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How to Fix a Lawn Mower That Won't Start

Eugene has a keen interest in DIY and gardening. Over a 30 year period he has also become self taught in garden power tool maintenance.

Help! My Lawn Mower Won't Start After Winter

It's summer again, you drag the mower out of storage and of course it refuses to start up. Or it starts once, then refuses to start again. TYPICAL!

In this article, you'll find a ten-point checklist to get your mower started. If that doesn't work, you'll learn more in-depth information about fuel, oil, and ignition system problems, carburetor fault-finding, checking for bad compression, and other issues which could be preventing the engine from starting.

Note that this second part of a two-part guide covers more advanced troubleshooting on four-stroke gasoline (petrol) engines. It might be a good idea to check out part one first, which is a 10-point quick troubleshooting checklist:

My Lawn Mower Won't Start? 10 Top Troubleshooting Tips for Small Engines

How Does a Lawn Mower Work?

A lawn mower engine is quite simple and basic compared to the one fitted to your car, although the principle of operation is the same. Generally, these type of engines are single cylinder and four-stroke (four cycle) and run on gasoline or "gas", which is known as petrol outside of North America. It's formal name is "motor spirit".
"Four Stroke" means there are four distinct phases or cycles which these engines go through before the process is repeated:

How a four-stroke engine works

First, fuel flows from the tank to a device called a carburetor (often shortened to carb in the USA or carby in Australia) which mixes a fine mist of gasoline with air to form an explosive mixture.

  1. 1st stroke (Intake): Intake valve opens and mixture is sucked into a hollow cylinder in the engine block.
  2. 2nd stroke (Compression): Intake valve closes and mixture is compressed.
  3. 3rd stroke (Power): Mixture is ignited by a spark and burned to generate power. The rapid increase in temperature and pressure forces a piston down the cylinder, and this rotates a crankshaft to turn the blade.
  4. 4th stroke (Exhaust): Exhaust valve opens and burnt gases are expelled from the cylinder.

The complete four-stroke process is repeated about twenty times per second when the engine is running full throttle. The piston of the engine is connected to a crankshaft via a connecting rod. A sharpened blade bolted to the end of the crankshaft spins at about 3000 RPM, cutting the grass.

4 stroke engine cycle. The reciprocating (up and down) motion of the piston is converted to rotation using a connecting rod and crankshaft. In the case of a lawn mower, a grass cutting blade is attached to the end of the crankshaft.

4 stroke engine cycle. The reciprocating (up and down) motion of the piston is converted to rotation using a connecting rod and crankshaft. In the case of a lawn mower, a grass cutting blade is attached to the end of the crankshaft.

What Are Four-Stroke Engines Used On?

Apart from lawn mowers, four-stroke engines are used on other garden power tools and building site tools such as:

  • air compressors
  • welders
  • pumps
  • cultivators (e.g. rotary tillers, commonly referred to by the brand names "Rototiller" and "Rotavator"),
  • snow blowers
  • compactors (wackers)
  • cement mixers
  • portable generators

Two-Stroke Engine Troubleshooting

If you've stumbled upon this page and need info on how to get a 2-stroke engine working (fitted to hedge trimmers, string trimmers/weed eaters, chainsaws, leaf blowers, small generators, and some lawn mowers), checkout my troubleshooting guide: String Trimmer Won't Work: 2-Stroke Engine and Carburetor Troubleshooting.

What Are the Main Causes of Engine Starting Problems?

For an engine to start, it basically requires two things: fuel and a spark. If you aren't getting a spark at the plug or fuel isn't getting through to the engine (known as fuel starvation), the machine will never turn over. After storage during winter, seals, pumps and valves in the carburetor could have become gummed up with varnish deposits if you left fuel in the tank. During the summer, dust and dirt can block the fuel system or clog filters.

