How to Fix a Lawn Mower That Won't Start
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!
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 can become gummed up with varnish deposits if you leave fuel in the tank. During the summer, dust and dirt can block the fuel system or clog filters.
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.
How Do You Start a Lawn Mower? Proper Startup Procedure
- Fill the gas tank using a funnel to avoid spilling gas over the engine (spilled gas could ignite if the engine is hot!).
- Check the oil level as described below.
- Use new fuel. Old fuel can make starting difficult, especially ethanol-based fuel 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.
- 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.
- 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.
- 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.
- 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.
- 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).
Reasons and Solutions for a Lawn Mower That Won't Start
Reason Your Mower Won't Start
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.
Dry and 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.
Below, you'll find many other possible causes and solutions.
Safety First: Before Working On Your Lawnmower
- 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.
- Keep the carburetor side of the engine facing upwards to avoid oil spills if the mower is placed on its side. Engine oil can also foul the carburetor and air filter, leading to difficulty in starting. The sump (the reservoir at the bottom of the engine which holds the 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.
- Don't forget to drain the gas tank if the mower is placed on its side. Otherwise, if the fuel level is higher than the vent in the cap of the tank, it may start to dribble out. You can collect gas in a jug or other suitable container (don't use one made from polystyrene which dissolves in gas). Alternatively, use a siphon pump designed for the purpose. If there is only a small amount of gas in the tank, then you don't need to do this.
The Golden Rule of Troubleshooting
Remember to only make one change at a time and try starting it again, because if you don't, your first brilliant flash of inspiration may sort the problem and the second dumb idea may stop things working again!
How to Get Your Mower Started: Troubleshooting Checklist
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.
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.
Make sure the "dead man's handle" control on the mower is held fully against the mower handle while starting.
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.
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.
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.
Check the cable connecting the "dead man's handle" (on the frame) to the engine is not damaged or snapped.
Check that the carburetor is tightly screwed or bolted to the engine.
Make sure there is no water lodged at the bottom of the gas tank.
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!
What If My Lawn Mower Still Won't Start?
Scroll down to learn more about how your machine works and other troubleshooting tips.
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). This means that there are four distinct phases or cycles which the engine goes through before the process is repeated:
First, fuel flows from the tank to a device called a carburetor, which mixes a fine mist of gasoline with air to form an explosive mixture.
- 1st stroke (Intake): Mixture is sucked into a hollow cylinder in the engine block.
- 2nd stroke (Compression): Mixture is compressed.
- 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.
- 4th stroke (Exhaust): 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.
Small Engine Repair and Four-Stroke Engines Used on Other Equipment
While these tips apply to lawnmower engines, they are equally valid for other small four-stroke engines used on yard equipment such as portable generators, 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, etc.
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.
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), a hand cleanser will do a better job than soap at removing grime.
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.
- Ensure the engine is cold or allow about 5 minutes for oil to settle if the engine has been running.
- Position the mower on a level surface.
- Remove the dipstick—this is usually attached to a small screw cap, smaller than the one on the gas tank.
- Wipe the stick clean with a cloth. Note the high and low level marks.
- Replace the dipstick.
- 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, checkout my guide How to Change Lawn Mower Oil.
Checking Lawn Mower Oil
Lawn Mower Troubleshooting and Tuning: Basic Hand Tools Required
- 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.
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.
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:
- 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.
- 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.
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.
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. Engines with electronic ignition (such as the Briggs & Stratton Magnetron™ system) work on the same principle, with mechanical points being 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.
Testing for a Spark at the Plug
- 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.
- 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!
- 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.
- 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.
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.
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.
Testing the Magneto Coil (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.
- 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.
- 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.
- 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.
What if My Lawn Mower Still, Still Won't Start?
Read on for more troubleshooting, but try all the easier stuff above before going any further.
Points and Electronic Ignition
Dirty contact points can also 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.
Nowadays, engines have electronic ignition system known as capacitor discharge ignition (CDI) and the modules can fail, requiring replacement.
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.
Removing the Flywheel
- 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.
- Next, use a pipe wrench (Stillsons) to remove this piece which holds on the flywheel. Loosen it counter-clockwise.
