Guide to Small Engine Technology: Power Equipment Engine Troubleshooting

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  • Understand how to systematically approach problems when working on power equipment engines
  • Understand troubleshooting procedures for the following conditions: engine problems, fuel system problems, electrical system problems, and abnormal noise problems


The ability to quickly and correctly trouble shoot power equipment engine problems is a sign of a competent technician. Proper diagnosis of a malfunction makes disassembly, repair, and reassembly much easier. If a malfunction has been improperly diagnosed, the repair process will become long and tedious, or even seemingly impossible. Therefore, in order to be a successful technician, you must possess proficient troubleshooting skills. First and foremost, troubleshooting begins with a thorough knowledge of the:

  • Components of a power equipment engine
  • Function of each component
  • Effect each component has on the overall operation of the power equipment engine
  • Types of failure symptoms that a defective component will cause

The previous sections in this guide were designed to help you to learn about these important areas pertaining to a working engine. Once you've gained this knowledge, trouble shooting becomes systematic and controlled.

You must understand that this knowledge does not always come easily. You must accept that you're bound to make mistakes along the way, when diagnosing and repairing a power equipment engine. But make sure you understand what you did incorrectly in those situations, and learn from those mistakes. It's true that everyone makes mistakes, including those who design and create the engines used in today's world. The key is to learn from our mistakes to gain a better understanding of what we are doing.

When troubleshooting, you'll need to:

  • Gather all available information about the malfunction
  • Analyze the symptoms related to the problem
  • Pinpoint the most likely cause of the problem

The repair process shouldn't begin until after you've gotten a clear picture of what's causing the problem. For this, you'll have to mentally divide the power equipment engine into sections (fuel, engine, and electrical systems):

  • Picture each component of every section
  • Picture what each component does
  • Picture each component and its relationship with the other components and decide if each part is functioning properly

For example, suppose a spark plug isn't firing correctly. You must envision the operation of the spark plug in relation to that of other power equipment engine systems. After doing this, you'll have a variety of possible problems identified, besides the obvious conclusion that the spark plug is bad. The problem could be due to a dirty air filter that's creating an excessively rich fuel mixture.

It's imperative that you understand what you're trying to repair before disassembling a machine. Once you begin the disassembly pro cess, the troubleshooting process is over. In our spark plug example, cleaning and replacing parts in the fuel system won't solve the problem because the cause is a faulty ignition. Therefore, ensure that you've isolated the problem before beginning any repair.

Because it's impossible to cover every type of power equipment engine problem, we'll discuss basic diagnostic and troubleshooting techniques and concepts that you can apply to many situations. Each topic in this section contains tables that can be used as a guide when troubleshooting. To get the most from this section, focus your attention on the basic concepts presented.

Learning from the information in this section will allow you to begin to build a solid foundation for developing your own troubleshooting expertise. This section comprises a set of tables that cover virtually every aspect of trouble shooting. Although you'll feel there are many tables here, they by no means contain every possible cause for each problem listed. These tables are intended to aid you in troubleshooting problems by giving you some of the more common causes of the problems listed.


It's important to understand that there are techniques that will expedite the process of troubleshooting power equipment engine problems. Developing a systematic approach to solving problems will help you perfect your skills as you gain more experience.

Types of Problems

A symptom is an indication of an abnormal condition that you can recognize and identify.

An example of a symptom would be a power equipment engine that's making a ticking sound when it's idling. The symptom helps you deter mine the cause of the problem. The following paragraphs cover three types of failures that you may encounter.

Constant Failures

A constant failure yields a symptom that is always present. For instance, a properly functioning power equipment engine stops without warning, the engine fails, and the crankshaft locks up and will no longer turn.

The locked crankshaft is considered a constant failure.

Intermittent Failures

An intermittent failure is one that isn't always present. This type of failure increases the difficulty of the troubleshooting process.

The following example illustrates an intermit tent failure. A lawn mower's engine functions properly, with the exception of occasionally shutting off when mowing uphill. The user restarts the engine and it runs trouble free until another hill is encountered. This intermittent problem could be caused by multiple factors.