How Do You Start a Lawn Mower? Proper Startup Procedure

Before doing any troubleshooting, make sure you're starting the engine properly:

  1. Fill the gas tank using a funnel to avoid spilling gas over the engine (spilled gas could ignite if the engine is hot, so ideally wait until it cools!).
  2. Check the oil level as described below.
  3. Use new fuel. Old fuel can make starting difficult, especially fuel containing ethanol which attracts moisture, potentially causing corrosion. As fuel evaporates, it may leave a deposit of gum, gel, or varnish which can clog or stick moving parts together. Use a fuel stabilization product such as STA-BIL to keep fuel fresh during storage. Once you get the mower running, you can add the old fuel to the fuel in the tank, a little at a time over the season to use it up.
  4. The throttle control should be set to the full revs position. Engines have a device called a choke (basically a plate which acts as a blockage in the air intake) which makes the fuel vapor sucked into the engine "richer" or more concentrated, and this aids starting. On some mowers, the choke is completely automatic and turns off when the engine runs for a few seconds. On other mowers, the throttle needs to be set to an initial start/choke position which turns on the choke mechanism. Once the engine starts, the throttle control is pushed back to the run setting. A third scenario is that the choke is completely manual and must be turned on. There is usually a small lever for doing this which must be slid into position. This is often the case with portable generators. If there is a separate choke, turn it on when starting a cold engine and turn it off after the engine has been running for about 5 seconds. Don't use the choke if the engine has been running in the last 10 minutes or so.
  5. If the engine has a primer button, press it in and release it about 5 times in total. The primer is a small pump (like the pump action on a bottle of window cleaner) whose function is to suck fuel up into the carburetor from the float bowl or tank. This allows easier starting, without requiring multiple pulls of the starter cord to suck fuel up into the carburetor.
  6. Pull the starter rope out until you feel resistance. Allow it to return gently and then pull the cord out sharply. This should be repeated until the engine starts but not more than about 4 times.
  7. If the engine hasn't started at this stage, push the priming button another few times and try to start the engine again by pulling the cord several times. If it doesn't start, it may have flooded so leave it for a while for fuel to evaporate.
  8. Once the engine has started and has been running for a few seconds, turn off the choke (if it is a manual control), or switch the throttle from the start to run position (if there is a start position on the throttle as described above).
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If you're trying to start the engine, check the spark connection. But if you need to clean under the deck or turn the blade, then remove the spark lead first. Make sure the lead can't spring back and touch the plug terminal.

If you're trying to start the engine, check the spark connection. But if you need to clean under the deck or turn the blade, then remove the spark lead first. Make sure the lead can't spring back and touch the plug terminal.

Safety First: Before Working on Your Lawn Mower

  • Important: When working on a mower, remember that gasoline/petrol is flammable. Remove all sources of ignition, such as naked flames, stoves, sparks, cigarettes, or any other hot objects in the vicinity when working on the tank or fuel lines. Also, adequate ventilation will prevent the build up of vapor. Don't leave rags moistened with gas lying about either as they could possibly ignite.
  • Remove the spark plug or disconnect the lead. Before working on the underside of an engine, make sure the controls are all off, that the spark plug lead is disconnected, and that the engine has cooled down (if you were previously able to get it started). If you are going to start turning the blade, remove the plug from the engine for absolute safety. In theory, it's possible, even if the plug lead is disconnected and the cylinder of a hot engine is flooded with gas, that the resultant vapor could explode when compressed by the piston, just like in a diesel engine, as the blade is turned during removal or while cleaning the underside of the deck. Although the engine won't run, it could give a "kick" while spinning the blade. This would be enough to chop off fingers or even a whole hand! Maybe it's a remote possibility, but removing the plug will vent the cylinder and prevent this from happening.
  • Check the manual to determine how your mower should be tipped for deck cleaning, oil changes or blade removal. Sometimes manufacturers recommend keeping the carburetor upwards, pointing skywards. Alternatively the plug may need to be uppermost with the handles tipped back onto the ground (you'll need a heavy weight such as a concrete block to keep the mower in this position). Don't turn the mower so that the carburetor and air filter face downwards as engine oil can foul the carb and filter, leading to difficulty in starting.
  • Drain the tank if you estimate the fuel level will reach the cap. The cap has a vent and fuel can trickle out through this.
  • Turn off the fuel tap if fitted. Then run the engine until it cuts out if the mower needs to be placed on its side or plug side up. The float bowl on some engines has an overflow and fuel can trickle out this vent. Also the float may not operate properly when the engine isn't horizontal, causing the carburetor to overflow into the intake manifold of the engine. If fuel leaks onto the outside of a hot engine, it can potentially start a fire. If the mower only needs to be raised a little to unclog the deck, you don't need to turn turn off fuel or drain the tank, but always disconnect the plug lead.
Some manufacturers recommend placing the mower on its side. The breather is usually on this side, so keeping it upright prevents oil from leaking out and fouling the carb or air cleaner

