How To Set Up Magneto On A Lawn Mower

Electrical equipment of internal combustion engines

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The principle of operation of magneto

Magneto is an alternating current apparatus (with excitation from permanent magnets), which combines a current source, transformer, chopper and distributor.

According to the device, magnetos are divided into the following main types:

1) with a fixed magnet and a rotating winding;

2) with a rotating permanent magnet and a fixed winding;

3) with a rotating magnetic switch, in which the magnet and the windings are stationary.

A magneto with a rotating magnet (Fig. 45) is used more often than other types, since they have a simpler device due to the lack of sliding contacts.

The magnetic flux of magneto is closed through an iron core 5, on which the primary 3 and secondary 4 windings are placed. When the rotor 6 is rotated, the magnetic flux generated by the primary current will change both in magnitude and direction.

Variable magnetic flux induces e. D.S in both windings of the core (e.M.S. Rotation). E.S. Rotation will reach a maximum at the moments of the highest rate of change of the magnetic flux (2 times per revolution of a bipolar magnet). E.S. Rotation in the primary winding of the core at high speeds reaches 50-100 in, and in the secondary 2000-3000 in. However, such an e. D.S clearly not enough to form a spark in the spark plug; besides, the spark created by her would not always slip exactly at the same given moment.

Fig. 45. Schematic diagram of ignition from magneto: 1. Capacitor; 2. Breaker; 3. Primary winding; 4-second winding; 5. Core; 6. Rotor; 7. Spark plug

To increase the secondary voltage and to be able to accurately ensure the moment of receiving a spark, a current chopper 2 is included in the primary circuit, the contacts of which close the primary circuit when e. D.S in the primary winding is close to zero.

After closing the contacts e. D.S in the primary winding begins to increase, this leads to an increase in current in it for a period of rotation of the armature by 90 °. The current in the primary winding reaches its maximum value when the rotor rotates by an angle slightly exceeding 90 °, i.E., with some delay from the maximum value of e. D.S idle move. When the breaker contacts open, the current in the primary circuit quickly drops to zero, and the energy of the magnetic field of the primary winding in this case passes into the electric energy of the spark on the spark plug 7. Thus, the magneto workflow is divided into the following stages: excitation of low-voltage alternating current in the primary winding, rupture of the primary circuit, cessation of current supply to the primary circuit and excitation of current in the secondary circuit, spark breakdown in the spark plug through the high voltage current distributor.

To get the maximum secondary voltage from magneto, it is necessary that the chopper open the primary circuit at the moment when the current induced in it reaches its maximum value. This occurs at a certain position of the rotor relative to the core. The angle determining the position of the magneto rotor at the moment of opening of the breaker contacts is called the magneto outline. Abris is set depending on the purpose of the magneto in the range of 7-14 °.

The current of the primary circuit of the ignition system from magneto and the intensity of the spark increase with increasing speed of the rotor. However, at high speeds of the rotor, this current will not increase, which is explained by a significant increase in the inductance of the winding with increasing current frequency.

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Any equipment, especially gardening, working in rather difficult field conditions, requires maintenance. Even its simple and minimal maintenance significantly increases the life of this equipment, and the work itself is made simpler and more hidden. And the engines start up “half a bump”, and work powerfully and without rattling and rattling.

Such a minimal service to its equipment can be provided by its owner himself, without the need to take it to a specialized service. Using the example of this article, I will talk about the necessary work that needs to be done on a gas mower. For other lawn mowers, everything will be about the same.

After last season and wintering in a cold shed, the lawnmower needs both cleaning and lubrication. External cleaning was done in the fall, before storage, but now it is necessary to carry out it more thoroughly and prepare the lawn mower for the battle with grass and weeds.

First of all, it is necessary to remove the protective covers from the engine of a gasoline lawn mower. This will provide easier access to the main nodes and facilitate the work. We will also be able to inspect the starter device, remove dust, dry grass particles and other debris from under the casing that interferes with normal cooling of the motor. And it, as you know, is airy and it is the cleanliness of the engine that affects its quality.

The engine cover (in my model MTD SP 48) is fastened with 4 self-tapping screws (pos. 1 in the photo). Pass the starter handle through the opening and remove the casing.

Now we can remove the starter device and gain access to the main engine components. To do this, unscrew the 3 screws that secure the starter starter and the flywheel fan cowl.