Therefore, a systematic approach to trouble shooting the problem is required. A systematic approach allows the problem (performance, electrical, mechanical, or fuel) to be diagnosed in a reasonable amount of time with a high degree of accuracy.

Improper-Service Failures

An improper-service failure, as the name implies, is caused by a mistake during the servicing of the machine. Suppose a customer brings a power equipment engine to your service department for a general service and the technician servicing the machine fails to properly tighten the engine drain bolt. After the customer gets the engine back and uses it, this mistake causes the drain bolt to come out while the engine is running. The problem will likely cause major engine failure.

Fortunately, most failures caused by improper service aren't this dramatic. It's important not to overlook problems resulting from incorrect ser vice when you're troubleshooting an engine.

The Troubleshooting Process

The proper method of diagnostic trouble shooting consists of four steps that must be followed in the proper sequence. Following these steps will ensure a foolproof approach to the troubleshooting and repair process.

1. Verify the problem

2. Isolate the problem

3. Repair the problem

4. Verify the repair

When troubleshooting, you must observe the failure and verify that all the information you've received is accurate and guides you to the problem area. After you've verified the condition, you're ready to isolate the problem.

Isolating a problem begins with the easiest and most obvious solution to the problem. If simple solutions fail to correct the problem, more involved and difficult checks are needed in a step-by-step manner. The most common diagnostic mistake is to overlook the obvious or easiest possible cause of a failure. Let's look at the following example. A power equipment engine was functioning properly, then stalled, and wouldn't restart. The owner took the power equipment engine to a service shop.

The technician removed and checked the spark plugs, checked the air filter, and performed compression and leak-down tests. When all was said and done, the problem was an empty fuel tank! Believe it or not, this situation isn't uncommon and results from poor troubleshooting skills (not starting with the simplest solutions first).

The symptoms of a problem guide you to the specific system you should troubleshoot, provided you have an understanding of how each system works and what it's responsible for. For example:

If the battery won't turn the engine over and just clicks when you turn the key, you can assume that the machine has a discharged battery and possibly a charging system that's failing to provide a proper charge to the battery.

If gasoline is leaking from the carburetor over-flow tube, you can assume that there's an internal carburetor problem that's causing excessive amounts of gasoline to enter the system.

As the severity of problems increases, the knowledge required to isolate problems increases.

An example of this is poor engine performance. A performance problem could be caused by an ignition system failure, a mechanical engine problem, or even a fuel-related problem. It's imperative to use all available resources and any information you can gather from your customer to assist you in identifying which system is responsible for the problem.

After you've isolated the problem, you can then repair the problem. In order to repair the problem, you must refer to the specific service manual for the particular engine you're servicing.

When you complete the repair, it's important to verify the repair. If you can't verify if that the repair was successful and the problem is still present, you must repeat the troubleshooting process, beginning with the verification stage.

Troubleshooting Guides

Virtually all manufacturers' service manuals contain troubleshooting checklists and/or tables of possible operating troubles along with their probable causes. These items are designed to aid the technician in troubleshooting and problem solving. All possibilities should be carefully checked because multiple factors may be causing the overall problem. Throughout the tables pro vided in this section, examples of typical problems and possible solutions are provided. These tables have been derived from current service manuals and technical guides to create a generic point of view. You should note that the tables provided in this section as well as in manufacturers' service manuals are intended only as a guide to diagnosing problems. Always read the detailed information in the specific sections of the appropriate service manual before performing service work on any system or major component. Remember to adhere to all cautions and warnings.

As you learn more about various power equipment engine systems, you may develop a tendency to troubleshoot problems based on your personal experience. This approach can be a gamble that may save you time; but if you guess wrong, it could cost you time and money. Don't be afraid to apply your experience to a good troubleshooting routine, but don't underestimate repairs only because the failure looks familiar.

