What is the gap on the lawnmower spark plug

Grass trimmer what clearance on the spool

Consider the most common breakdowns associated with the chainsaw’s ignition system.

Soot or moisture on the spark plug

If there is no ignition, there is no way to start the chainsaw motor. When inspecting the system, you see that black soot has formed on the spark plug. This indicates that the malfunction should be looked for in the carburetor, piston group or reconsider the ratio of oil to gasoline.

If the spark plug is wet, it indicates excessive fuel. Check all hoses for leaks, replace those with holes and cracks. Clean the spark plug of soot, dry it. Try starting the engine.

If the spark plug is dry and not splashed with any spark fluid, check for spark. To do this, unscrew the spark plug, put the high-voltage ignition cable on it and press it against the body of the motor. Then sharply start the starter. If there is no spark, change the spark plug and repeat everything from the beginning. Spark appeared. start the engine!

Breakage of HV wire

If the new spark plug also does not give the spark, it is necessary to test the BB wire. To check the integrity of the wiring, it is best to use a special tester. a multimeter, which was talked about at the beginning of the article.

Breakage of the circuit in the coil can be seen by checking it with a multimeter on the resistance mode, the value will tend to infinity.

Large gap between the ignition module and the flywheel magnet

In the case of a large gap formed in the coil, you must independently set the necessary value, using the algorithm given above.

Reduced clearance

The gap between the electrodes less than 0.4 mm is also problematic. Because of that the spark is weak and can not ignite the combustion mixture. At best the fuel will burn, but not completely.

Naturally, in this mode, the car engine will begin to experience overload, and the amount of harmful substances in the exhaust will increase.

“I was changing Saturn Sky plugs. condition and color were fine, but the bottom contact burned out. As it turned out, the gap was larger than it should have been. After the change, the car really flew, and it was a lot more fun. So everything in the car has to be how the engineers thought it would be. Phill, auto mechanic

Adjustment specifics on different trimmer models

The principle of operation

Adjusting the ignition system is necessary to ensure that the fuel ignites in the combustion chamber at the right time.

If the flywheel is shifted on the shaft relative to its original position, fuel ignites faster than the piston crosses the top dead center. As a result, the piston moves backwards and the crankshaft rotates in the opposite direction. Determine early fuel ignition by a broken starter cord, broken starter assembly, or other breakdowns.

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Sometimes the ignition is late. The piston compresses the unignited fuel and then goes down and only then does the spark form. The result is that the engine either does not run or its power is reduced and revs are picked up much slower.

Usually the flywheel with magnets is mounted in a perfect position, precisely coordinated with the piston and carefully secured to avoid misalignment. This allows you to assert that the incorrect installation of the part is impossible. But if the displacement of the flywheel is a confirmed fact, it is a failure or an accident.

The adjustment of the ignition system is simple and identical on different models of trimmers, whether STIHL FS55, AL-KO BC 4125 or another lawnmower. Firstly the engine hood must be removed by first unscrewing the fasteners that hold it in place. This disassembly step may be different on different models of brushcutter (described below). Next, use a screwdriver to partially unscrew the fasteners that fix the magneto, and rotate the flywheel so that the magnets are at the top.

To make the optimal gap between the coil and the magneto, it is necessary to use a special template. It is not practical to buy it for a single use, so as an alternative it is possible to use a business card or a small calendar, as well as a sheet of A4 paper folded in four.

Then the coil will be attracted to the flywheel by the magnetic field. Holding the magneto in this position, you need to tighten the fasteners securing it, and then pull out the card. After that, you need to crank the flywheel a few times to make sure it is not hitting the coil at all. The process is over. the ignition on the brushcutter is set. You can learn more about adjusting the trimmer ignition system on the Internet, where many videos on the topic are available.

Installing the ignition system.

The flywheel has an impeller that works as a fan for cooling the engine. Also on one side of the flywheel are permanent magnets, shown with an arrow in the figure.

The rotation of the flywheel produces an electric current in the coil to produce a spark on the spark plug. Spark appears at the moment when the magnets are directly under the magneto.

  • Loosen the spark plug bolts.
  • Point the magnets on the flywheel at the ignition coil
  • Clamp the flywheel between the flywheel and coil and tighten the mounting bolts.
  • Remove the strip
  • Rotate the flywheel 360 degrees several times, making sure it is not clinging to the coil anywhere.

All. Clearance between coil and flywheel is set.

lawnmower, spark, plug

Is impossible and impractical. Below is a list of ignition coils that can be purchased on our website

Repair of ignition coil is not possible or practical. Below is a list of ignition coils that are available on our website

BC-43E brushcutter spark plug replacement

Spark plugs and malfunctions are associated with occurrence:

  • Wear, more often the side electrode.
  • Cracked insulator housing.
  • Melting of both electrodes.
  • Erosion. thinning of the side element.
  • Fouling.

Not timely replacement of plugs leads to distortion of the gap and as a natural result, the spark formation with the consequences of a terrible start-up. Wear of lateral electrode is caused by additives in gasoline. As a result. spark gaps, detonation of engine. Products change.

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Even inconspicuous cracks or otherwise visually visible chips on the body of the insulator are the reason for replacement.

