If You use a electronic ignition system watch out for the Zener Diode!! If the Zener Diode is badly grounded or have worn out, it could give more than 20 Volt!!, and make the electronic system to set the ignition timing too high with the result of a hole in the piston.
How to Check your Zener Diode.

Contact Breaker & Spark Plug.

The correct gap on Contact Braker is .015 in (.381 mm)
Spark timing, degrees fully advanced: 30
Spark plug: N4 ( MX N3 )


Maintenance and testing

The rectifier is a silicon semi-conductor device which allows currellt to flow in one direction only. lt is connected to provide full-wave rectification of alternator output Current.

The rectifier requires no maintenance beyond checking that the connections are clean and tight. The nuts clamping the rectifier plates together must not under any circumstances be slackened.
A separate nut is used to secure the rectifier and it is important to check periodically that the rectifier is firmly attached andI therefore well earthed.
When tightening the rectifier securing nut, hold the rectifier with a second spanner, for if the plates are twisted, the internal connections will be broken.

Testing the rectifier on the machine

(1) Disconnnect the Zener Diode by removing the straight Lucar connector with the Brown/ Blue cable from the 2 MC capacitor.

(2) Locate the snap connector junction for the Brown/Blue cable to the box and disconnect.

(3) Connect a D.C. voltmeter (with the 1 ohm load in parallel) with the red lead to earth and the Black lead to the Brown/Blue cable from the box.

(4) Locate the White/Yellow cable in the other snap connector junction from the box. and using a jumper lead connect the cable from the box to the negative (-) terminal of the battery.

(5) Start the machine and run at approximately 3.000 rev./min., and take a reading from the voltmeter. This should not read less than 7. 75 volts, which indicates the rectifier is operating satisfactorily. A lower reading indicates a fault in the rectifier which can be confirmed by a bench test.

(6) Stop engine before disconnecting the voltmeter.

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The capacitor is an electrolytic polarised unit, which will be irreparably damaged if incorrectly connnected.

Terminal identification

Looking at the terminal end of the unit, two Lucar terminals of different sizes will be observed, the small terminal being the positive earth terminal : for identitication the rivet has a red Spot. The double Lucar terminal is the negative connection. The basic object of using the electrolytic capacitor in the system is to enable the motorcycle to be run without a battery giving the rider the advantage of using the machine for competition work. and refittilng the battery for normal road use. If the battery should be disconnected and the machine run on capacitor ensure that the negative (brown/blue) lead is well insullated.

Periodic check

Disconnect the battery. Start and run the engine. Full lighting should be available.


If engine will not fire and run. Proceed to next check.

Efficiency check

(l) Disconnect the capacitor.
(2) Connect the capacitor direct to a 12 volt battery for 5 seconds (see polarity note).
(3) Disconnect the battery and let the charged capacitor stand for 5 minutes.
(4) Connect a D.C. voltmeter across the terminals (see polarity note) and note the steady reading after the initial swing, which should not be less than 9 volts for a serviceable unit.

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Zener Diodezener diode


The Zener diode is mounted within the electrical box at the front. Providing the diode is kept clean, to ensure maximum efficiency, no maintenance will be necessary. The "earthing" stud which secures the diode must not be subjected to a tightening torque greater than 24 - 28 lb./in. The electrical box lid acts as a heat sink and it is most important that the diode makes good contact with the mounting surface.

Checking performance of the Zener Diode

The battery should be fully charged before starting. If there is any doubt about the state of charge of the battery, it should be recharged before commencing the test. Isolate the Zener Diode by disconnecting all leads from the 2MC capacitor. Connect a D.C. voltmeter Black lead to the straight Lucar with the Brown/Blue cable and the voltmeter Red lead to earth. Connect a D.C. ammeter Red lead to the straight Lucar with the Brown/Blue cable, and the Black lead to the right angle Lucar with a Brown/Blue cable. Check that all electrical equipment other than the ignition is switched off. Start the engine and raise r.p.m. to approximately 3.000. Take a careful note of the readings. As the system voltage rises to 12.75 volts no reading should occur on the ammeter. The voltage will then continue to rise and after 12.75 volts the ammeter should start to read. The next check occurs when the ammeter rises to 2 amps, at this point the voltmeter should read between 13.5 and 15.5 volts.


The Zener Diode must be replaced if:-

(1) Current flow commences before 12.75 V is reached.

(2) Voltmeter registers more than 15.5 V. before 2 amps is shown on the ammeter.

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Electric drawing 96 kb in pdf file (original)
Electric drawing 148 kb in Power Point slides by John Pugh (Homemade)

Updated 2001-03-07
© Rickard Nebrér