Section 1:

Operation of the Motoplat Electrics

 1. All OSSA motorcycles are fitted with the Motoplat solid state electronic ignition system.  This system is comprised of four basic pieces.

(a) A rotating magnet called a magneto fly-wheel.

(b) A set of stationary coils, wires and diodes encased in epoxy resin. This is called a magneto stator.

(c) A device similar to an ignition coil called an electrical converter.

(d) A spark plug.

2. The magneto flywheel contains permanent magnets. When these magnets pass the low voltage coils on the magneto stator an AC current is formed in the coils.

3. The AC current formed in the low voltage coils then goes through a diode, which is a small electrical component that, among other things, allows current to pass through it in only one direction. Therefore, the current on the other side of the diode is DC current.

4. This DC current flows to the electrical converter or high voltage coil on the frame and charges a capacitor housed within the coil.

5. The capacitor is connected to a silicone controlled rectifier or Thyristor. This Thyristor will not allow the current to discharge from the capacitor until it is triggered by a separate voltage.

6. When it is time for the spark plug to fire, a special magnet in the flywheel passes by a 11 pickup" coil on the stator. This generates a small current in this coil and it flows to the Thyristor and triggers it, allowing the capacitor to discharge its current through the primary windings of the high voltage coil. As a result, a high voltage is formed in its secondary windings and the spark plug fires.

7. To regulate the time at which the spark plug fires, you simply rotate the magneto stator in one direction or the other on its mounting bosses. This changes the time at which the special magnet passes the pickup coil on the stator. See Part A, Chapter 3, Section 6, for details on regulating the engine timing.

8. On models fitted with lights, the magneto stator is also wired to provide charging current for the lighting system.

Section 2:

Testing the Ignition System

1. If the engine won't run, is difficult to start, or runs very poorly, and you suspect the ignition system, remove the spark plug from the engine. Fit the plug into the spark plug cap and ground the metal base of the plug against one of the fins on the cylinder head, as shown in Fig. 142. Operate the kick starter and watch and listen for a healthy spark. It should make a snapping sound. The engine should be rotated at least 500 R.P.M. to get a good spark.

2. If you get a weak spark or no spark at all, fit a new spark plug to the spark plug cap and check it again.

Page 115