How to Set Magneto On a Trimmer

On gas trimmers, both two-stroke internal combustion engines (ICE) and four-stroke are used. But the ignition system in them is no different from each other, so the repair and adjustment of this fuel ignition system will be the same for different types of ICE.

How the trim system works

The ignition system of a two-stroke ICE consists of 2 elements: a flywheel and a coil (magneto). The image below shows how they look.

How to Set Magneto On a Trimmer

An impeller is located on the flywheel, which acts as a fan to cool the engine. Also on one side of the flywheel are permanent magnets, shown in the figure by an arrow.

When the flywheel rotates, an electric current is generated in the coil, which serves to produce a spark on the spark plug. A spark appears when the magnets are directly below the magneto.

Why do I need ignition adjustment

In some cases, when the engine does not start, ignition adjustment will be required. Of course, first you need to check the serviceability of the candle and high-voltage cable with a cap.

Ignition control on a two-stroke engine is necessary in order to ensure timely ignition of fuel in the engine combustion chamber. Below is a diagram from which you can understand the principle of the internal combustion engine.

A spark in the candle should appear at the moment of compression of the fuel by the piston, when it does not reach the top dead center (TDC) just a little. When the piston passes TDC, the fuel mixture ignites, as a result of which the piston goes down under the action of the explosion energy.

Therefore, if for some reason (mainly due to the displacement of the flywheel on the shaft relative to its initial position), the fuel ignites before the piston passes TDC, then it goes back and the crankshaft scrolls in the opposite direction. This movement can damage the starter assembly, cut off the starter cord, etc. In this case occurs early fuel ignition.

May also be late ignition on the trimmer: the piston after compression of the fuel, which did not ignite, goes down, and at this moment a spark appears. In this case, the engine will either not start, or it will significantly lose power and will not gain momentum well.

But such problems with early or late ignition on two-stroke ICEs are quite rare, since the flywheel with magnets is already installed in the correct position, which is perfectly synchronized with the movement of the piston. In addition, the flywheel is fixed to the shaft with a dowel and nut. Therefore, incorrect installation of the part is excluded.

How to check the health of magneto

Magneto is the main part in the engine ignition system and is a coil with primary and secondary windings located around the core. Often, due to a magneto malfunction, a spark disappears on the spark plug, and ignition of the fuel does not occur. To check the coil, you will need to use a tester, and perform the following steps.

  1. On the tester, going into resistance measurement mode, you need to set 200 Ohms. In this way, the primary winding will be checked.
  2. Connect one tester electrode to the “mass” of the coil, and the second to the contact coming out of the magneto. Normally, the resistance should be between 0.4 and 2 ohms.
  3. To check the secondary winding, you will need to set the switch of the device to 20 com. Further, one probe of the device is inserted into the cap of the candle, and the second is connected to the “mass”. Normally, resistance indicators should be in the range from 4-6 kom and above (depending on the model).

If you don’t have a tester, then you can use the following method, with great care so that the electrode does not touch the cylinder, otherwise the coil will burn out.

  1. Remove the cap from the candle. You do not need to twist the candle to maintain compression.
  2. Insert a nail without a cap into the cap. The nail should be of such a diameter that it holds well in the cap.
  3. Tie the cap with the nail to the cylinder using a dielectric so that the gap between the electrode and the cylinder body is between 5.5 and 7 mm (but by no means touches the “mass”).
  4. Using a starter, simulate the engine starting several times. The ignition button must be on.
  5. Observe whether the spark appears and what color it has.

Video: How to Set Magneto On a Trimmer

Normally, the spark should be strong, white or have a bluish tint. If the spark is weak, and its color is orange or yellow – this indicates a faulty magneto.

It should also be borne in mind that sometimes a cold magneto can produce a good spark. But after heating due to internal malfunctions, it ceases to produce current. As soon as the coil cools down, the trimmer starts again and works for a while until the magneto is again heated. Such a coil must be replaced.

Ignition Algorithm

Ignition on the trimmer is fairly easy to adjust.

    Remove the casing from the engine by unscrewing all the bolts holding it. For example, on the Patriot (Patriot) PT 2540 lawn mower, to remove the casing, you will need to unscrew the air filter housing and remove it from the latches with a flat screwdriver. After that, the plastic case can be easily removed.

