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When a tree falls on a saw, there is rarely any doubt what happened, but when the engine fails, it is sometimes difficult for pro users to understand what has occurred and why. The following images of damaged pistons illustrate what can happen inside a saw's engine. While the piston is not the saw's only internal engine part, it is often the part that "pays the price" when a saw is not operated or maintained correctly. We hope this information helps explain what can occur and why, and more over, provide knowledge of how to avoid common causes of failure in the first place.

Damage From Running Unmixed Fuel

The piston above has severe scouring on the exhaust skirt with the heaviest damage on the clutch side of the piston. All of this damage was caused from running straight fuel in a STIHL MS360. The lack of lubrication on the piston has caused it to seize to the cylinder wall. The damage you see was caused in the moments before the piston "stuck," which seized the engine.

This kind of piston damage can also be found on a saw that was run with the carburetor set too lean or one that was run with an air leak. If you didn't know this saw had been run with no oil in the fuel, how would you know it wasn't a heat seizure? To fully understand the cause of this failure, it is important to look at the rest of the piston. The photo below is of the same piston. It shows additional damage that's usually only found on a saw engine that had been run with unmixed fuel.

On this piston, notice the scouring in the wrist pin area. (red arrow) You can also see it is dry under the piston, in the cir clip area, under the rings, and in the transfer ports -- no oily residue. A heat seizure will show similar damage on the piston skirt (photo one), but the conditions under the piston will look normal. On this piston, the scouring and other dry conditions provide the evidence to suggest this seizure was caused by no lubricant in the fuel.

If you tear down a saw and find this kind of damage, don't forget to replace the fuel in the saw's fuel tank before you test run it after the repair. It is also important to check the contents of the fuel container that was last used to fill it. Since the repair required the replacement of both the barrel and piston this is a repair you don't want to do twice or on more than one saw.

Damage From Debris Getting Through the Air Filter

The damage on this piston skirt is caused by debris getting through the air filtering system. Notice the horizontal machine marks have been scrubbed off all across the bottom indicating extreme wear on the lower part of the skirt. Not shown, but the other side of the piston looked perfect. This damage was only found only on the intake side of the piston. This is typical for damage caused by intake debris. The other side of the piston is not exposed to an intake port, so it isn't affected at early stages.

What damages the intake skirt is debris from a leaking filter wedging between the piston and cylinder wall causing scuffing on the piston skirt. Since the piston is made of softer material, the damage is more pronounced on the skirt than on the cylinder bore's hard surface. This wear on the piston increases the clearance, which allows the piston to "rock" in the cylinder's bore. As the skirt becomes thinner and weaker, rocking increases. Eventually the piston will break. When it does, the engine seizes.

On a pro saw, the piston skirt performs another important function. Not only does it guide the piston, the skirt serves as the engine's intake valve. As the piston travels up and down the cylinder, its base opens and closes the intake port as it passes. For the engine to run its best, it is important for this valve to function well.

Intake Skirt Damage

Some intake skirt damage is not uncommon on a pro saw's piston after it has run hundreds of hours. No air filtering system is perfect, so you can expect to see the affects of debris damage even on saws whose filters have been well maintained. The important thing to learn is this damage can happen quickly when the filtering system is leaking debris.

Piston damage is not the only consequence of bad filtration. This debris can also collect in the bottom end of the engine. This leads to premature bearing and seal failure.

When a faulty air filter is replaced, this puts a temporary stop to further debris damage, but it does not reverse damage once it has occurred. This is why it is important to change air filters before they fail. Most pro saw manufacturers suggest changing the air filter after thirty days of use or before if a regular inspection reveals any leakage.

Damage From Bearing Failure

These fine scratches and "peppering" on the exhaust skirt and lower intake skirt is caused by the failure of the lower rod bearing or main bearings. Small, but hard pieces of the bearings and retention cages are breaking loose, causing this piston damage. If you are lucky enough to catch a piston in this condition, stop running the saw until you find which bearing is giving up material. If you keep running the saw, eventually the bearing(s) will completely fail. This usually releases larger pieces of bearing material.

When this occurs, sometimes the crank shaft locks up. But if it keeps running, loose pieces in the bottom end will travel up through the transfer ports and into the engine. All the parts won't make the complete trip. Some won't pass through the upper transfer port and when the piston goes by, it will drive these parts into the cylinder wall, destroying both. To repair this damage, both the crankshaft assembly and the cylinder and piston must be replaced -- two very expensive components.

We typically see this kind of damage on saw engines that have been over-revved.

Damage From Over-Speeding The Engine

The piston above has been damaged by over-speeding. Look at the piston material between the ring-lands. You can see a big chunk of it is missing and some has been "squished" thinner, creating a super-wide ring-land. Look at the top ring (bottom of photo). You can see the edge is rounded-over, a sure sign the rings were catching in the exhaust port. When this occurs, this sets off a high frequency vibration, eventually breaking the ring-land.

Damage From Detonation

The piston above has been damaged by detonation. Notice the damage on the top and the edges of the piston. The heat caused by detonation made the piston so hot, the rings stuck and the piston seized in the cylinder. You can see the seizure marks on the side of the piston. This damage usually ruins both the cylinder and piston. Detonation can be caused by a number of things. In this case, changing to higher octane supreme grade fuel was the answer.