Some manufacturers recommend placing the mower on its side. The breather is usually on this side, so keeping it upright prevents oil from leaking out and fouling the carb or air cleaner

Alternatively lower the mower back onto its handles. A weight such as a block will keep it in place.

Alternatively lower the mower back onto its handles. A weight such as a block will keep it in place.

How to Check the Oil Level in a Lawn Mower

Lawn mower engines are generally 4-stroke although 2-stroke engines are available. A 4-stroke engine has a crankcase or sump located at the lowest point of the engine. Oil in this reservoir is thrown onto the cylinder walls, crankshaft, and all other moving parts by a splasher or "flicker." The oil level in an engine should be checked before use. If the level is too low, or there is no oil in the engine whatsoever, it will rapidly seize up.

  1. Ensure the engine is cold or allow about 5 minutes for oil to settle if the engine has been running.
  2. Position the mower on a level surface.
  3. Remove the dipstick—this is usually attached to a small screw cap, smaller than the one on the gas tank.
  4. Wipe the stick clean with a cloth. Note the high and low level marks.
  5. Replace the dipstick.
  6. Now remove the dipstick again and check the level is between the marks.

Get into the habit of checking oil level regularly. Ideally this should be done every time you use the mower, but if the engine isn't burning oil, a check every month or so is sufficient (depending on usage). If you are cutting on sloped ground, oil consumption can be greater if oil gets blown through a vent/baffle arrangement for the crankcase called the breather. Actually it's no harm keeping the carburetor on the high side when mowing on a slope.

Lawn mowers should have an oil change after every 20 to 50 hours of use (consult your manual or look for info on the engine block for details). If you don't know how to do this, check out my guide How to Change Lawn Mower Oil.

If you buy a new mower or engine, the oil sump will usually be empty. Manufacturers drain oil out after testing and before shipping so that it doesn't leak if crates/boxes get turned upside down. Don't forget to add oil before use! Usually there's a label to remind you to do this before starting!

Checking Lawn Mower Oil

Oil level should be between the high and low marks on the dipstick.

Oil level should be between the high and low marks on the dipstick.

Reasons and Solutions for a Lawn Mower That Won't Start

Reason Your Mower Won't StartPossible Solutions

Engine is not getting the fuel it needs.

Empty and refill tank with fresh gas; Check the fuel intake system: the carburetor bowl; the choke, throttle, primer button, fuel filter

It's not getting the spark it needs.

Clean the spark plug, check its connections, or replace it; check the ignition system.

It needs oil.

Check oil level. Some newer engines have a low oil level float switch to disable starting

It's not getting the air it needs.

Check, clean, or replace the air filter.

Starter rope issues.

Check flywheel brake; make sure there's nothing jamming the blade.

You put diesel into the engine instead of gasoline

Have you really used gasoline? You can tell by smell whether fuel is diesel, kerosene or gas. Ask someone who can tell the difference.

You ran over something that got tangled in the blade

The key in the flywheel may have sheared or the key in the blade carrier. Alternatively the blade carrier's internal surface may have been damaged by the key, requiring replacement.

Battery is flat or not charging

Ride on mowers and some of the newer walk behind self-propelled mowers have batteries for easy starting. Battery voltage should be at least 12 volts and over 13 volts when the mower is running and the battery is charging.

