Ignition System-Function And Service; Ignition Timing - Yamaha JT1 Series Service Manual

ELECTRICAL SYSTEM-Ignition System, Ignition Timing
5-3 Ignition System-Function and Service
1. Function
The ignition system consists of the components as shown in Fig. 5-3-1. As the flywheel
rotates, the contact breaker points begin to open and close, alternately. This make-and-
break operation develops an electromotive force in the ignition power source coil, and
produces a voltage in the ignition coil primary windings. The ignition coil is a kind of
transformer, with a 1 :50 turn ratio of the primary to the secondary winding. The voltage
(150-300 V) which is produced in the primary coil, is stepped up to 12,000-14,000 V by
mutual-induction, and the electric spark jumps across the spark plug electrodes.
High-tension wire
Ignition coil
Flywheel
Fig. 5-3-1
5-4 Ignition Timing
Remove the spark plug and screw the dial indicator holder into the plug hole. Next, insert the
dial indicator into the holder. Bring the piston up to T.D.C. and set the zero on the dial face to
line up exactly with the dial indicator needle. The crankshaft should then be turned backwards,
so that the piston travels down past 1.8 mm B.T.D.C. and slowly brought back up to precisely
1.8 mm B.T.D.C. (This removes any ,lack), Adjust the points so that they are just
beginning to open with the piston in this position. A low resistance point checker ( 100 Ohms
or less) should be used to determine the opening and closing of the ignition points.
Ignition Timing, 1.8 mm. B.T.D.C.
Maximum ignition point gap 0.3 to 0,4 mm. (0.012"-0.015")
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