache. With some simple diagnostics, and a few facts about how things work, most issues can be repaired easily. Keep in mind you may always contact a certified technician to inspect the backfiring problem for you.

This article originally appeared on YourMechanic.com as How to Troubleshoot a Small Engine Problem and was authored by Jessica Howe.


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