Carburetor has flooded

If you keep pulling the starter cord multiple times without the engine starting, the carburetor can flood with fuel, making starting even more difficult. Wait for 10 minutes for fuel to evaporate and try again.

Other.

Below, you'll find lots more possible causes and solutions.

How to Get Your Mower Started: Initial Troubleshooting Checklist

Tip

#1 Use fresh gas. Don't use old gasoline which can cause difficulty starting. Make sure there is enough fuel in the tank and check to make sure the vent in the tank cap is unblocked.

#2 Make sure the spark lead is firmly attached to the plug, and the plug is tightly screwed in. Try replacing the spark plug with a new one.

#3 Make sure the "dead man's handle" control on the mower is held fully against the mower handle while starting.

#4 Turn on choke if there's a manual choke fitted. Don't turn on the choke if the engine has been running in the last 5 minutes.

#5 Make sure the primer bulb is pressed about 5 times (if fitted). If the mower runs out of gas during cutting, it will need to be primed again.

#6 Check that the air filter isn't dirty. Wash and dry a foam type air filter (see below for details), or replace a dirty paper filter.

#7 Check the cable connecting the "dead man's handle" (on the frame) to the engine is not damaged or snapped.

#8 Check that the carburetor is tightly screwed or bolted to the engine.

#9 Make sure there is no water lodged at the bottom of the gas tank.

#10 If the starter cord is hard to pull, check that there are no clumps of grass clogging the underside of the deck. These can jam the blade. Disconnect the spark lead before attempting to move the blade to remove clippings!

Protecting Your Hands

Ideally you should wear gloves to protect your hands from grime which can irritate sensitive skin, especially if you have to change engine oil or your hands are in contact with gasoline or diesel. Disposable latex gloves aren't recommended, and vinyl gloves are only supposed to have "fair" resistance to gasoline/oils, according to specs. From experience, I've found that vinyl disposables seem to be much more durable than latex though, and withstand contact with oil, grease, and gasoline for short periods. Nitrile rubber gloves are the most resistant to these chemicals.

If you dispense with gloves and work barehanded (which inevitably happens because it's difficult to handle small parts with gloves), an abrasive hand cleanser will do a much better job than soap at removing grime and is pretty much essential. I use Dreumex anti-bacterial Pumice Heavy Duty Hand Cleaner, available from Amazon for removing oil and grease from my hands. It's also good for removing oil paint, tar, soot and general garden grime. (even used it once for taking black bicycle grease out of a whitecarpet!)

Dreumex Pumice Heavy Duty Hand Cleaner Orange Lotion Bottle

Dreumex Pumice Heavy Duty Hand Cleaner Orange Lotion Bottle

What Tools Do I Need for Small Engine Repair?

  • Socket wrench set. Inch sizes for American engines and metric for European or Japanese engines. You don't need to spend a fortune on a set, since you are not going to be using them every day. However, don't buy rubbish either as you can use them for working on other equipment. A 3/8 inch ratchet is fine, or a smaller 1/4 inch size for use in confined spaces.
  • Combination wrenches, open at one end and ring at the other end, come in handy also. These can be used for situations when a nut needs to be undone but the threaded section of the bolt extends too far beyond the nut to fit into a socket. Wrenches are also known as spanners in the UK. Sockets or wrenches don't need to be greater than 3/4 inch AF (across the flats) or 19mm in size.
  • Plug spanner for removing spark plugs.
  • Screwdrivers. Flat head and Philips, both the big and small sizes.
  • Magnetic tray. This is useful for stopping all those small parts from going AWOL!
  • Torque wrench for tightening the bolt which holds on the blade and spark plug.
  • Feeler gauge for checking spark plug or points gap.

Note: Tuning an engine refers to the process of adjusting the engine to give optimum performance. This includes setting the spark plug and points gap and adjusting the fuel/air mixture and the idling speed.

Combination wrenches (spanners) - Open at one end and ring at the other

Combination wrenches (spanners) - Open at one end and ring at the other

Long nose pliers, flat blade screwdriver, Philips screwdriver, and socket wrench.