- 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.
- 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 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!)
Testing Correct Operation of Points and Setting the Gap
- Connect one probe or a crocodile clip to the capacitor.
- Connect the other probe to the rocker arm.
- Set your multimeter to the lowest resistance range.
- 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.
- 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.
- 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.
If the ignition system is okay, you'll need to check out the fuel system next.
Troubleshooting books and guides often advise checking whether fuel is actually reaching the engine by removing and examining the spark plug after several pulls of the starter cord. It should be damp but a wet plug indicates flooding. However by the time the plug is removed and if weather is warm, any dampness can evaporate, so this isn't a totally reliable method of identifying a problem with the carburetor.
Fuel flows from the tank to the carburetor. The function of this device is to create an explosive mist of fuel mixed with air, which is sucked into the engine to be burned and provide power. An air filter cleans the incoming air to prevent malfunction and damage to the engine. Several problems in this section of the engine can cause difficulty in starting or loss of power. These include:
- Dirty air or fuel filters
- Clogged jets
- Parts that are sealed poorly
- Punctured or worn components
- Moving parts sticking
- Water in the tank
Schematic of a Carburetor
Carburetor Seal or O-Ring
The carburetor should make a good seal with the engine by being screwed or clamped on tightly, and any gaskets or o-rings shouldn't be worn or damaged.
Water in the Tank
- Water in the gas tank can cause an engine to splutter and stall. Maybe you left your mower exposed all winter, causing water to leak through the vent in the tank cap, or you could have gotten bad fuel from your gas station. Leaving gas in the tank over winter can result in an accumulation of moisture, especially if you use fuel with an ethanol content. Ethanol attracts some moisture which normally exists as a suspension in gas (like cream in milk). If a mower is left sitting for months, this can eventually settle out to the bottom of the tank, and only a couple of teaspoons may be enough to cause difficulty starting.
- Water is denser than gas and will sink to the bottom of the tank. If you shine a torch inside, you may see globules of water at the bottom (a maglite is useful for this sort of thing because the beam can be focused to light up the bottom of the tank).
- If you drain the tank of gas, you can use a cloth or piece of sponge pushed onto the end of a long screwdriver to soak up any remaining blobs of water at the bottom of the tank. If a lot of water has lodged at the bottom, it may also have passed through the fuel line into the carburetor. In this case you will have to remove the float bowl as described below, and drain the contents of the bowl plus whatever remains in the fuel tank.
The Air Filter or Cleaner
The function of an air filter is to remove dust, grit, grass, and other grime from the airstream before it enters the engine. Without a filter, this material would scour the piston and cylinder, causing premature wear of the engine over time. Dirt would also clog the tiny jets which fuel flows from inside the carburetor.
The air filter must be clean for optimum engine operation. A dirty, clogged air filter will starve an engine of oxygen, causing a reduction in power, excess fuel consumption or flooding of the carburetor, and difficulty in starting. Foam elements can be washed in hot water and soap to remove oil and dirt. Next, squeeze the foam dry in a cloth. Pour a teaspoon of motor oil onto the foam and massage it evenly through the element (the function of the oil is to catch dust passing through the foam). It is possible to clean some of the dust from a paper filter by blowing from the inside with compressed air. These type of filters, however, should ideally be replaced.
The choke is simply a flat disk mounted on a shaft in the air intake of the carburetor. When you turn on the choke, the shaft is turned so that the disk blocks the airflow, and there is greater suction in the intake. This allows more fuel to be sucked in and also makes for easier starting. Chokes are usually automatic but can be manual on some engines. In this case, they must be turned on when starting an engine and then turned off after the engine has been running for several seconds. If an engine is hot, leaving the choke on will likely flood the carburetor and cause difficulty starting.
A kinked or blocked fuel line between the tank and float bowl can restrict fuel flow to the engine.
Float Bowl Type Carburetor
If the fuel tank is located above the carburetor, it is likely to be a float bowl type. (See photo below.)
