Fuel/air mixture adjustment : lawn mower

Fuel/air mixture adjustment : lawn mower

Another difficult starting cause is an incorrect fuel/air mixture ratio. The function of the carburetor is to mix fuel and air in a fog, which is then absorbed by the engine to burn it. A mixing screw adjusts a needle valve/jet, which controls the flow of gas to optimize the air/fuel mixture, maximizing the output power (some motors have no set screw and the mixture is factory set). The jet works exactly like the nozzle in a garden hose, producing a fine pulverization of gas particles. Unless someone has turned this screw since the last time you used your lawn mower, this is unlikely to be a problem. However, sometimes the dirt or gums may clog the valve. To adjust the mix:

•     If you are not sure if the screw was tampered with, turn it fully clockwise and then unscrew 1 1/2 turns counterclockwise.
•     Allow the engine to run for five minutes until it is completely hot.
•     Using a flat-head screwdriver, turn the screw clockwise, 1/4 back at a time, pausing for about 5 seconds after each adjustment until the motor starts to jam. Do this carefully, because if the spout is exceeded and screwed firmly into your seat, there is always a danger of damaging the soft tip of the needle valve.
•     Now repeat this procedure, but this time turn the screw counterclockwise until the motor begins to jam again.
•     Finally, place the screw halfway between the two loss positions. If the mixture is too poor, this can cause a phenomenon called "hunting", where the engine is put "brmmm brmmm brmmm" and accelerates and decelerates repeatedly. Hunting can also be caused by a stretched Governor's Wharf or an obstructed carburetor that restricts the flow of fuel. A poor mixture makes an engine run excessively hot and "spit". If the mixture is too rich, the symptoms are black smoke and an unburned gas odor.
•     Make sure that the fiber or rubber gaskets on the shaft of the mixing screw are in good condition. If poorly sealed, it reduces the engine's ability to absorb fuel.

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Automatic choke, Punctured diaphragm : lawn mower

Automatic choke, Punctured diaphragm : lawn mower

Automatic choke

The carburetor in these photos has an automatic choke. After starting the engine, the choke is kept closed by a bond attached to the diaphragm membrane. Once the engine starts, the carburetor venturi suction gradually pulls a section of the membrane for a few seconds, opening the choke through the linkage. The suction force acts through a small crest on the surface of the tank, which is seen in the picture above. If this flange is blocked, the choke plate will remain closed, flooding the carburetor.

Punctured diaphragm

A perforated diaphragm has symptoms that include a lack of response when the mixing screw is rotated clockwise, and the motor also continues to operate even when the mixing screw is fully tightened in the direction of the Clock needles. The engine will run erratically, intermittently, and fail due to excess fuel, and black smoke may also occur. The diaphragm is normally spent and pierced in the circular section that pumps the fuel. You can search for holding holes against a bright light or the sun. The only solution is a spare part.

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Carburetor type pump and weir : lawn mower

Carburetor type pump and weir : lawn mower

Some tanks are located under the carburetor, where a pump and weir system is used. This is the situation in the small engines of Briggs and Stratton.

•     The fuel is pumped from the tank by a suction-operated diaphragm of the air inlet.
•     It then flows into a small chamber/tank with a landfill that allows the gas to overflow into the tank once the camera is full. This ensures that the fuel level is constant, regardless of the fuel level in the main tank.
•     The mesh fuel filters cover the ends of the tubes that siphon fuel from the tank to the Chamber and the Chamber/tank to the carburetor venturi, and these can be covered with dirt, restricting the flow of fuel.
•     The diaphragm pump of this type of system can become worn and punctured, which causes the carburetor to absorb too much fuel. The symptoms are an engine that accelerates repeatedly up and down, produces a black ray, plus an odor of unburned gas. Alternatively, the pump can be stretched and "idle, in which case it will not be pumped at all. The only solution is to replace the diaphragm.
•     In addition, single-sense reed valves (which are only fins that form part of the diaphragm) may become clogged on tank/carburetor surfaces due to rubber deposits if fuel is left in the tank during the winter. This is why it is important to dry the lawnmower before storing it during the winter, and add a fuel stabilization product like STA-BIL to the gas to keep it cool. Ethanol in fuel attracts moisture that can be harmful to carburetors, causing corrosion over time and requiring carburetor reconstruction.

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Bad Seals, Solenoid Valves : lawn mower

Bad Seals, Solenoid Valves : lawn mower

Bad Seals

O-rings and gaskets can shrink, crack and lose their elasticity over time. Replace any part that shows signs of ageing.