Long nose pliers, flat blade screwdriver, Philips screwdriver, and socket wrench.

Two styles of plug wrench: Socket wrench and box wrench/spanner.

Two styles of plug wrench: Socket wrench and box wrench/spanner.

Before Dismantling

Before you start dismantling everything, take some high resolution photos with a digital camera if there is any chance you are going to forget how to reassemble the parts.

Also, if lots of washers, bushings, springs, and nuts on a shaft or bolt need to be removed, you can string them onto a piece of wire to keep track of the reassembly sequence.

Avoiding Oil Leaks Through the Breather

The sump (the reservoir at the bottom of the engine which holds the lubricating oil) is vented to the atmosphere through a baffle/reed valve arrangement called the breather. Venting is necessary so that air can enter and leave the sump as the piston moves in and out of the cylinder. The baffle prevents drops of oil from getting blown out of the vent. Also, some unburned gases may get by the piston and into the crankcase. This would eventually cause a rise in pressure without the inclusion of a vent. The breather is sometimes connected to the carburetor through a tube so that any droplets of oil which manage to get through the valve, and unburned gases are sucked into the engine and burnt. (A faulty breather valve can cause excess oil consumption.) Oil can leak out through the breather if the engine is turned on its side with the breather facing downwards and the blade is turned.

If the Starter Cord Is Hard to Pull, Slips, or Won't Wind Up

If you find it difficult to pull the starter cord, there are three likely causes:

  1. Firstly, clumps of wet grass or moss can jam the underside of the deck and prevent the blade from turning. Before removing clippings and attempting to turn the blade to clear the deck, disconnect the spark plug lead.
  2. The flywheel brake may be preventing the engine from turning. When you release the "dead man's handle" on the handle of the mower, a switch is closed and cuts the engine. A brake also springs into place and slows down the engine. If this brake has seized in place or the cable which operates it has snapped, this will prevent the engine being turned. You will need to remove the cowling on top of the engine to get at this brake.
  3. Recoil starters on some engines have about half a dozen steel balls which act as part of the ratcheting mechanism. These can get rusty and either cause the starter to slip and not crank the engine or prevent it from winding back up properly. You can clean the balls with fine wire wool— add a squirt of light machine oil to the compartment which houses the balls when replacing them. (Scroll down for instructions.)

The Ignition System: Best Place to Start Troubleshooting

The function of the ignition system is to create a spark at the plug and ignite the fuel/air mixture in the cylinder. A device called a magneto generates the high voltage, and either mechanically operated points or an electronic switch will trigger the spark at the appropriate moment in the four-stroke cycle.

Over 90% of the time, problems with gas engines are due to an issue with the carburetor. However, the spark plug is easy to check out first.

See the ignition system diagram below.

The Magneto

On a push mower and other engines without a battery (such as chainsaws, trimmers, and motorbikes without batteries), a device called a magneto is used to generate the high voltage spark. Magnetos are even used on piston-engined aircraft for safety reasons, so that battery or alternator failure doesn't result in the engine cutting out. A magneto is sort of a cross between a generator and a transformer and has a primary and secondary coil.

The magneto is mounted adjacent to the flywheel (the thing with fins at the top of the mower, which spins). A magnet embedded in the edge of the flywheel induces a pulse of current in the primary coil as it moves rapidly past the poles of the magneto. When the current reaches a peak, a set of switch contacts called points opens, interrupting the current. This causes the magnetic field in the primary coil to rapidly collapse, inducing a spike of voltage in the secondary coil. This coil, which has lots of turns of wire, steps up the voltage to about 10,000 volts (or more). A capacitor, also known as a condenser, absorbs the current produced by the primary coil enabling the magnetic field to collapse rapidly. Modern engines have electronic ignition and points are replaced by an electronic switch called a thyristor or SCR (Silicon Controlled Rectifier).

Some engines (such as those on ride-on mowers) don't have magnetos, and instead they'll use an on-board battery and an ignition coil to produce a spark—similar to the system on a car.