The starter is carefully cleaned of debris and inspect the starter cable. If it is damaged, you will have to change it to a new one, having disassembled the launcher itself. Light “disheveled” cable can be restored using any elastic glue (rubber, PVA). Those. One that would stick together the villi, but at the same time the place of gluing remained elastic.

Having removed the fan cowl, the cowl itself and the fan blades are thoroughly cleaned of debris using a hard brush with a long pile. Now let’s move on to the engine nodes.

Ignition system

The mower’s ignition system is designed like this. A permanent magnet is integrated in the engine flywheel. When it “flies” past the magneto poles (pos. 2 in the photo), the magnetic flux closes and an electromotive force is induced in the magneto coil. Using an electronic circuit, this pulse is shortened in time and a powerful pulse is formed, sufficient for electrical breakdown of the spark plug gap (item 1)

Namely, the quality of the spark provides ease of starting the lawnmower engine. Therefore, we do such operations.

First, unscrew the spark plug and inspect it. If in the previous season the lawnmower had to work hard, then it is better to immediately replace it with a new one, but if the old one still looks decent. The new one will still be better. Of course, first you need to check the gap between the electrodes of the candle. It is optimal to expose it by bending the L-shaped electrode to 0.5. 0.6 mm, no more. Otherwise, the magneto power may not be enough, and the spark will be either weak or it will not be at all.

Now let’s get magneto. If before that the lawn mower started without problems, then it will be a brain to clean and check the wires. If there were problems before, check the gap between the magneto and the flywheel. The smaller it is, the better the spark will be. But you can not press magneto close to the flywheel. There is an easy way to check and set such a gap. We take an aluminum beer can and cut a strip out of it. After loosening the magneto fastening and laying a strip of aluminum between its poles and the flywheel, we press the magneto and tighten the bolts. After that we take out a strip with pliers. I used this method many times and it always gave a good result.

Now we will examine the brake system. (item 3 in the photo). It is designed to quickly stop the engine when it is turned off. The brake pad is pressed against the flywheel of the motor, while simultaneously closing the ignition wire to ground.

Of course, both the block and the flywheel should be beaten clean. If necessary, the brake pad is replaced or adjusted. The cable and brake spring are also cleaned and lubricated. It is convenient to use grease in aerosol cans.

Changing the oil in the crankcase. It must be produced according to the instructions for the gas mower, but at least once a season, even if you mowed a little. Because the oil is oxidized and its lubricating properties deteriorate.

To drain old oil, the lawn mower is tilted towards the drain hole (item 1 in the photo), placing a block of wood or something else under the wheels. In order to prevent the old oil from draining into the protective casing, a small groove is cut out of the plastic bottle along which the old oil drains into the installed container without staining the casing. When all the oil drains, the hole is closed and new oil is poured through the oil filler neck to the required level. Use oil with a viscosity recommended by the manufacturer. Usually it is 10W40 for 4-stroke engines. It makes no sense to use more liquid and expensive grades, since the engine does not have an oil pump and lubrication is done by oil mist. And the operation of the lawn mower occurs at positive temperatures. And expensive oils show their properties only in extreme situations.

Assembly and installation of all casings is carried out in the reverse order.

The final stage of the “inspection” is the revision of the knives. At various forums one can meet mutually exclusive opinions, sharpen them or not sharpen them. I am a supporter of “do not sharpen.” Of course, if there are bulges and bends on them left by stones that you tried to mow, then they must be removed. But to sharpen the edge to razor sharpness does not make any sense. And the arguments here are. Firstly, for example, the trimmer’s fishing line is generally round. And nothing, he cuts the grass perfectly. Secondly, the grass itself is better when it is not cut, but plucked. The cut off grass tip usually turns yellow and dries. And nipped off. Quickly overgrows, healing the wound. And thirdly, no matter how you sharpen your knives, after half an hour of work they will go into their normal working condition. Those. It will be the edge of the metal with a rounding of 0.5 mm in diameter. In this state, they will be workers for a very, very long time. The knife cuts the grass with the force of impact, inertia, and not sharpness. Therefore, only the most visible burrs with a file or an abrasive bar should be removed from the knife, and this will be enough.

Well, that’s basically it, our lawn mower is ready for summer work.