Solenoid Valves

Larger engines, for example, those mounted on the mowers can incorporate a solenoid valve into the float cup. The solenoid is an electromagnet that opens a valve when voltage is applied to the coil, allowing the fuel to flow. Check with a multimeter that the voltage is being applied to the bobbin. (This also applies to starter solenoids.) In addition, solenoids may become clogged, or the valve may become clogged.

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Float, bowl and jet check : lawn mower

Float, bowl and jet check : lawn mower

•     First of all, if there is a gas faucet installed, turn. Alternatively, hold the fuel line with a vise or something similar (this may not completely cut the flow). You can do without holding the line, but don't panic when the fuel starts to run out, in any case it should be just a trickle. Have a container ready and hold it under to catch any fuel.
•     Loosen and remove the nut from the cup (which looks like a bolt, don't ask me why it's called a nut!) At the bottom of the cup, being careful not to lose any fiber washer.
•     Remove bowl completely.
•     Loosen the vise screws or open the faucet and the fuel must flow freely from the inlet valve. Gently press the float up and be sure to cut the flow. If it does not, debris may be blocking the needle valve, which prevents the needle from settling properly. In addition, the float could be drilled and not be able to float properly. Both conditions will cause the carburetor to flood and possibly the fuel outlet of the carburetor air inlet through the air filter.
•     If no fuel came out of the needle valve when the floating container was removed or the fuel tap was activated, there could be two causes. First of all, the fuel filter may be blocked. This can be a separate component in the fuel line or in the fuel line inlet inside the tank. The second cause is a blocked valve that prevents the fuel from flowing into the container, so it should be cleaned. If the container is slowly filled due to a partially blocked inlet valve, the symptoms are a lawn mower that starts and then dies after a couple of minutes as the fuel is consumed faster than it can flow into the container from the T Anque. Another possibility in larger engines, such as lawn mowers, is a fuel pump that is not working properly.
•     If there seems to be no problem with the flow of fuel in the container, the fault could be a clogged main fuel jet. If you have an air compressor, you can try to blow through the jet with an air pistol. However, it might be gummed up, and in this case you may try using an aerosol carburetor cleaner. Insert the flexible spray straw into the stream and use some puffs to clear any debris. Do not use steel wires, pins or needles to hit the jets, as soft bronze surfaces may be damaged (in theory, the soft copper wire strands of a power cord should be fine because they are softer than brass).
•     Finally, remove any debris from the Filter cup and check if the nut in the brass box is clogged by dirt. In some carburetors, this performs two functions, keeps the container in place and also acts as part of the jet system. Once all this has been done, replace the cup, being careful not to overtighten the Brass cup nut and try to start the engine again.

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Fuel line, Floating Bowl type Carburetor, How Float Bowl Works : lawn mower

Fuel line, Floating Bowl type Carburetor, How Float Bowl Works : lawn mower

A bent or obstructed fuel line between the tank and the Float cup can restrict the flow of fuel to the engine.

Floating Bowl type Carburetor

If the fuel tank is located above the carburetor, it is likely to be float type. (See photo below)

Dirt and gum are the carburetors ' greatest enemies, as they can contain several narrow passageways through which the fuel flows. Over time, rubber deposits can reduce these passageways. In addition, the tiny dust particles contained in the fuel will bypass a fuel filter and be directed to the carburetor. This can also obstruct everything. A patented carburetor cleaner can be used to clean everything.

How Float Bowl Works

The float and bowl function as a toilet cistern, keeping the fuel at a constant level so that it can be sucked into the carburetor. Float buoyancy causes it to rise. When the gas in the container reaches an established level, a needle at the top of the float is forced into a valve seat, closing the fuel inlet to the container. Without this system, the gas would simply feed by gravity to the carburetor, flooding. If the lawnmower does not run for more than a few minutes, the float Cup may not be filling or filling very slowly. If it is slowly filled, the mower is cut, but can often be restarted after it has been left for a while (because the container refills).

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The choke : lawn mower

The choke : lawn mower

The choke is simply a flat disc mounted on a shaft at the carburetor air inlet. When you turn on the choke, the shaft is rotated so that the disc blocks the airflow, and there is greater suction in the inlet. This makes it possible to vacuum more fuel and also makes it easier to start. 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 will probably flood the carburetor and cause difficulty starting.

Carburetor air intake showing choke plate (air filter has been removed) on a small 3 1/2 HP Briggs and Stratton engine.