Schematic of lawn mower ignition circuit with mechanical points. The points are connected to the primary of the magneto, the spark plug to the secondary.

Schematic of lawn mower ignition circuit with mechanical points. The points are connected to the primary of the magneto, the spark plug to the secondary.

Modern engines have electronic ignition and there are no points to clean. If the ignition module fails, the complete unit needs to be replaced.

Modern engines have electronic ignition and there are no points to clean. If the ignition module fails, the complete unit needs to be replaced.

Engine flywheel, visible when the cowling or cover over the mower is removed.

Engine flywheel, visible when the cowling or cover over the mower is removed.

Flywheel, magneto, and governor vane.

Flywheel, magneto, and governor vane.

Testing for a Spark at the Plug

  1. First, check the cable feeding high-voltage to the spark plug—this must not be loose and should be attached tightly. Sometimes, the cable can pull out of the rubber boot and terminal which pushes onto the spark plug. Then, using a spark plug wrench, remove the plug, re-attach the plug lead to the plug, and place the threaded part of the plug in contact with the engine block to ground it.
  2. Now, pull the starter cord and check for a spark. You will need to hold the plug tight against the engine somehow as it may bounce around when you pull the starter cord. This is where an assistant might come in handy as it may be difficult for you to see the plug from where you are pulling the starter cord. Try pushing the plug tight against the mower with a piece of plastic or similar, such as a plastic clothes peg. The plug could be held by the lead but if this is damp or the insulation is bad, you could end up getting a shock!
  3. If you don't see a spark at this point, there might be a crack in the plug insulation or it could be dirty and need to be cleaned with an old toothbrush and some gas. A toothbrush-sized wire brush is even better. Allow it to dry and try for a spark again. If you are still not successful, try another plug and check again. There is no harm having a spare plug for this purpose or using a plug out of a car, but don't use this plug in the engine unless it is the correct type as it could hit the piston causing damage if it is too long.
  4. The outer electrode of the plug should be filed square with a small file if it has become rounded. The spark plug gap should usually be set to 0.030" (30 thousandths of an inch) / 0.75 mm, but check the recommendations of your engine manufacturer. The gap is measured with a feeler gauge (see photo below). These are usually in the form of a set of steel strips of varying thickness from about 0.002 inch to to 0.030 inch.

Caution: The threads on a plug are steel, but the engine block and cylinder head are usually aluminum alloys, which is softer than steel. So the threads can be damaged if you aren't careful. Check that there is no grit or other debris on the threads of the plug or the engine before screwing it in. Make sure the plug doesn't become cross-threaded and don't over tighten it. Ideally, you should use a torque wrench to tighten the plug or when re-attaching the blade on a mower.

If you do damage the threads and the plug won't tighten, you can get a Helicoil fitted in a repair shop. This involves tapping the cylinder head and screwing an insert into place which has internal threads to suit the plug.

Removing the plug. Keep one hand on the plug wrench to stop it slipping off and damaging the spark plug.

Removing the plug. Keep one hand on the plug wrench to stop it slipping off and damaging the spark plug.

Hold the spark plug against the cylinder head and pull the starter cord to check for a spark. You may need an assistant. A plastic clothes peg may be a good idea to prevent shock in case the lead is damp!

Hold the spark plug against the cylinder head and pull the starter cord to check for a spark. You may need an assistant. A plastic clothes peg may be a good idea to prevent shock in case the lead is damp!

A "toothbrush" style wire brush is useful for cleaning soot from plugs.

A "toothbrush" style wire brush is useful for cleaning soot from plugs.

The outer electrode of the plug should be filed square if it is excessively rounded.

The outer electrode of the plug should be filed square if it is excessively rounded.

The plug gap (electrode spacing) is measured with a feeler gauge. These come as a set.

The plug gap (electrode spacing) is measured with a feeler gauge. These come as a set.