Dirt and gum are the greatest enemies of carburetors, since they may contain several narrow passageways through which fuel flows. Over time, gum deposits can narrow these passageways. Also, minute particles of dust contained in fuel will bypass a fuel filter and make their way into the carburetor. This can also clog everything up. A proprietary carburetor cleaner can be used to clean everything out.
How the Float Bowl Works
The float and bowl work like a toilet cistern, keeping the fuel at a constant level so that it can be sucked up into the carburetor. The buoyancy of the float causes it to rise. When the gas in the bowl reaches a set level, a needle at the top of the float is forced into a valve seat, shutting off entry of fuel into the bowl. Without this system, gas would just gravity-feed into the carburetor, flooding it. If your lawn mower won't stay running for more than a few minutes, the chances are that the float bowl isn't filling or is filling very slowly. If it fills slowly, the mower cuts out, but can often be re-started after it's been left for a while (because the bowl re-fills).
Checking the Float, Bowl, and Jet
- Firstly, if a gas tap is fitted, turn this off. Alternatively, clamp off the fuel line with a vice-grips, or something similar (this mightn't totally cut off flow). You can dispense with clamping the line, but don't panic when fuel starts to run out, in any case it should be just a trickle. Have a container ready and hold it underneath to catch any fuel.
- Loosen and remove the bowl nut (which looks like a bolt, don't ask me why its called a nut!) on the underside of the bowl, taking care not to lose any fiber washers.
- Fully remove the bowl.
- Loosen the vise-grips or turn on the tap and fuel should flow freely from the inlet valve. Gently press the float upwards and ensure it cuts off the flow. If it doesn't, debris may be blocking the needle valve, preventing the needle from seating properly. Also, the float could be punctured and not able to float properly. Both of these conditions will cause flooding of the carburetor and possibly fuel flowing out of the carburetor air intake via the air filter.
- If no fuel flowed out of the needle valve when the float bowl was removed or the fuel tap turned on, there could be two causes. Firstly the fuel filter may be blocked. This can be a separate component in the fuel line or at the intake of the fuel line inside the tank. The second cause is a blocked valve preventing fuel from flowing into the bowl, so you need to clear this out. If the bowl fills slowly due to a partially blocked inlet valve, the symptoms are a mower that starts then dies after a couple of minutes as fuel is used up quicker than it can flow into the bowl from the tank. Another possibility on larger engines such as those on lawn mower tractors is a malfunctioning fuel pump.
- If there doesn't appear to be any problem with fuel flowing into the bowl, the fault could be a clogged main fuel jet. If you have an air compressor, you can try blowing through the jet with an air gun. However, it could be gummed up, and in this case you can try using an aerosol carburetor cleaner. Insert the flexible straw of the aerosol into the jet and use a few puffs to clear any debris. Don't use steel wires, pins, or needles to poke at the jets as the soft brass surfaces can be damaged (strands of soft copper wire from a power cable should in theory be ok because they're softer than brass).
- Finally remove any debris in the filter bowl and check the hollow brass bowl nut for any blockages caused by dirt. On some carburetors, this serves two functions, it holds the bowl in place and also acts as part of the jet system. Once you have done all this, replace the bowl, taking care not to over-tighten the brass bowl nut and try starting the engine again.
O-rings and gaskets can shrink, crack, and lose their elasticity over time. Replace any parts that show signs of aging.
Larger engines, for example, those on ride on mowers may incorporate a solenoid valve on the float bowl. The solenoid is an electromagnet that opens a valve when voltage is applied to the coil, allowing fuel to flow. Check with a multimeter that voltage is being applied to the coil. (This also applies to starter motor solenoids.) Also, solenoids can get stuck, or the valve can become clogged.
Pump-and-Weir Type Carburetor
Some tanks are located below the carburetor, where a pump-and-weir system is used. This is the scenario on small Briggs and Stratton engines.
- Fuel is pumped from the tank by a diaphragm operated by suction from the air intake.
- It then flows into a small chamber/reservoir with a weir that allows gas to overflow back into the tank once the chamber is full. This ensures the fuel level is constant, independent of the fuel level in the main tank.