Locating and fixing a problem is very self rewarding, provided you use good trouble shooting techniques. Furthermore, the more difficult the problem, the greater the satisfaction when you've solved it. To be successful, the most important barrier to overcome is the lack of self-confidence required to perform the job. Here are some points to keep in mind when you're troubleshooting a problem:

Always think the problem through

Never overlook the obvious

Never assume anything

Never take shortcuts

Never make more than one change or adjustment at a time

Always use the appropriate service manual(s) for all removals, replacements, and adjustments

Remember to always verify the problem, isolate the problem, repair the problem, and most important, verify the repair


There are various problems that require troubleshooting. These cover a wide spectrum of problems, from an engine that does not start to engine overheating. You must have a thorough knowledge of all components of the engine before you can begin troubleshooting. You must know what parts are used, understand how they work, and be aware of their relation to one another.

Refer to the following tables as a guide to the conditions listed.

--------Engine Does Not Start/Starting Difficulty

--------Runs Poorly at Low Engine Speeds

--------Runs Poorly or No Power at High Engine Speeds

--------Runs Poorly or No Power at High Engine Speeds (continued)

--------Engine Overheating

---------Does Not Reach Operating Temperature (Liquid-Cooled Engines)

-----------Excessive Exhaust Smoke

-----------Transmission and Clutch Problems

Drivetrain Troubleshooting

Although not discussed in detail in this guide, a transmission is employed by many power equipment engines to operate the machines they power. This section guides you in determining a problem with the transmission of an engine.

The two most common symptoms found when diagnosing power equipment engine trans mission problems are as follows:

The transmission is hard to shift. The transmission jumps out of gear. There are other drivetrain-related problems as well. The following table provides the most common causes of drivetrain-related problems.


Fuel system troubleshooting is one of the most common power equipment engine repair jobs. It can be a simple, straightforward, rewarding procedure or a tedious, complicated, unrewarding chore. The difference between these two extremes lies with your approach to problem solving. You can randomly disassemble and replace components, or you can take a systematic, step-by-step approach.

When troubleshooting fuel-system-related problems, start with simple tasks, such as verifying if there is fresh fuel in the gas tank and fuel flow to the carburetors and injectors.

A plugged gas-tank vent, a fuel shut-off valve, or a pinched fuel line can be responsible for restricting the fuel supply to the fuel sys tem. Trace through the system in search for blockage. Begin with the vent, then the shut off valve, and finally the gas line. If any of the components is plugged or restricted, it must be repaired.

Does It Run Rich or Lean?

Usually, fuel-system problems arise because of improper air-fuel mixtures: either too rich or too lean. Observe the engine exhaust and check the condition of the spark plug to determine if the mixture is too rich or too lean. Always keep in mind that a rich or lean mixture can have more than one cause. Too much fuel or not enough air can cause a rich mixture. Too much air or not enough fuel can cause a lean mixture.

Either condition can become bad enough that the engine will not start. The following table shows some common carburetor-related symptoms and some likely causes.

When Is the Problem Apparent?

After determining whether the mixture is too rich or too lean, you must determine in which throttle position the problem occurs to know which circuit needs repair. This means the physical position of the throttle such as at idle or at full throttle.

Before starting to work on the fuel system, you should always check out some of the external components that can affect carburetion. If the mixture appears rich, check the air filter and the cable to the carburetor choke. If the air cleaner is excessively dirty, air will have difficulty getting to the engine. If the choke cable is too tight, the choke will be cutting off the air supply to the carburetor. If the mixture is too lean, ensure that the fuel is flowing properly from the fuel tank. Also, inspect the intake manifold for air leaks.

If everything on the external side of the engine is in proper working order, the carburetor will most likely need repair. The following tables are divided into throttle ranges and pro vide common causes of rich and lean mixtures.

Also included are common repairs for each situation, as well as suggestions to follow if none of the common problems is present.

----------Carburetor-Related Problems

----------Carburetor-Related Problems (continued)

----------Surging at Idle

----------Full Throttle Operation

Other Fuel-System-Related Problems

Aside from the carburetor problems previously discussed, other common problems also appear from time to time. The most common of these is water in the fuel system. Water is heavier than gasoline and penetrates the circuits of the carburetor. When this occurs, the engine runs rough or not at all. Draining the float bowl of the carburetor may correct the problem temporarily, but ultimately, the underlying cause needs to be determined and the problem fixed.