Hairline cracks in the insulation of plugs can short out high voltage and result in no spark across the gap

Hairline cracks in the insulation of plugs can short out high voltage and result in no spark across the gap

Wiring and Connections

If you don't get a spark, inspect the wiring on the engine. Just like on a car, the ignition system uses the engine block as a ground, so check to make sure that all ring crimps are screwed down tightly and not loose. Damaged insulation can short voltage to the engine block. Spades can pull off, and screws holding ring crimps can work their way loose due to vibration.

Check for loose or dirty connections.

Check for loose or dirty connections.

The Kill Switch

If you still can't produce a spark, there could be a problem at the kill switch. This shuts off the engine by shorting out the coil on the magneto and preventing a spark from being created when the mower is turned off. The switch is operated when you release the "Dead Man's Handle" on the mower or set the throttle to the stop position. Trace the wire from the magneto coil to find this switch. You will more than likely have to remove the cowl from the top of the engine and possibly the flywheel to check this.

Check the switch using the continuity or the lowest ohms range on your meter. The switch should be open-circuit when the engine is running and a short circuit when the engine is off. Remove one of the connections to the switch when checking. Otherwise, the resistance of the magneto coil will give a false reading.

Electronic Ignition/Capacitor Discharge Ignition (CDI)

Nowadays, engines have electronic ignition system known as capacitor discharge ignition (CDI) and the modules can fail, requiring replacement. If the ignition coil doesn't produce a spark after you've tried a new plug and you've checked the kill switch to make sure it's not shorting out, there's nothing more you can do but replace the module.

Testing the Magneto Coil on Older Engines With Points (See Schematic Above)

The resistance of the secondary coil (which connects to the spark plug) can be measured with a DMM (digital multimeter). If you don't know how to use one, read How to Use a Digital Multimeter (DMM) to Measure Voltage, Current, and Resistance.

  1. First, you'll need to connect the probes of the meter between the two ends of the coil. One end of both the primary and secondary coils is grounded, so connect one probe to the engine and the other to the end of the spark lead. This should give a reading between 2.5 and 5 kiloohms. However, this doesn't necessarily mean that the coil is okay, since high voltage could be tracking to ground through a breakdown in insulation. An open circuit reading indicates a fault, however.
  2. Check the grounding connection from the end of the coil to the engine block. This is likely a ring crimp terminal, so make sure it is tightly connected and not an open circuit at this point. The resistance of the primary coil should be about one ohm, dropping to less than an ohm as the flywheel is turned, causing the points to close.
  3. If the resistance doesn't change when the flywheel is turned, the points could be at fault. Make sure the engine is set to the start position (which opens the kill switch) when measuring. Otherwise, the kill switch will short out the coil, giving a false reading.

Cleaning Points on an Old Engine

Most modern engines have an electronic ignition system. According to Briggs and Stratton, this typically applies to engines manufactured after 1983. The engine below is probably about 40 years old and the points need to be cleaned.

Dirty contact points can prevent a spark occurring at the plug. Points in an engine are basically a switch which opens when the current through the coil of the spark generating device (the ignition coil or magneto) is at its maximum. This creates a spike of voltage at the spark plug. These points can be come tarnished or corroded and pitted over time and need to be cleaned. A rubber oil seal is fitted at the exit point of the crankshaft from the sump, and at the top of the engine block. If the engine tends to misfire (no sparks occurs in a cycle), it can be due to oil getting past this seal and splattering over the points. A pool of oil in the points compartment is evidence of this. It is somewhat of an ordeal to get at the points which are usually under the flywheel of the engine.

This mower has been stored in a leaking shed and rusted up for over 20 years without being started. I'm going to check out the points and carburetor to see if it's ok. This machine has a pump-and-weir type carburetor.

This mower has been stored in a leaking shed and rusted up for over 20 years without being started. I'm going to check out the points and carburetor to see if it's ok. This machine has a pump-and-weir type carburetor.

First, remove the mesh on the flywheel.

First, remove the mesh on the flywheel.

Prise off the plate covering the ratchet mechanism.

Prise off the plate covering the ratchet mechanism.