Damage From Heat Seizure

The piston above shows the most common severe piston damage we see - the exhaust side has damage caused from excess heat. This damage looks similar to piston damage caused by running straight gas shown in the first image, but with this piston, conditions under the piston looked normal.

This kind of damage can be caused by over-revving the saw, running the carburetor adjustment too lean, by ignoring an air leak in the saw's engine, or a combination of factors. The best way to avoid a such a seizure is to use good quality fuel and mix oil, avoid over-revving the engine, and always stop running a saw that shows signs of a potential air leak. This kind of damage can also be caused by a partially plugged fuel filter, which is another reason fuel filters should be replaced regularly.

Damage From Heat Seizure Due to Blocked Cooling

The piston above also shows damage from excess heat. What makes this piston damage interesting is that this damage is on the side of the piston - near the transfer port, and not on the exhaust side where heat damage is found more frequently. In this case, the damage occurred on the back side of the cylinder, where less air passes over the cylinder's cooling fins.

On all saws, the cylinder causes a sort of circulation "shadow" reducing air volume to the cooling fins behind the bore.This is one reason it is important to keep the cylinder's cooling fins clean and free from debris. When wood chips build up around the cylinder, they can block air circulation that cools a saw's cylinder. Today's pro saw engines need all the cooling they can get, especially when a saw is making a big cut in a tree on a hot day.

Our advice

Hopefully these images help you understand more about your pro saw's engine, and in turn, help you avoid running conditions that can damage it. The manner in which a saw is operated and the maintenance it receives greatly affect its performance and working life. Now you know some things to look for and to avoid.

When operating a saw, don't over-rev it. Some saw users like the sound of a "screaming" saw, but it shortens its life and causes repairs. Proper fuel mix also plays a role in both the performance and working life of a saw. Fuel and the use of properly formulated mix oil is extremely important. The way the fuel is blended and its age are also factors. Maintaining the air filter is also important. Inspect it, clean it, and replace it often. Postponing air filter and fuel filter replacement is not a good way to save money.

  1. While disassembling your engine, make sure to 100% clean all areas as this will reduce the risk of dirt and debris from previous engine failure falling into the engine. Once you remove the cylinder, stuff a clean rag down into the crankcases.
  2. After identifying the piston and cylinder as the problem and following step 1 please make sure all other parts in the saw are 100% perfect, bearings, crankshaft, seals, ignition, connecting rod, carburetor, crankcases and the list goes on..... If not 100% or mistakenly thought good you may cause the new assembly to fail within hours.
  3. Never re-use old gaskets. They may cause leakage and or no pressure/vacuum.
  4. Always install the circlips with the opening facing straight up or down, that way inertia will hold it tight into the clip groove. Place one clip in the groove before installing the piston on the connecting rod. Its easier to install a clip with the piston in your hand rather than on the rod. There's also less chance that you'll drop the circlip in the crankcases.
  5. Always install the rings on the piston with the markings facing up. Coat the rings with pre-mix oil so they can slide in the groove when trying to install the piston in the cylinder.
  6. Always install the piston on the connecting rod with the arrow on the piston crown facing towards the exhaust port.
  7. The traditional way to assemble the top end is to install the piston assembly on the connecting rod, compress the rings, and slide the cylinder over the piston. A piston ring clamp set it perferable.
  8. Cross torque the cylinder head as required by manufacturer.
  9. Make sure your Decompressor and or Spark Plug isn't leaking.
  10. After doing a top end rebuild please dont neglect to replace filters and spark plug.
  11. Always use the right oil to petrol mix as per manufacturer and only use a quality 2 stroke oil and if installing a new Cylinder and Piston Assembly make sure the Carburetor is set correctly, not to rich and not to lean as the wrong setting even with the right 2-Stroke mix could cause engine failure within minutes/hours.
  12. Gauge engine speed with a tachometer(available from us Euro Small Engine Spares). Each brand/model and Carburetor has specific settings please refer to your manual.
  13. Inspect the choke plate at the opening of the carburetor's throat If your chainsaw has trouble starting. If it doesn't close properly or if it moves freely, this could create starting problems. Loosen any grit by spraying carburetor cleaner on the choke's shaft and into the venturi.
  14. Examine the spark plugs to repair running problems. A wet plug may be caused by water in the fuel, so you'll need to service the fuel tank. An excessively dry plug could mean a clogged fuel filter, which can be fixed by servicing the filter. Use a micrometer caliper to make sure the spark plug gap is .025 of an inch.
  15. Excess Choke or fuel issues: If the engine is flooded from excessive choke/fuel, sometimes if you give it a few minutes to dry off you can try the starting sequence this time without choke. If this fails you can also try starting fluid. Find the air intake and remove the air filter. Spray a one-second burst of starting fluid into the air intake. Reassemble the filter and try to start the engine.
  16. Avoid old Oil/Fuel Mix: The major reason for balky engines is old fuel. If your gas is more than a few months old, it’s likely the cause of your starting problems. Empty it out and refill your 2 stroke mix with fresh mix. (add a fuel stabilizer, optional). If you’ve been running old fuel through the engine, you may have to take the machine in to remove the varnish from the carburetor.
  17. Have the right tools: 1. Compression Tester to test cylinder/piston/rings etc. 2. Crankcase & Carburetor Pressure Tester to test seals gaskets and daiphragms etc. 3. Vacuum Pump to test crankcase gaskets and seals etc. 4. Ignition & Spark Plug Tester to check coil/module and plug. 5. Bearing Puller Set. 6. Carburetor Adjusting Tools, splinned, d-shaped and double d-shaped to adjust (H) and (L) screws. 7. Zama & Walbro Metering Tools to adjust carburetors. 8. Piston Stop and Flywheel Puller. 9. Piston Ring Clamp Set. 10. RPM Tachometer is used to set the exact rpm's on your machine, both idle and peak. These are the basics and aren't as expensive as you may think. You can start by buying 1 or 2 items at a time and all are available from us Euro Small Engine Spares.
  18. Carbon build up is in the top 5 killers of Cylinders & Pistons and should be cleaned regularly from the exhaust port .Carbon is caused by to rich a mix and or poor scavenging through exhaust port.
  19. A 2- Stroke Saw should not be over revved when not under load or cutting, this can cause over-heating and seizing.
  20. A dull chain causes over revving and stressing the engine out eventually causing failure.
  21. Air Filters must be cleaned and or replaced regularly and this is just as important as adding 2-Stroke oil to your Petrol.
  22. If the Rings on you Piston are stuck in the grooves then your machine has over-heated and you will have lost compression.
  23. Carbon/Detonation and or colouration should not occur below the first piston ring.
  24. If either the under belly of the Piston Crown and or Piston Pin have Blue/Dark Brown Colouration you have over-heated your machine.
  25. Always run in a new Cylinder/Piston/Rings Assembly before putting a load on the saw. I strongely recommend running 1/2 tank fulls of 35/1 mix through the assembly while still in the workshop. You'll need to increase the RPM(using the idle screw) and varying the rpm over the time period of the run in.