Water in the Fuel System

Often, cleaning a power equipment engine with a high-pressure washer causes water to penetrate the sealing area between the air filter and carburetor or the seal of the fuel cap.

If water penetrates either area, it will eventually enter the fuel system and cause the problems just mentioned.

Clogged Fuel-Tank Vent

Another widespread and potentially baffling problem is a clogged fuel-tank vent. Normally, air enters the tank through the vent and replaces the space left by the fuel when it's burned. If the vent is clogged, a vacuum is formed in the fuel tank, which can restrict the flow of fuel. This restriction can sometimes be enough to cause the engine to stall and baffle a user. With the vacuum inside the fuel tank, assume that the user opens the fuel cap to just check if there's gas in the tank. Opening the fuel tank removes the vacuum (the user has unwittingly removed the problem, although only temporarily); the user sees fuel and starts the engine. The machine functions properly for a few minutes until the vacuum is formed again, and the fuel flow is slowed or stopped.

When confronted with a baffling fuel-system problem, exercising common sense is the only way to rectify the problem. Follow these basic tips when you've reached an impasse:

Remove yourself from the situation and consider the total process of carburetion.

Thoroughly think out all options before doing anything drastic.

Most important, implement one change at a time; multiple adjustments made simultaneously many times complicate the problem.

Electronic Fuel-Injection System Troubleshooting

As mentioned previously, fuel-injection power equipment engines are becoming more prevalent nowadays in larger engines. Fortunately, there are relatively few problems with fuel-injection power equipment engines, and when a problem does arise, in most cases, the fuel-injection light (FI light), also known in the automotive industry as the malfunction indicator lamp (MIL), lights up to let the user know that there is a problem.

When properly activated, the FI light displays a code as to what is causing the problem. Along with the FI light, most power equipment engine manufacturers now provide electronic diagnostic tools to assist with the troubleshooting of fuel-injection power equipment engines. This is due to the high level of technology involved.

However, these tools are not regulated or customized as to what information is required to be provided to the technician, as they're in the automotive industry. Therefore, it's necessary to understand each specific manufacturer's special tool. Consequently, the information provided here has been made as generic as possible.

Symptoms on fuel-injection engines are different from those found on carbureted engines.

The following example illustrates this point. A carbureted engine with low compression tends to run lean. This means that it takes longer to warm up and may not perform as well as a similar machine that has proper compression. Why is this? An engine with lower compression will have less intake port vacuum for the carburetor(s), and less fuel will be drawn into the engine. This makes for a lean running condition. But, fuel-injection engines don't act the same when the compression is low because fuel is injected into the port regardless of intake vacuum. Fuel-injection engines will generally run rich when they have low compression, just the opposite of a carbureted engine. Remember that recommended compression pressures vary between engines; so always check the appropriate service manual specification for the engine you're testing.

The real issue here is that even though the power equipment engine will seem to have a fuel-related problem, the fact of the matter is that the problem may be mechanical, and no FI light indicator will tell you that! The real test for the technician with fuel injection engines is determining the problem when the engine runs poorly and appears to have a fuel-related problem but there is no indication from the FI light. The following table shows some fuel-injection engine problem symptoms and some known problems that have been found with these symptoms that will not trigger an FI light.

---------Symptoms in Fuel-Injection Engines

--------Symptoms in Fuel-Injection Engines (continued)


Of all problems that come into a power equipment engine service department, electrical system problems are usually considered the most difficult to troubleshoot and repair. One of the reasons for this is that many technicians don't fully understand electrical systems, and they can't actually see the electrical system working.

They only know the symptoms. For instance, if a charging system stops functioning, you can't see that electricity isn't being produced. All you know is that the battery is dead. But, if a tire goes flat, you can see the result of the problem as well as the nail that caused it! After you've mastered the ability to properly and quickly analyze electrical problems, you'll become a valuable asset to any service department. With a complete understanding of how the electrical systems in power equipment engines work, you'll rarely take more than an hour or so to diagnose any electrical problem.