Remove the steel balls which form part of the ratchet mechanism. If they are rusted, clean them with fine wire wool and add a little light machine oil when replacing.

Remove the steel balls which form part of the ratchet mechanism. If they are rusted, clean them with fine wire wool and add a little light machine oil when replacing.

Removing the Flywheel

  1. First, remove the balls from the ratchet mechanism piece. Check to see that they're not rusted and stuck in the mechanism, and add a little light oil to the balls later on when you replace them.
  2. Next, use a pipe wrench (Stillsons) to remove this piece which holds on the flywheel. Loosen it counter-clockwise.
  3. Don't put pressure on any lugs in case they break off. I used a flat steel bar under the flywheel wedged against the engine body to immobilize it. (There's actually a special flywheel holding tool for doing this.) Don't wedge anything into the fins of the flywheel which could end up snapping them off if excessive torque is required to loosen the fitting.
  4. The flywheel is wedged onto a tapered shaft, and a special lawn mower puller is available for removing it. You're not really supposed to do this, but if you keep gently tapping the underside of the flywheel all the way around with a light hammer, it should be easy to release it. (Gently tap the thickest part with the magnet, but don't hit it hard!)
Use a pipe wrench (Stillson) to remove this piece. Keep the jaws away from the screw lugs to avoid damaging them.

Use a pipe wrench (Stillson) to remove this piece. Keep the jaws away from the screw lugs to avoid damaging them.

Lightly tap the flywheel all round the perimeter to release it. First tap the heavy part with the magnet inserts as shown, and the similar piece on the other side. Don't lose the iron key in the flywheel shaft.

Lightly tap the flywheel all round the perimeter to release it. First tap the heavy part with the magnet inserts as shown, and the similar piece on the other side. Don't lose the iron key in the flywheel shaft.

Ideally, remove a flywheel with pullers.

Ideally, remove a flywheel with pullers.

The points are inside this compartment.

The points are inside this compartment.

Points and capacitor (originally called a condenser): If the rubber seal on the crankshaft is faulty, oil can accumulate in this compartment and splash onto the points, causing misfiring.

Points and capacitor (originally called a condenser): If the rubber seal on the crankshaft is faulty, oil can accumulate in this compartment and splash onto the points, causing misfiring.

Clean any tarnish and pitting from the points with a fine cut file or fine sandpaper. Take care, as the points are coated with special material to withstand arcing. Only lightly file. If you overdo it with the file, you'll end up removing the coating

Clean any tarnish and pitting from the points with a fine cut file or fine sandpaper. Take care, as the points are coated with special material to withstand arcing. Only lightly file. If you overdo it with the file, you'll end up removing the coating

Finally, remove any residue or oil with a piece of cloth.

Finally, remove any residue or oil with a piece of cloth.

The points gap is set on this Briggs and Stratton engine by loosening the bolt and moving the capacitor. It should be about 0.02 inch (check it with a feeler gauge).

The points gap is set on this Briggs and Stratton engine by loosening the bolt and moving the capacitor. It should be about 0.02 inch (check it with a feeler gauge).

Testing Correct Operation of Points and Setting the Gap

  1. Connect one probe or a crocodile clip to the capacitor.
  2. Connect the other probe to the rocker arm.
  3. Set your multimeter to the lowest resistance range.
  4. With the controls on the mower set to the "on" position, and with the points open, resistance should be less than one ohm and is effectively the resistance of the primary coil of the magneto and the leads of your meter in series.
  5. Now turn the crankshaft until the points close. The resistance should now drop to a lower value and is just the resistance of the leads. The points gap is set either by releasing a screw and moving the capacitor, or turning a screw. The gap should be typically 0.02 inches, but check the specs for your engine to get a specific value.
  6. If you're getting a weak spark, thin and not very blue, it's possible that the capacitor (condenser) is faulty. It's not really possible to test this component without special equipment, however if you test it with a multimeter set to a low ohms range, a shorted capacitor will have a definite resistance. A good capacitor should give an open circuit reading, i.e. usually an overload reading shown as "1" on the display.