NB: Machines suffer Engine Failure 99.9% of the time not due to parts failure but due to the following:

  1. Improper use/over stressing.
  2. Wrong oil/fuel mix and or poor quality oil.
  3. Wrong carburetor settings, to rich/to lean.
  4. Dirty air filters or lack of servicing.
  5. Installing errors, this is most common, eg, piston arrow pointing the wrong way/Cir-clips installed with the opening in the wrong direction/Piston rings fitted upside down/Debris and fragments of metal and dirt not cleaned out from Crankcase/Crankshaft before installing new Cylinders/Pistons etc.
  6. When doing a repair/rebuild other components not found faulty(Bearings/Oil seals/Gaskets/Crankshaft/Conrod/Crankcase) before installing new parts, this almost certainly causes new parts to break straight away.
  7. Lending 2-Stroke Machines to people who are not familiar with the correct way to fuel or use them.
  8. Carbon build up.
  9. Over Revving
  10. Dull Chains and Blades.
  11. Bearings/Gaskets/Oil Seals not replaced with new ones when for example replacing a Cylinder & Piston Assembly which is most common

If your 2/4 Stroke Machine requires a repair/rebuild, for example a Cylinder & Piston/Carburetor/Ignition etc unless you are 100% confident you know what you are doing please bring it to a good mechanic.

Please see our Mechanics Directory Listing

Nobody likes to be told there chainsaw has seized up, the very nature of the word seized implies the machine is solid and will not turn over, the reality is the piston has lost the oil film between it and the cylinder wall, it now has metal to metal contact, material from the piston skirt now transfers to the cylinder wall, the cylinder fins are very effective and the piston cools very quickly, and the engine will now rotate,so technically it is not seized up, unfortunately the damage is done, the material from the piston has scuffed up over the piston ring, which is now stuck in, compression is lost , and the machine now refuses to start.


Stock answer from the dealer is usually a simple one line answer, no oil in the fuel, this in a broader sense is known more technically as "Lubrication Breakdown", most engine seizures come under the heading of a lubrication breakdown, the reasons for this are as follows.


  1. Insufficient two stroke oil in the fuel.
  2. Poor quality two stroke oil in the fuel.
  3. Incorrect grade of oil in the fuel.
  4. Incorrect carburetor settings, causing over speeding.
  5. Inadequate servicing.
  6. Poor chain maintenance, causing over loading.
  7. Fuel system blockages, causing lean mixture.
  8. Blocked cooling fins.
  9. Operator miss use.
  10. Engine air leaks, causing lean mixture.
  11. Stale fuel, water contaminated fuel.

As you can see there are many reasons for a lubrication failure, you must remember this small engine is revving at anything up to 14000 rpm probably three times the rpm of your motor car engine flat out,and in a very hostile environment, the machine is air cooled, and basically a stationary engine, so all the cooling air is supplied by the vanes on the flywheel via the starter cover grill, if you allow dirt and debris to collect in the starter cover vents,you have now effectively cut off the cooling air to the cylinder.

You can have perfect oil/fuel ratio and pin point accurate carburetor settings and still experience a lean mix resulting in scoring and low compression usually from seal/gasket leakage and or other reasons.