  • Using a screwdriver, unscrew the 2 bolts (it is not necessary to completely unscrew) holding the magneto.
  • Turn the handwheel so that the magnets are at the top.
  • Further, to correctly set the best clearance between the flywheel and the coil, you can use the usual business card or pocket calendar. If you don’t have either one at hand, then to set the ignition, you can use a sheet of A4 paper, folding it 2 times (in the end, you will get a thickness of 4 sheets). This thickness will be enough to get the optimal clearance and adjust the ignition. Insert a business card or paper between the spool and magnets.
  • A magnetic field will pull the coil to the flywheel. Holding magneto with your fingers in this position, tighten the bolts holding it.
  • Remove the business card. After that, the ignition on the trimmer will be adjusted.
  • The principle of operation of magneto

    Before checking the coil and adjusting it, let’s understand how the unit works. When the magnet rotates, current is excited in the primary winding, which is closed using the contacts of an interrupt device. At the moment when the current strength in the primary section reaches its maximum value, the contacts open on the interrupt mechanism. Accordingly, this leads to a rupture of the primary current.

    As a result, the magnetic field created by the primary current also disappears. Due to changes in the magnetic field in the secondary section of the circuit, the formation of high voltage. This voltage can break through the entire gap between the spark plug electrodes. As the magneto rotor continues to rotate, a new spark appears.

    Rare magneto for cars 1920-1930 onwards

    Diagnostics of the technical condition of the unit

    As for the diagnosis, it is carried out as follows:

    1. First you need to connect the high voltage cable to the voltage output.
    2. The second end of the cable should be kept at a distance of about 0.5-0.7 cm from the device.
    3. In this position of the wire, it is necessary to sharply rotate the rotor in the direction of rotation. If the magneto is adjusted, then a spark should slip through the rotation of the rotor between the wire contact and the housing. If it is absent or too weak, barely noticeable, most likely, the device must be checked for malfunctions and, if necessary, adjusted.

    Typical faults and solutions

    Now consider the main malfunctions of magneto:

    1. Failures in sparking. There can be several reasons, as well as ways to solve them. This is the oxidation or burning of the contacts, violation of the clearance adjustment, wear of the arm cushion of the interrupt device, punched capacitor element. Defective items must be replaced, and the adjustable clearances should be adjusted. If the problem is in the contacts, they need to be changed or cleaned.
    2. Lack of spark. The reason may be a break in the wiring of the transformer, a short to ground, or breakdown of the insulation layer on the high-voltage cable. If the problem is in the transformer, then the node changes, if the circuit is closed, then it should be eliminated, and if the reason is a breakdown of insulation, then the cable just needs to be changed.
    3. If the spark is too weak, then most likely the reason is a punctured capacitor, which also needs to be changed.

    Photogallery “Mechanism device”

    Instructions for disassembling and assembling magneto

    To repair a magneto, it must be dismantled and disassembled, for this, follow these steps:

    1. First, the device is removed from the power unit.
    2. The unit must be thoroughly cleaned of dust, as well as traces of motor fluid, if any. Magneto will be dirty, so it must be cleaned. Do not allow dirt to get on the internal elements when disassembling the device.
    3. The next step will be parsing. Using a socket wrench, it is necessary to unscrew a nut which fixes the ignition timing automatic machine. Remove this element, and then remove the key from the groove.
    4. Then pull the latch a little to the side, after which you can dismantle the cover of the interrupter assembly. To remove, unscrew another four bolts that fix it.
    5. When the cover is removed, the rotor can be removed from the housing itself.
    6. The final step will be to unscrew the studs that secure the transformer assembly. Having done this, the transformer can be removed from the housing. Thus, you have gained access to the constituent elements of magneto. Now the mechanism is being repaired with the replacement of all failed components. For further assembly and installation of magneto, all the steps described above will need to be repeated in the reverse order.

    Adjustment Features

    Magneto is adjusted if the node cannot fulfill the functions assigned to it, while all the elements of the mechanism are intact. Magneto is tuned by measuring the gap between the contacts of the interrupting unit, while the crankshaft of the motor should be rotated by the flywheel. The shaft rotates until the contact divergence is greatest. Adjust the gap by loosening the bolt securing the contact strut and turning the strut of holes that is installed in the slot of the eccentric.

    When the gap is adjusted, it is necessary to test the mechanism – this will determine the correctness of the process. If everything was done correctly, then failures in sparking can be avoided.