Carburetor Air inlet showing choke plate (air filter removed) on a small 3 1/2 HP Briggs and Stratton engine.

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The air filter or cleaner : lawn mower

The air filter or cleaner : lawn mower

The function of an air filter is to remove dust, sand, grass and other dirt from the air stream before it enters the engine. Without a filter, this material would scrub the piston and cylinder, causing premature motor wear over time. Dirt will also clog small streams that flow from fuel from inside the carburetor.

The air filter must be clean for optimum engine operation. A dirty, clogged air filter will deprive an oxygen engine, causing a reduction in power, excessive fuel consumption or carburetor flooding, and difficulty starting. The foam elements can be washed with hot water and soap to remove oil and dirt. Then squeeze the dry foam into a cloth. Pour a teaspoon of motor oil into the foam and massage evenly through the element (the function of the oil is to catch the dust passing through the foam). It is possible to clean some of the dust from a paper filter blowing from the inside with compressed air. However, this type of filter should ideally be replaced.

• Paper air filter.
• Foam air filter.

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Water in the tank : lawn mower

Water in the tank : lawn mower

•     Water in the fuel tank can cause the motor to agitate and lock. Maybe you left your lawn mower exposed all winter, causing the water to seep through the vent on the tank lid, or you could have gotten bad fuel from your gas station. Leaving gas in the tank during the winter may cause moisture to accumulate, especially if you use fuel with an ethanol content. Ethanol attracts some moisture that normally exists as a suspension in the gas (such as cream in the milk). If you leave a lawnmower sitting for months, it may eventually settle down to the bottom of the tank, and only a couple of teaspoons may be sufficient to cause difficulties at the start.
•     The water is denser than the gas and will sink to the bottom of the tank. If you light a flashlight inside, you can see water globules at the bottom (a maglite is useful for this kind of thing because the beam can be focused to light the bottom of the tank).
•     If you drain the gas tank, you may use a cloth or sponge pushed to the end of a long screwdriver to absorb any remaining water at the bottom of the tank. If a lot of water has accumulated at the bottom, it may also have passed through the fuel line in the carburetor. In this case, you must remove the floating container as described below, and drain the contents of the container plus whatever is left in the fuel tank.

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The carburetor : lawnmower

The carburetor : lawnmower

If the ignition system is OK, you must check the fuel system below.

Books and troubleshooting guides often advise you to check if the fuel is actually coming to the engine by removing and examining the spark plug after several pulls of the starter cord. It must be moist, but a damp plug indicates flooding. However, at the time the plug is removed and if the weather is warm, any moisture can evaporate, so this is not a totally reliable method to identify a problem with the carburetor.

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The fuel flows from the tank to the carburetor. The function of this device is to create an explosive fuel mist mixed with air, which is aspired to the engine to burn it and provide energy. 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 starting difficulties or loss of power.

These include:

•     Dirty air or fuel filters
•     Clogged jets
•     Parts that are badly sealed
•     Punctured or worn components
•     Moving Parts sticking
•     Water in the tank

Diagram of a carburetor
A carburetor filters air and mixes it with fuel, forming a fine mist which gets sucked into the engine.
A carburetor filters the air and mixes it with fuel, forming a fine mist that is absorbed into the engine.

Carburetor seal or O-ring

The carburetor must be properly sealed with the motor screws or tightened, and the O-rings must not wear out or be damaged.

Test the correct functioning of the points and establish the gap : lawnmower

Test the correct functioning of the points and establish the gap : lawnmower

•     Connect a probe or alligator clip to the condenser.
•     Connect the other probe to the rocker.
•     Set your multimeter to the lowest resistance range.
•     With the controls on the mower in the "on" position, and with the open points, the resistance must be less than one ohm and is effectively the resistance of the Magneto's primary coil and the cables of its serial meter.
•     Now turn the crankshaft until the points are closed. The resistance should now fall to a lower value and is only the resistance of the wires. The point gap is set by releasing a screw and moving the capacitor, or by turning a screw. The gap should typically be 0.02 inches, but check the specifications for your engine to get a specific value.
•     If you get a weak, thin, and not very blue spark, the capacitor (capacitor) may be defective. It is not possible to test this component without a special equipment, however, if you test it with a multimeter set to a low ohm range, a short capacitor will have a definite resistance. A good capacitor should give an open-circuit reading, i.e., usually an overload reading that is displayed as "1" on the display.

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