Poor chain sharpening can and will cause over loading, and in turn increased cylinder temperature.

Clogged fuel filters, dirt in the carb, dirty fuel, incorrect carb settings ,all increase engine revs, increased revs and lean fuel oil mix to the cylinder results in engine over heating and possible seizure.

Engine air leaks, either through leaking gaskets or crank seals will also increase engine speeds and possible cylinder failures. If you suspect an engine seizure, or your saw will unexplainable not start, remove the muffler, this is normally only held with a couple of screws, check the condition of the pistons exhaust skirt ( this is the hottest part of the engine, between the centre of the piston and the opposite side to the flywheel, as the flywheel side runs a little cooler ) any signs of piston scuffing,and the engine has seized, and will require further investigation.

Other possible reasons for cylinder seizures are main bearing failures, or foreign objects entering the saw through the inlet port. When removing clutches, or flywheel nuts, you should never enter anything into the cylinder to lock the piston, piston stops, bits of starter rope etc, these items can damage the piston, or get caught between the cylinder and the piston.

A final thought on two stroke oil, there are many different makes and colours of oils out there, what you need to ask yourself is, how many of these oil suppliers actually produce a chainsaw engine? how do they know the lubrication requirements of the engine? why would anybody spend there hard earned money on a good saw, and then buy the cheapest oil they can find, always buy the best two stroke oil you can, preferably from a chainsaw manufacturer, and to be mixed at 50-1 (remember the environment, if not that remember you are breathing an amount of the exhaust gas in)

And finally a piece of trivia, an engine running for two hours a day, five days a week, running a 9500rpm the crank has rotated 1.7 million times in the first day, the diaphragms have cycled 3.42 million times, an engine running at 13500 rpm the spark plug sparks 225 times a second. I hope you find this interesting and informative.

2 Stroke Theory

The main difference between the two stroke and four stroke engine is that the two stroke engine has only two cycles; as compared to the four stroke, where it has four. The two cycles in the two stroke engine are, intake/compression and power/exhaust stroke. Okay, during the intake/compression stroke, the fuel/air mixture is injected into the crankcase. When the piston moves down, it opens a port in which the fuel can enter the cylinder. As the piston moves up, it closes the port and compresses the fuel. The final cycle is power/exhaust cycle. When the piston reaches TDC, the spark plug ignites the fuel. When the piston is thrown down, it opens a different port, causing the exhaust fumes to be propelled out of the cylinder. At the same time but the other port, the fuel is injected once again. These two ports are located 180 degrees away; and because they are both open at the same time, some of the fuel does escape through the exhaust port. Although this happens, it is a minuet amount. Because the two stroke engine only has two cycles, it runs a lot faster than the four stroke engine.

A two-stroke in its purest form is extremely simple in construction and operation, as it only has three primary moving parts (the piston, connecting rod, and crankshaft). However, the two-stroke cycle can be difficult for some to visualize at first because certain phases of the cycle occur simultaneously, causing it to be hard to tell when one part of the cycle ends and another begins. Several different varieties of two-strokes have been developed over the years, and each type has its own set of advantages and disadvantages. The subject of the animated GIF (and this dissertation) is known as a case-reed type because induction is controlled by a reed valve mounted in the side of the crankcase. The easiest way to visualize two-stroke operation is to follow the flow of gases through the engine starting at the air inlet. In this case, the cycle would begin at approximately mid-stroke when the piston is rising, and has covered the transfer port openings:

As the piston moves upward, a vacuum is created beneath the piston in the enclosed volume of the crankcase. Air flows through the reed valve and carburetor to fill the vacuum created in the crankcase. For the purposes of discussion, the intake phase is completed when the piston reaches the top of the stroke (in reality, mixture continues to flow into the crankcase even when the piston is on its way back down due to the inertia of the fuel mixture, especially at high RPM):

During the down stroke, the falling piston creates a positive pressure in the crankcase which causes the reed valve to close. The mixture in the crankcase is compressed until the piston uncovers the transfer port openings, at which point the mixture flows up into the cylinder. The engine depicted here is known as a loop-scavenged two-stroke because the incoming mixture describes a circular path as shown in the picture below. What is not readily apparent in the picture is that the primary portion of the mixture is directed toward the cylinder wall opposite the exhaust port (this reduces the amount of mixture that escapes out the open exhaust port, also known as short-circuiting):

Mixture transfer continues until the piston once again rises high enough to shut off the transfer ports (which is where we started this discussion). Let's fast-forward about 25 degrees of crank rotation to the point where the exhaust port is covered by the piston. The trapped mixture is now compressed by the upward moving piston (at the same time that a new charge is being drawn into the crankcase down below):

Somewhat before the piston reaches the top of the stroke (approximately 30 degrees of crank rotation before top-dead-center), the sparkplug ignites the mixture. This event is timed such that the burning mixture reaches peak pressure slightly after top dead center. The expanding mixture drives the piston downward until it begins to uncover the exhaust port. The majority of the pressure in the cylinder is released within a few degrees of crank rotation after the port begins to open:

Residual exhaust gases are pushed out the exhaust port by the new mixture entering the cylinder from the transfer ports.

hat completes the chain of events for the basic two-stroke cycle. The discussion is not complete. The animated demonstration has an added device commonly known as an expansion chamber attached to the exhaust port. The expansion chamber (an improperly named device) utilizes sonic energy contained in the initial sharp pulse of exhaust gas exiting the cylinder to supercharge the cylinder with fresh mixture. This device is also known as a tuned exhaust.