- Mesh fuel filters cover the ends of the tubes that suck fuel from the tank to the chamber and chamber/reservoir to the carburetor venturi, and these can become coated with dirt, restricting the flow of fuel.
- The diaphragm pump on this type of system can become worn and punctured, causing the carburetor to suck in too much fuel. Symptoms are an engine which repeatedly speeds up and down, produces black spoke, plus a smell of unburnt gas. Alternatively the pump can become stretched and "baggy in which case it won't pump at all. The only solution is to replace the diaphragm.
- Also the one-way reed valves (which are just flaps forming part of the diaphragm) can become stuck to the surfaces of the tank/carburetor due to gum deposits if fuel is left lying in the tank over winter. This is why it is important to run a mower dry before storage over winter, and add a fuel stabilization product such as STA-BIL to gas to keep it fresh. Ethanol in fuel attracts moisture that can be detrimental to carburetors, causing corrosion over time and requiring a carburetor re-build.
Checking a Pump-and-Weir Carburetor on a Briggs Engine
The carburetor in these photos has an automatic choke. Upon engine start-up, the choke is held closed by a linkage attached to the diaphragm membrane. Once the engine starts, suction from the carburetor venturi gradually pulls down on a section of the membrane over a period of a few seconds, opening the choke via the linkage. The suction force acts through a small ridge in the surface of the tank, seen in the photo above. If this ridge becomes blocked, the choke plate will stay closed, flooding the carburetor.
A punctured diaphragm has symptoms that include a lack of response when the mixture screw is turned clockwise, and also the engine continues to run even when the mixture screw is fully tightened all the way clockwise. The engine will run erratically, splutter, and misfire due to excessive fuel, and there may also be black smoke. The diaphragm normally becomes worn and punctured in the circular section which pumps the fuel. You can check for holes by holding it up against a bright light or the sun. The only solution is a replacement part.
Adjusting the Fuel/Air Mixture
Another cause of difficult starting is an incorrect fuel/air mixture ratio. The carburetor's function is to mix fuel and air into a mist, which then gets sucked into the engine to be burned. A mixture screw adjusts a needle valve/jet, which controls the flow of gas so that the air/fuel mixture is optimized, maximizing the power output (on some engines there is no adjustment screw and the mixture is factory set). The jet works exactly the same as the nozzle on a garden hose, producing a fine spray of gas particles. Unless someone has turned this screw since the last time you used your mower, this is unlikely to be a problem. However, sometimes grime or gum can clog the valve. To adjust the mixture:
- If you're not sure whether the screw was fiddled with, turn it fully clockwise and then unscrew 1 1/2 turns counter-clockwise.
- Allow the engine to run for five minutes until it is fully warmed up.
- With a flat screwdriver, turn the screw clockwise, 1/4 turn at a time, pausing for about 5 seconds after every adjustment, until the engine begins to stall. Do this gently because if you overdo it and screw the jet tightly into its seat, there is always the danger of damaging the soft tip on the needle valve.
- Now, repeat this procedure, but this time turn the screw counter-clockwise until the engine begins to stall again.
- Finally, set the screw midway between the two stall positions. If the mixture is too lean, this can cause a phenomenon called "hunting" where the engine goes "brmmm brmmm brmmm" and speeds up and slows down repeatedly. Hunting can also be caused by a stretched governor spring or clogged carburetor restricting fuel flow. A lean mixture causes an engine to run excessively hot and "spit." If the mixture is too rich, symptoms are black smoke and a smell of unburnt gas.
- Make sure any fiber or rubber seals on the shaft of the mixer screw are in good condition. If they seal badly, it reduces the ability of the engine to suck in fuel.
Small engines use a simple gravity feeding system to fill the float bowl. Larger engines, such as those on lawn tractors, require a greater flow rate of gas. Also, the gas tank may be located lower than the carburetor. So a fuel pump may be required. These pumps can be electric powered, use a rod from the camshaft to operate a diaphragm in the pump or rely on the suction in the carburetor via a hose to operate the diaphragm (a diaphragm is a disk of flexible material pushed backwards and forwards to pump fuel).