While operating spark plugs with cracks the current leakage occurs through the insulator to the plug body

The defects appear for a reason:

  • Wear (natural).
  • Detonation due to an early ignition angle.
  • Low-octane fuel.
  • Insulator of low quality.
  • Misalignment.

Insulator defects are the reason of current leakage. This fact leads to a decrease in the discharge of spark between the electrodes. This already constitutes a hard start of the engine, low injectivity, jolts when moving from a place, unbalanced idling speeds.

lawnmower, spark, plug

Micro cracks are easy to find in the dark or in the darkness of the garage.

Electrode melting. a manifestation of serious overheating

Occurrence of rough, porous fouling. Occurs due to thermal stress, low octane gasoline, early ignition cycle. Distributor deficiencies (tramler) on other valves are not excluded. Mixing, ignition, and spark plugs are checked.

Influence of the gap on ignition

To understand how this factor affects the efficiency of the gasoline engine, it is necessary to briefly consider the work of the ignition system. The algorithm for converting fuel energy into mechanical work looks like this


  • Having expelled exhaust gases through a valve in the cylinder head, the piston moves downward. When it is at its lowest point, the second valve opens, from where the fuel-air mixture enters the cylinder.
  • The piston rises to the combustion chamber and compresses this aerosol several times.
  • When the bottom of the piston is at its top dead center, a high voltage pulse is applied to the spark plug electrodes. A spark flashes between them, igniting the compressed mixture of fuel and air.
  • When the fuel burns at a certain rate, it releases energy, pushing the piston down. The mechanical work is done.
  • The last stroke is the emission of combustion products into the exhaust system, after which the cycle repeats.

A high voltage pulse (over 20 kilovolts) is produced by a coil on the signal of the controller. Its winding is designed to produce a spark of a certain intensity which is sufficient for the ignition of a quality portion of the fuel mixture. If the gap in the spark plugs is too large or too small, the combustion process will be impaired.

lawnmower, spark, plug

A large gap between the electrodes

An abnormally large gap is considered to be over 1.3 mm. What happens if you bend the electrodes to this extent:

  • the power of the coil will not be enough to make the spark penetrate the increased distance with each compression stroke, there will be missed cycles;
  • Engine operation will be unstable, there will be vibration;
  • one part of unburned gasoline will drip down the cylinder walls and fall into the crankcase, the other will be thrown into the exhaust tract;
  • fuel consumption will increase;
  • the walls of the combustion chamber, the bottom of the piston and the working elements of the spark plugs quickly become soiled.

These processes result in the engine not producing its full power, and the piston-cylinder assembly wears out more quickly in the long term. too much clearance affects the quality of fuel combustion, you will feel it while driving: the acceleration of the car will be sluggish and the vibration of the engine will be increased. Because of skipping the cycles, there will be problems with cold starting. the engine will start with several attempts.

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Electrode spacing is too small

the distance between the spark plug pins less than 0.6 mm is considered unacceptably small. True, skipping work cycles will not be observed, but there will be another problem. too weak ignition. A short spark, breaking through a small gap from one electrode to the other, is not able to ignite the air-fuel mixture occupying the whole volume of the combustion chamber normally.

Due to the reduced speed of combustion, the air and gasoline aerosol does not have time to burn completely, as the stroke of the piston comes, and then. the emission of exhaust gases. As a result, the consequences described above occur. a drop in power of the power unit, increased fuel consumption, soot in the exhaust and soot inside the cylinders.

Improper adjustment of the spark plug gap is often exacerbated by other factors: wear of the piston group, problems in the fuel supply system, punctured insulation of high-voltage wires and so on. Then the problem of gap adjustment recedes into the background and often falls out of the motorist’s sight.

How to Clean a Spark Plug on a Chainsaw?

If you do not have a sandblaster, wipe the candle with a cloth, take a fine sandpaper and clean the electrodes to a shine, but without much fanaticism, then determine from the chainsaw manual what the gap between the electrodes should be and set it with a dipstick.

How to Gap a Spark Plug

You can adjust the gap by bending the outer electrode towards the inner contact to decrease the distance, or by bending the outer contact outward to increase the gap. After adjusting, the electrode spacing is checked again.

Do I need to adjust the gap on new plugs??

If the spark plugs fail they must be replaced with new ones, having previously cleaned the place where they are fastened with a dry and clean rag. Once new spark plugs are installed, the gap should also be checked and controlled.

How to check the spark on a chainsaw spark plug A very simple. Again, unscrew the spark plug, put the spark plug wire on it and make sure the spark plug housing (i.e. its skirt, where the thread is) is in good contact with the cylinder block. After that, you twist the starter arm like you do when starting. You should clearly see the spark.

Is it possible to adjust the gap on the spark plugs?

You can adjust the distance by bending the outer electrode toward the inner contact to reduce the distance, or by bending the outer contact outward to increase the clearance. After adjusting, check the electrode spacing again.

frequent fouling of the electrodes occurs if the gap is too large. The spark suffers from a large gap, and here there is also a black dense plaque. The ignition on such a candle may not work at all. It is important to periodically unscrew the spark plugs and change or clean them.

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