Picking up the discussion at the point shown by the exhaust blowdown picture above, an extremely high energy pulse of exhaust gas enters the header pipe when the piston begins to open the exhaust port:

The sonic compression wave resulting from this abrupt release of cylinder pressure travels down the exhaust pipe until it reaches the beginning of the divergent cone, or diffuser, of the expansion chamber. From the perspective of the sound waves reaching this junction, the diffuser appears almost like an open-ended tube in that part of the energy of the pulse is reflected back up the pipe, except with an inverted sign (a rarefaction, or vacuum pulse is returned). The angle of the walls of the cone determine the magnitude of the returned negative pressure, and the length of the cone defines the duration of the returning waves:

The negative pressure assists the mixture coming up through the transfer ports, and actually draws some of the mixture out into the exhaust header. Meanwhile, the original pressure pulse is still making its way down the expansion chamber, although a considerable portion of its energy was given up in creating the negative pressure waves. The convergent section of the chamber appears like a closed-end tube to the pressure pulse, and as such causes another series of waves to be reflected back up the pipe, except these waves are the same sign as the original (a compression, or pressure wave is returned). Notice that this cone has a sharper angle than the diffuser, so that a larger proportion of energy is extracted from the already weak pressure pulse:

This pulse is timed to reach the exhaust port after the transfer ports close, but before the exhaust port closes. The returning compression wave pushes the mixture drawn into the header by the negative pressure wave back into the cylinder, thus supercharging (a bigger charge than normal) the engine. The straight section of pipe between the two cones exists to ensure that the positive waves reaches the exhaust port at the correct time:

Since this device uses sonic energy to achieve supercharging, it is regulated by the speed of sound in the hot exhaust gas, the dimensions of the different sections of the exhaust system, and the port durations of the engine. Because of this, it is only effective for a very narrow RPM range. This explains why two-stroke motorcycles equipped with expansion chambers have such vicious powerbands (especially in the old days before variable exhaust port timing existed). With the design illustrated here (i.e. a single divergent stage and a single convergent stage), the powerband of the engine will be akin to a 'light switch' - once the expansion chamber goes into resonance, there will be a HUGE, almost instantaneous increase in power.

The powerband can be softened somewhat by reducing the angles on the cones, but this is simply due to a lower degree of supercharging. In order to get the best of both worlds (a large power increase and a wide powerband), the cones should consist of several sections, with a different angle for each section. Proper design of even a simple expansion chamber is somewhat of a black art, even though formulae exist that will get you in the ballpark (there is quite a bit more to this than simply choosing the appropriate angles and lengths based on sonic velocity - everything about the pipe comes into play, including the headpipe diameter and length, and the tailpipe ('stinger') diameter and length). Design of a multi-stage expansion chamber becomes incredibly difficult - it basically comes down to the old 'cut and try' approach in the end. This of course is not even considering whether or not the exhaust and transfer port timings and outlet areas have been optimized for expansion chamber use.

This drawing reviews the typical components of a fuel system on a two-stroke engine.

Small gas engines serve us in many ways. They power lawn mowers, tillers, cultivators, trimmers, edgers, snowblowers, chain saws, pumps, generators, air compressors, and other useful home tools. They also power our fun: outboard boats, snowmobiles, motorcycles, all-terrain vehicles, ultralight aircraft, and other toys. To keep them operating efficiently, an owner of these tools and toys should know about small engines: how they work and what to do when they don't.

Small gas engines are made up of individual systems that work together to produce power. Each system has many components. Internal combustion gasoline-powered engines require six systems: fuel, exhaust, ignition, combustion, cooling, and lubrication. In this article, we will discuss the systems and components that make small engines work.

The fuel and exhaust systems are critical to operation. They furnish the fuel for combustion and remove exhaust gases. The following are components of a fuel and exhaust system.