Typical faults in pumps are a loss of voltage to the pump, a clogged filter in the pump, disconnected air hose, punctured diaphragm, or the seizing up of components of the pump.
Low Oil Level
Larger engines with an electrical system sometimes have a low oil level float switch. This prevents the engine from being started and damaged if the oil in the sump is below a threshold level.
While fuel and a good spark are essential for an engine to start, good compression is also important. Compression is simply the ability of the fuel/air vapor to be compressed in the cylinder—also known as the combustion chamber—by the piston prior to ignition, without leaking out somewhere due to bad sealing.
You can get a rough idea of compression by sticking your thumb over the hole into which the plug screws and pulling the starter cord or turning the flywheel if this is easier. You should be able to feel the pressure on your thumb. However with the spark plug back in place, if compression is severely compromised, the starter cord will be easy to pull, with no resistance.
Bad compression can have several causes:
- Loose Spark Plug. It could be as simple as a loose spark plug, so tighten it up. Don't use excessive force when tightening.
- Bad Cylinder Head Gasket. The cylinder head is the piece of the engine into which the plug is fitted. This is bolted onto the cylinder block with the gasket sandwiched in between. You may notice burn or soot marks around the edges of the gasket.
- Bore Wear. Another cause of bad compression is bore wear, causing the piston to seal badly in the cylinder. This can happen as an engine reaches old age, or if it has been run without an air filter, or on low oil.
- Stuck Open Valves: Backfiring and Spitting. Another cause of bad compression is a stuck open or badly seating valve. During the 4-stroke cycle, an intake valve opens and allows air/gas mixture to flow into the cylinder, and an exhaust valve opens to allow burnt gases to be expelled from the cylinder. During the compression stroke, both valves should be closed. Now if the valves don't seat and seal fully due to wear, build up of deposits, or the exhaust valve becomes burned over many years of engine use, compression can suffer. Another scenario is if an engine comes to a standstill with one of the valves happening to be open. If the engine is stored for years without use, what can happen is that the valve seizes in this position. Now, normally, a camshaft opens each valve via rods in contact with the valve stems called tappets, and a spring returns the valve to the closed position. If the valve gets stuck, the spring force isn't strong enough to return the valve to the closed position. Symptoms of a stuck or badly worn valve can be spitting or backfiring through the carburetor or exhaust without the engine starting up, or the starter cord being really easy to pull. So the moral of the story is to "exercise" machines in storage every so often to prevent this happening. This can be as simple as pulling the starter cord of the engine gently so that everything internally moves. This applies to anything with moving parts including engines, motors, tools, and workshop equipment.
I actually experienced a compressions issue with my mower this morning even though it isn't terribly old. The cord was easy to pull with no resistance, so I assume a valve got stuck. After leaving the engine for about an hour, the problem resolved, maybe because oil worked its way into the stuck tappets or valve stems.
By looking into the cylinder through the spark plug hole, or removing an inspection cover over a compartment (known as the valve chest) which provides access to the tappets, you will be able to see whether a valve is sticking open. Some engines have OHV or overhead valves, and again access is possible by removing an access cover. Valves can be freed by spraying WD40 into the valve seat and onto the stems. However, this may be beyond the ability of the amateur. If the cylinder head needs to be removed, on replacement, bolt tightening should be in a staggered sequence and bolts should ideally be tightened back into place with a torque wrench.
Spitting can also be caused by a mixture which is set too lean, requiring the mixture screw to be turned anti-clockwise to richen it (see procedure above). Another cause of spitting is a sheared flywheel key, causing the timing to be off and the spark plug to fire at the wrong instant.
Smoke is a Tell-Tale Symptom
The color of smoke from the exhaust is a tell-tale symptom of problems with an engine.
White or blue smoke?
This is caused by oil being burnt in the engine. While a certain amount of oil is consumed and burned under normal circumstances, clouds of blue smoke indicate a problem. Old engines emit smoke as the cylinder wears (bore wear) and the piston and rings no longer fit so well. Excess oil gets up into the cylinder and is burned. Fractured piston rings can allow lots of oil to get into the cylinder.
