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Chapter 9 to 10

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9. Description of ignition

There are conventional contact type magneto and recent contact less magneto (C.D.Ignition & T.C.Ignition).


Contact type magneto serves to achieve high voltage at primary. Contact type magneto serves to achieve high voltage at primary side coil by disconnecting electric current, generated at primary coil in magneto, at contact. By this, high voltage is generated at secondary side, and sparks are ignited at spark plug.

The C.D.Ig. (Capacity Discharge Type Contact-less Ignition) of contact-less magneto will store electric current generated at excited coil in capacitor. This stored current is applied to coil when voltage is caused at pulsar coil, instead of contact, to generate high voltage at secondary side by means of thyristor.

T.C.Ig. (Inductive Discharge Type Contact-less Ignition) Type is based on a contact type concept. Principle of operation is: High voltage can be generated at secondary side by disconnecting primary current through control unit incorporating semiconductors such as transistors, using voltage generated at primary side of ignition coil as an ignition signal. The features of the contact-less magneto are as follows:

Advantages:
(a) There are no troubles such as, damage, wear, burn out, or corrosion during storage.
(b) If turned, it will always generate sparks, and there is no limit on high speed rotation.
(c) Since sparks are strong because of high voltage, it is advantageous against fouling of spark plug, (especially C.D.Ig.).
(d) Since the discharging time is longer, the start-up and ignition are better (especially T.C.ig.).

Disadvantages:
(a) It is expensive compared to contact type.

10. Description of spark plug


1) Standard Plugs are specified depending on types of engines, heat range, thread diameter, reach, etc.

2) Burning of Plug

a. If color of ignition unit of electrode after operation is light brown or light gray, it is acceptable.
b. When ignition unit is black or damped, ignition temperature is too high. In that case, replace plug with a hot type plug with small heat range.
c. When ignition unit becomes white and abnormal particles adhere on its surface, it is recommended to replace plug with a cold type with a large heat range.
d. Causes of burn-out of plug
  * Fuel is too thin. * Insufficient cooling.
  * Operating under overload. * Too small heat resistance.
e. Causes of poor ignition
  * Choked too much. * Over-fuel.
  * Low-speed operation is continued excessively * Excessive heat resistance of plug.

3) Spark Gap
Adequate spark gap is 0.5 to 0.6 mm. If electrodes are worn, correct them to proper shape and dimension. If too wide, it leads to poor start-up and acceleration. If too narrow, it leads to fouling, bridging and unstable idling

4) Tightening Torques

Aluminum cylinder head 2.5 to 3.0 kg-m
Cast iron cylinder head 2.5 to 3.5 kg-m
(After tightening manually, tighten the plug 1/8 turn with a wrench.)


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【参】BSI-EN:フッターナビ