  1. Gasoline: Gasoline is a combustible liquid that burns relatively slowly. However, when sprayed as a mist and mixed with air, it is quite explosive. All it needs is a spark. Two-stroke engines require that oil be mixed with the gasoline to lubricate internal parts. Four-stroke engines use a fuel-air mixture.
  2. Fuel Tank: The fuel tank stores fuel in preparation for mixing by the carburetor and use by the engine. Some fuel tanks are pressurized with air to help deliver fuel to the carburetor. Other tanks are non-pressurized and depend on a fuel pump to deliver fuel to the carburetor.
  3. Fuel Line: Fuel is moved from the tank to the pump and/or carburetor through a fuel line. Pressurized fuel systems often have a squeeze bulb in the fuel line for building pressure.
  4. Filter: A carburetor jet has a small opening that can easily become clogged. A fuel filter traps dirt and sediment from the gas before it is delivered to the carburetor.
  5. Pump: A fuel pump produces a vacuum that pulls the fuel from an unpressurized tank, then delivers it to the carburetor.
  6. Carburetor: The carburetor has one job: to mix the correct proportion of gasoline and air for the engine. Too much gasoline in the mixture makes it rich; too little gas makes it lean.
  7. Throttle: The throttle controls the amount of fuel-air mixture that enters the engine from the carburetor. The throttle thus controls the speed of the engine.
  8. Primer: A primer injects a small amount of gasoline into the carburetor throat to make the initial fuel-air mixture rich. A primer is used to help start a cold engine.
  9. Choke: Some engines control the richness of the fuel-air mixture at startup by controlling the air rather than the fuel. A choke reduces the amount of air in the fuel-air mixture.
  10. Governor: A governor is a device that automatically opens the engine's throttle when more power is needed and closes it when the load is light.
  11. Muffler: Small gas engines, especially two-stroke engines, are noisy when they operate. A muffler reduces the sound of the exhaust gases by passing them through baffles.
  12. Spark Arrestor: A spark can exit the exhaust port of a small gas engine, potentially starting a fire on nearby combustibles. A spark arrestor on the exhaust port can reduce the chances of such a fire. Spark arrestors are especially important on chain saws, motorcycles, and all-terrain vehicles operated in dry woodlands.

The ignition is a primary system within all small gas engines. It produces and delivers the high-voltage spark that ignites the fuel-air mixture to cause combustion. No spark means no combustion, which means your engine doesn't run. Below are the components found in small engine ignition systems. Some systems will include breaker point ignitions while others depend on solid-state ignitions. Magneto-Powered Ignition System: A magneto uses magnetism to supply electricity in ignitions where there is no battery. The magneto is turned by the crankshaft, which rotates when the manual recoil starter is pulled. The three types of magneto ignition systems are mechanical-breaker, capacitor-discharge, and transistor-controlled.

  1. Battery-Powered Ignition System: If your small engine includes a battery for starting, the ignition coil will also use it to supply spark to the spark plugs. A battery stores electrical energy until needed. Battery ignition systems also use mechanical-breaker, capacitor-discharge, and transistor-controlled ignitions.
  2. Mechanical-Breaker Ignitions: High-voltage electricity must be sent to the spark plug at the appropriate time. In mechanical-breaker ignitions, this job is performed through the contact points and a condenser.
  3. Points: As the crankshaft rotates, a cam opens and closes a set of contact points. These points function as an on/off switch: Closed is on, and open is off.
  4. Condenser: Because the spark moving across points can damage their surfaces, the condenser stores voltage to reduce arcing between points.
  5. Capacitor-Discharge Ignitions (CDI): A capacitor is a large condenser. A CDI stores and delivers voltage to the coil using magnets, diodes, and a capacitor
  6. Transistor-Controlled Ignitions (TCI): Transistors are electronic controllers. A TCI uses transistors, resistors, and diodes to control the timing of the spark.
  7. Coil: An ignition coil is simply two coils of wire wrapped around an iron core. The coil changes low voltage (6 or 12 volts) into the high voltage (15,000 to 30,000 volts) needed by the spark plug.
  8. Spark Plug: A spark plug is an insulated electrode that is screwed into the top of the engine cylinder. High-voltage timed electricity from the magneto travels by wire to the spark plug. The base of the plug has an air gap of about 0.030 inch (30 thousandths of an inch), which the current must jump.
  9. Wires: The primary wire from the coil to the breaker point and secondary wire from the coil to the spark plug(s) deliver electricity to the ignition components.
  10. Distributor: A distributor is an ignition system for engines with more than one cylinder and spark plug. It distributes the spark to the appropriate cylinder using a rotor, cap, and individual spark plug wires.

The combustion system of a small gas engine is where the work gets done. Components of the combustion system include the cylinder block, cylinder head, camshaft, valves, piston, connecting rod, crankshaft, timing gears, and flywheel. To better understand small gas engines, let's look at how this vital system works.

  1. Cylinder Block: The largest single part in a small gas engine is the cylinder block. It is a piece of metal in which the cylinder hole is bored or placed.
  2. Cylinder Head: The cylinder head is the top, or ceiling, of the cylinder and is attached to the block with bolts. Depending on the type of engine, the head may or may not include valves.
  3. Piston: A piston is the movable floor in the combustion chamber. Its upward movement compresses the fuel-air mixture. After combustion, its downward movement rotates the crankshaft.
  4. Crankshaft: An engine's crankshaft is a metal shaft with an offset section onto which the connecting rod is attached. Rotation of the crankshaft moves the piston up in the cylinder. Movement of the piston down in the cylinder then rotates the crankshaft.
  5. Connecting Rod: Between the piston and the crankshaft is a connecting rod. At the larger end of the connecting rod is a bearing that allows rotation around the moving crankshaft. The small end is attached to the piston pin.
  6. Valves: Valves simply open and close passages. A reed valve in a two-stroke engine is activated by changes in air pressure.
  7. Flywheel: At the end of the crankshaft is a circular weighted wheel called a flywheel. The flywheel delivers the engine's power to devices (wheels, blades, etc.) and helps keep the crankshaft turning smoothly.