Burning oil can also be caused by a faulty breather allowing oil to get blown out from the sump and then sucked into the engine via the carburetor (if there is a tube linking the breather to the carburetor).
Yet, another cause is an over-filled sump. On one occasion, my neighbor and her helpful friend decided to top-up the mower with oil, and being generous, they decided to fill the oil to the brim of the oil fill plug. The result? Thick clouds of white smoke which I could see in the distance! Yours truly had to save the day and drain some oil out of the mower!
Black smoke is an indicator of too much gas being burned. This can be caused by an over-rich mixture (adjust the mixture screw), a faulty or punctured diaphragm in the carburetor, a choke not opening when the engine starts or a dirty air filter.
Throttle Linkages and Governor
- Check that the throttle cable is actuating the throttle plate via the linkages. These can sometimes get seized or obstructed by grass or small twigs.
- Ensure that the governor linkage isn't keeping the throttle closed. A governor is a device that regulates the speed of an engine, keeping it constant independent of the load on the engine. On a lawn mower, two types of a governor are used. One system consists of a vane pivoted at one end and connected to the throttle plate in the carburetor via a spring and linkages. Air is directed from the fins on the flywheel onto the vane. If the engine slows down (due to long grass), this reduces the force of the air on the vane. This allows a spring to open the throttle further, letting more fuel into the engine, giving more power and this increases the speed to compensate. The governor also limits the engine from over-revving by closing off the throttle when the load is reduced. Another type of governor works by centrifugal force. A spool that is able to slide on a shaft is acted on by arms which move further outward as engine speed increases. The spool actuates the throttle via linkages.
- With the air filter removed and using a torch, you can determine the position of the throttle and choke valves. These are plates which open or close in the intake. The outermost plate is the choke which almost fully obstructs the airway when turned on during a cold engine start. The throttle plate creates least obstruction on full throttle and the governor linkage shouldn't be closing it before the engine starts. Check that the governor can move freely and isn't sticking.
Governor Vane and Linkages
Don't Try Starting an Engine Without the Blade!
Small engines, such as those on lawn mowers, usually have an aluminum flywheel, and the added weight of the blade actually forms part of the flywheel system. If you are familiar with mechanics (a branch of physics), you will know that imbalance increases the moment of inertia of the system. If the blade is removed, it will be difficult to start the engine because of insufficient spin momentum when you pull the starter cord.
Routine Mower Maintenance Checks During the Summer
- Check oil every month or more regularly depending on usage.
- Remove any grass or leaves from the cooling fins on the cylinder head. Air blown through the fins by the flywheel conducts heat away from the engine so its important that they are not clogged.
- Sharpen the blade a few times during the season, again depending on usage. If you have a small lawn, a single sharpening at the start of the year should be sufficient.
- Check for any loose nuts or screws on wheels or other parts of the mower.
- If possible, drip some light machine oil onto the control cables for the throttle and drive. If you do this a little at a time, it will run down along the cables into the cable sleeves, preventing corrosion and seizing.
- Try to avoid cutting grass when it is wet. Wet grass clippings stick to the underside of the deck along with mud and other grime. This clogs the pathway to the collection bag and eventually the engine can struggle while cutting. After use in these conditions, wash the underside of the deck with a hose, but try not to get any water up onto the engine and air filter.
End of Season Storage
To prevent starting problems next season, run the mower until it cuts out from lack of fuel. This should clear most of the fuel from the carburettor and bowl and help to prevent everything gumming up. To prevent corrosion, let the mower cool down, remove the spark plug and put a teaspoon of oil through the plug hole. Pull the starter cord a couple of times and replace the plug. Remove all caked-on grass from the underside of the deck as this can ferment, become acidic and eat its way through the metal over time.
If you can't get your mower started, maybe it's time to buy a new one! Checkout my guide to Choosing and Buying a Lawn Mower which discusses the pros and cons of electric, gas, and battery models.
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My Personal Experience with Lawn Mowers
I don't fix lawn equipment professionally, but I learned all this stuff by messing around with engines since I was a teenager, and have gained about thirty years experience with lawn mowers, string trimmers, and hedge cutters.
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© 2012 Eugene Brennan
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