To help you categorize electrical-system problems, this section has been divided into four basic areas:

  1. Charging-system troubleshooting
  2. Ignition system troubleshooting
  3. DC circuit troubleshooting
  4. Electric starter-motor troubleshooting

Charging-System Troubleshooting

The symptoms found in a charging system that's not operating properly are simple and straightforward: The engine's charging system is either undercharging or overcharging! In the case of a system that's undercharging, the battery will eventually go dead, and the electrical components will no longer function. Charging systems in most of today's power equipment engines are designed to provide more than adequate electrical output whenever the engine is running. If a battery constantly discharges even though it's been properly maintained and the vehicle has been used frequently, check the charging system before replacing the battery. Batteries can be quite expensive.

In the case of an electrical charging sys tem that's overcharging the battery, there will undoubtedly be a faulty component in the charging system-most likely the voltage regulator, which in most cases is integrated with the rectifier as a single unit, as explained in Section 14.

In a way, troubleshooting electrical problems isn't difficult. As a matter of fact, it's one of the cleanest jobs you'll be required to do! In most cases, the causes of the problems are as simple as a dirty or loose connection. One manufacturer has let it be known that out of every 150 charging-system components that are returned for warranty purposes, only one is actually defective! This tells us that as the technician is diagnosing the problem in the charging system, he or she is fixing the problem without even knowing it! Over 95% of all charging-system related problems are connection related, and not actual component problems.

Be sure you know the color codes used for wires before beginning to work on an electrical problem. Manufacturers use different color wires for their electrical circuitries. As you per form each step in the troubleshooting process, check to see if you've corrected the problem.

Use the following tables to supplement the basic charging-system troubleshooting procedures we've discussed. The troubleshooting procedures in the tables can be used for any charging system.

----------Symptom in Discharging or Weak-Charging System

------------Symptom in an Overcharging Charging System

Ignition-System Troubleshooting

The most common issue found with ignition systems is a no-spark condition. To check for spark, the spark plug can be removed or a known good spark plug can be placed on the pug wire and ground to the engine. Once you've determined that an engine's ignition system isn't producing a spark, the next step in the troubleshooting procedure depends on the type of ignition system. If the ignition system uses a breaker-points assembly, the points and con denser are the most likely cause of the problem.

To check the points, remove all necessary covers and components. Check the contacts for pitting; check for dirt or moisture between the contacts.

In an electronic ignition system, the problem of no spark may be caused by several components. Fortunately, all these components are easy to check. First, check the spark plug to see if it's fouled. Then check to make sure that the engine stop-switch wire is properly connected and functioning correctly. This switch may be a switch that goes to ground or one that completes the ignition circuit. Check the service manual wiring schematic to be sure. If that is OK, then check for proper connections at all ignition-related components. If these connections appear to be OK, check for proper resistance and AC voltage at the pulse generator and exciter coil (in the case of capacitor discharge ignitions). If all these components are in proper working order, the problem is probably a failure in the ignition control module (ICM). Replace the ICM with a known good component and test the engine. If the engine operates properly, you can assume that the ICM was the problem.

In most power equipment engines, it's easy to remove and replace ICMs. But this component is usually quite expensive; so it's important to check all other components before replacing an ICM. Typically, ICMs are reliable, and the problem is likely to be found in another area of the ignition system.

In a battery-type ignition system, a weak battery can cause ignition failure. Check the battery using a voltmeter to see if the proper voltage (should be at least about 12 V) is present. Remember that the ignition switch or safety interlock switches can also be the cause of spark failure.

The following tables offer steps to follow with an AC-powered electronic ignition system and a battery-powered electronic ignition system.

---------CDI Ignition Troubleshooting: No-Spark Condition

---------CDI Ignition Troubleshooting: No-Spark Condition (continued)

----------Battery-Powered Electronic-Ignition-System Troubleshooting: No-Spark Condition

DC Circuit Troubleshooting

The battery in a power equipment engine provides electrical energy to operate the ignition and many other electrical components. We'll focus on two components that you'll frequently encounter: lights and switches.