Small Engine Gallery

Cooling and Lubrication

Combustion and friction produce heat. Heat and friction -- if not controlled -- can quickly damage an engine's components. Small gas engines are typically cooled by air. Friction is reduced using movable bearings and lubricants.

  1. Air-Cooling Fins: For simplicity, most smaller gas engines are cooled by air. Metal fins around the outside of the combustion chamber help dissipate the internal heat.
  2. Friction: Friction is resistance that occurs when one surface rubs against another. Friction causes wear. In an engine with many moving parts, friction is reduced with bearings and lubricants.
  3. Bearings: A bearing is a replaceable part that takes the brunt of the friction. A friction bearing relies on lubricants to minimize friction. A nonfriction bearing uses hard steel rollers or balls to prevent wear, though it too requires some lubrication.
  4. Lubricants: Lubricants such as oil and grease reduce surface friction by coating parts with a film. Lubricants in two-stroke engines are applied to surfaces by mixing oil with fuel.
  5. Viscosity: An oil's viscosity is its resistance to flow. The thicker a lubricating oil or grease is, the higher its viscosity number.
  6. Filters: Friction happens. Moving parts wear, even with the best lubricants. The resulting metal as well as carbon from the combustion process must be cleaned from the oil to ensure long lubrication. Some small engines use oil filters to remove contaminants from the circulating oil.

Regularly servicing your small engine will ultimately save you money and time. In the next section, we'll review how, where, and when to service this engine.

Purchasing a small engine-driven implement can make a dent in your budget. Tools and toys powered by small engines can cost anywhere from $100 to $10,000. That's why it's a good idea to invest in periodic servicing of your small engine. Replacing an engine every couple of years is an annoying and needless expense. Below we will review detailed information on how to service two-stroke gas engines. Following these procedures could help you put more money in the bank and less into your mechanic's pocket.

Servicing your small engine tool or toy on a regular basis offers many advantages over the Wait-Until-It-Breaks Maintenance Program. By establishing a service schedule, you will gain confidence that whenever you need the unit it will be ready for use. By performing a number of service functions together, you will save time. You can pick up all needed parts and lubricants in one trip to the parts store. Then you need to disassemble a component only once to perform numerous service procedures rather than taking it apart many times. Regular service gives you a chance to visually inspect the entire engine and related components for damage, wear, and other potential problems.

Knowing how to service is as important as knowing when. Some service procedures can be performed wherever you store your tool or toy: in a garage, storage shed, or tool shed. If the unit is heavy, you can build a ramp up to a sturdy table that is at a handy height for working. Or you can use a ratchet winch to lift the engine.


Units weighing less than 40 pounds may be lifted to a workbench or table as long as you lift with your legs rather than with your back. Get help if you need it, and make sure that the unit will remain sturdily in place as you service it. Remember to always put safety first!Servicing a small engine is easy once you know what to do and when to do it.


A service chart can help you determine common service requirements as well as track what service has been done. Your engine-powered unit may have a service chart in the owner's manual or service manual. Typical recommendations include changing engine oil every 25 hours of use and tuning up the engine at least once a year.


The purpose of ongoing service, also known as preventive maintenance, is to keep your engine-driven tool or toy in good operating condition. Ongoing service procedures include air cleaner service, crankcase breather service, cooling system service, muffler service, lubrication, and tune-up.Lubrication service means making sure that all moving parts have sufficient lubrication (oil and/or grease) to minimize wear.


Lubrication service procedures include mixing oil with fuel in two-stroke engines, and lubricating other moving parts.A tune-up consists of the adjustment and/or replacement of parts critical to smooth and efficient engine operation. Those parts include components in all engine systems: fuel, exhaust, ignition, combustion, cooling, and lubrication. Ignition tune-ups are more important for mechanical-breaker ignitions than they are for self-contained solid-state ignitions.


Regular tune-ups will keep your small engine running smoothly and reduce the need for repairs.In addition, you should check other systems and make adjustments as needed to keep them operating smoothly. This includes adjusting the throttle, choke, and governor linkage, and cleaning off debris.Engine-driven tools and toys usually come with an owner's manual. While some manufacturers' manuals are more complete and better written than others, most manuals include basic information on safe operation and service. Unfortunately, product manuals are often written to reduce the manufacturer's liability for accidental misuse rather than to help the owner service the product. In addition, manuals for engine-driven products typically show how to service the nonengine components: the grass catcher, wheel adjustments, blades, chains, and other parts. Service information for the engine may be minimal or nonexistent in the owner's manual.


What can you do about this lack of information? Fortunately, there are numerous after-market publishers of service manuals for specific models of small engines. If you don't have an owner's manual, you can contact the manufacturer directly to purchase one; manufacturers also sell service manuals. Most manufacturers keep product manuals for up to 20 years. If they only have one original copy left, you can often request a photocopy for a small charge.


Knowing how to service the fuel system is an important part of caring for a small engine. Learn how to care for fuel filters, carburetors, and other major fuel system parts in the next section.

This fuel tank has two filters: one at the opening and one at the entry to the fuel line.

