The 1994-2001 Yamaha YZ125 motocross bikes repair manual by Clymer, covers models indicated.
Book Excerpt: 1994-2001 Yamaha YZ125 Repair Manual
TWO-STROKE LEAK DOWN TEST
CAPACITOR DISCHARGE IGNITION
Many owners of two-stroke bikes are plagued by hard starting and generally poor running, for which there seems to be no cause. Carburetion and ignition may be good, and compression tests may show that the engine's upper end is satisfactory.
What a compression test does not show is lack of primary compression. The crankcase in a two-stroke engine must be alternately under pressure and vacuum.
After the piston closes the intake port, further downward movement of the piston causes the entrapped mixture to be pressurized so it can rush quickly into the cylinder when the scavenging ports are opened.
Upward piston movement creates a slight vacuum in the crankcase, enabling the air/fuel mixture to be drawn in from the carburetor.
When servicing the CDI system, note the following:
The CDI unit is a sealed unit. Do not attempt to open it, as this causes permanent damage.
- Keep all connections between the various units clean and tight. Be sure that the wiring connectors are pushed and locked together firmly (Figure 2).
- Never disconnect any of the electrical connections while the engine is running. Otherwise, excessive voltage may damage the coils or the igniter unit.
- When kicking the engine over with the spark plug removed, make sure the spark plug is installed in the plug cap and grounded against the cylinder head (Figure 3). If not, excessive resistance may damage the igniter unit.
- The CDI unit is installed in a rubber vibration isolator. Make sure that the CDI unit is installed correctly. Handle the CDI unit carefully when removing and installing it from the frame.
Subject: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001 Yamaha YZ125 service, maintenance, repair. ISBN-10: 0892878398 | ISBN-13: 9780892878390 | Clymer M497-2