Burned-out lightbulbs are replaced and not repaired. To check a bulb that has been removed from a circuit, you can use a battery and two wires.

Connect one wire to the negative side of the battery and to the ground on the lightbulb. Connect the other wire to the positive side of the battery and to the insulated side of the lightbulb. If the bulb lights up, it's good. An ohmmeter can also be used to check lightbulbs that have been removed from the circuit. Connecting one lead wire to the ground of the lightbulb and the other to the insulated side of the bulb causes the ohmmeter to show continuity, that is, a complete circuit.

Lightbulbs can go bad due to excessive vibration. Excessive vibration can cause the filament inside the lightbulb to break. When this occurs, the bulb must be replaced.

Another problem that you may encounter results from a loose connection in the light bulb socket or circuit. This condition can cause the bulb to get brighter and dimmer, flicker, or not light at all. This problem is corrected by locating the problem and repairing the faulty connection.

Lightbulbs of different wattages and volt ages are of the same size; so always be sure that the replacement bulb is of the same voltage and wattage as the one removed. Check the service manual if you aren't certain about what size bulb should be installed.


Switches are designed to open and close a circuit. You can check a switch using an ohm meter. An ohmmeter indicates continuity when the switch is in the On position and does not indicate continuity when the switch is in the Off position. If a switch is defective, it must be replaced.

Electric Starter-Motor Troubleshooting

There are four main troubleshooting problems that occur with power equipment engine electric starter systems:

The starter motor turns slowly. The starter solenoid makes a clicking sound, but the engine doesn't turn over.

The starter motor turns without turning over the crankshaft.

The starter motor doesn't turn at all. Refer to the following table to troubleshoot these starter-motor problems.

---------Starter-Motor Problems

--------CDI Ignition Troubleshooting: No-Spark Condition (continued)


Abnormal noise complaints can be for virtually any part of a power equipment engine. The following table indicates classic symptoms and possible problem areas for abnormal noise.

---------------Abnormal Engine Noise

---------------Abnormal Engine Noise (continued)


This section has covered a wide variety of troubleshooting symptoms as well as possible solutions. The information has been derived from various power equipment engine manufacturer service manual suggestions and compiled into one general area.

Use of the tables in this section will help you determine the problem quickly and ensure that you've repaired the machine correctly the first time itself. The following list summarizes the section.

Proper diagnosis of a malfunction makes disassembly, repair, and reassembly much easier.

An improperly diagnosed problem renders the repair process long and tedious.

Developing a systematic approach to solving problems will help you perfect your skills as you gain more experience.

There are three types of failures that you may encounter: constant failures, intermit tent failure, and improper-service failures.

Always verify the problem, isolate the problem, repair the problem, and most importantly, verify the repair.


1. Name the tool that's used to test a lightbulb after it has been removed from its socket.

Voltmeter a.

Coil tester b.

Ammeter c.

Ohmmeter d.

2. If the air-fuel mixture to a power equipment engine is too rich, the problem could be caused by empty gas tank.

b. an over full gas tank.

c. an air leak between the carburetor and the intake manifold.

d. leaving the choke on after the engine has warmed up.

3. What are the two most common symptoms found when troubleshooting a transmission problem? ___ and _______

4. If you turn on the choke and the engine runs better, this generally indicates a carburetor mixture problem.

5. A possible cause of a transmission that jumps out of gear is

shifting at too low an engine speed.

a. damaged shift fork. b. excessive play in the clutch pedal. c. incorrect oil level. d.

6. An excessive amount of ____ exhaust smoke indicates worn piston rings.

black a.

brown b.

red c.

white d.

7. A four-stroke power equipment engine with insufficient valve clearance will have crankshaft seizure.

a. incorrect ignition timing. b. low compression. c. extra power at higher speeds. d.

8. What are the four procedures that must be followed in proper order when developing the proper method of diagnostic troubleshooting?





9. What are the two main problems associated with charging systems? and

10. If a charging system is overcharging, the ____ is most likely the faulty component.

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