The function of a small engine fuel system is to store and deliver fuel to the combustion chamber. Maintaining a fuel system includes servicing the fuel filter, air cleaner, fuel tank, and fuel lines; adjusting the carburetor; and adjusting the governor. Of course, not all small engines have all of these components.

Some small engines have a fuel strainer in the bottom of the fuel tank. Others have a removable fuel strainer in the fuel line. Still other small engines use disposable in-line fuel filters made of pleated paper. To clean sediment from a tank:

  1. Step 1 Drain or siphon all fuel from the tank.
  2. Step 2 With a flashlight, find the lowest point in the tank: the sediment reservoir. Clean all sediment from the reservoir indentation.
  3. Step 3 Wipe sediment from the end of the filter element.
  4. Step 4 Wipe the inside of the tank with a clean rag.
  5. Step 5 Refill the fuel tank.

Here is how to clean sediment from a fuel strainer:

  1. Step 1 Find and close the shutoff valve on the fuel line.
  2. Step 2 Loosen the lock nut on the bowl retainer and remove the sediment bowl.
  3. Step 3 Empty and clean the sediment bowl. Clean the filter screen. Refill the sediment bowl with fresh fuel.
  4. Step 4 Reinstall the sediment bowl and bowl retainer, tightening the lock nut.
  5. Step 5 Open the shutoff valve.

Here is how to replace an in-line fuel filter:

  1. Step 1 Find and close the shutoff valve on the fuel line or use a clothespin to pinch the fuel line closed.
  2. Step 2 Disconnect the fuel filter from the fuel line.
  3. Step 3 Replace the in-line fuel filter with an exact replacement part.
  4. Step 4 Open the shutoff valve.

The purpose of an air cleaner on a small engine is to keep large particles in the air from clogging the carburetor. The two types of air cleaners used on small engines are oil bath and dry. Hereis how to service an air cleaner:

  1. Step 1 Remove the cover of the air cleaner, typically by unscrewing a nut on top of the cleaner.
  2. Step 2 If it's an oil cleaner, remove all oil and contaminants from the center channel of the cleaner, wipe it clean, then replace oil to the indicated level. A dry cleaner cannot be cleaned and therefore must be replaced.
  3. Step 3 Replace the air cleaner cover and make sure all fasteners are securely tightened.

Fuel systems with pumps use nonpressurized fuel tanks. Outboard engines typically use pressurized tanks. Fuel lines are usually made of neoprene. Here's how to service a fuel tank and line:

  1. Step 1 Remove the cap from the fuel tank.
  2. Step 2 Using a flashlight, check for sediment in the fuel tank. If sediment is found, clean the tank and replace the fuel. Replace the fuel cap when done.
  3. Step 3 Check the fuel line and siphon bulb, if there is one, by squeezing them and inspecting for cracks. If damaged, replace with a line or a bulb of the same inside diameter. Make sure it is approved for use with fuel.

Fuel systems with pumps use nonpressurized fuel tanks. Outboard engines typically use pressurized tanks. Fuel lines are usually made of neoprene. Here's how to service a fuel tank and line:

  1. Step 1 Remove the cap from the fuel tank.
  2. Step 2 Using a flashlight, check for sediment in the fuel tank. If sediment is found, clean the tank and replace the fuel. Replace the fuel cap when done.
  3. Step 3 Check the fuel line and siphon bulb, if there is one, by squeezing them and inspecting for cracks. If damaged, replace with a line or a bulb of the same inside diameter. Make sure it is approved for use with fuel.

A governor is a device that controls the speed of the engine as the load changes. As the load slows the engine down, the governor opens the throttle to return the engine to a set speed. Governors are commonly used on engine-powered electrical generators where constant speed is important. Two types of governors are installed on small engines: mechanical and air-vane.

Caution: An incorrectly adjusted governor can cause the small engine to operate at excessively high speeds and damage or destroy it.

A mechanical governor responds to the centrifugal force created by the engine's revolution.

Unfortunately, there is no universal adjustment sequence for small engine governors. Much depends on the type of governor, whether the crankshaft is horizontal or vertical, the complexity and pivot points of the linkage, and the intended operating range. Because of these factors, refer to an owner's manual or service manual for your specific engine and application to adjust the governor.

In the next section, learn how the ignition system needs to be routinely serviced so it can deliver a high-voltage spark to help start a small engine.

Here are the parts of a typical flywheel magneto.

An ignition system in a small engine produces and delivers the high-voltage spark that ignites the fuel-air mixture to cause the combustion. Some small engines require a battery to supply electrical power and the ignition spark. Others develop the ignition spark using a magneto.

A small engine ignition includes the ignition controller (mechanical-breaker, capacitor-discharge, or transistor-controlled), spark plugs, flywheel, and wiring. Servicing the ignition system of your small engine depends on which types of components it has. Below are step-by-step instructions for servicing ignition systems found in modern small engines.

A magneto applies magnetism to supply electricity in ignitions where there is no battery. The magneto is turned by the crankshaft, which rotates when the manual recoil starter is pulled. Here's how to service a non-battery ignition system:

  1. Step 1 Service magneto.
  2. Step 2 Service ignition controller.
  3. Step 3 Service spark plug(s).
  4. Step 4 Service flywheel.
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