The 1985-1990 Yamaha YZ125, YZ250, YZ490 motocross bike repair manual by Clymer, covers the following dirtbike models:

• Yamaha YZ125, 1985-1987
• Yamaha YZ250, 1985-1987
• Yamaha YZ490, 1985-1990

Book Excerpt: Yamaha YZ125, YZ250 1985-1987, YZ490 1985-1990 Repair Manual FUEL, POWER VALVE AND EXHAUST SYSTEMS

The fuel system consists of the fuel tank, shutoff valve, a single Mikuni carburetor and air cleaner. There are slight differences among the various models and they are noted in the various procedures.
AIR CLEANER The air cleaner must be cleaned frequently. Refer to Chapter Three for specific procedures and service intervals.
CARBURETOR OPERATION For proper operation, a gasoline engine must be supplied with fuel and air mixed in proper proportions by weight. A mixture in which there is an excess of fuel is said to be rich. A lean mixture is one which contains insufficient fuel. A properly adjusted carburetor supplies the proper mixture to the engine under all operating conditions. Mikuni carburetors consist of several major systems. Two floats and a float valve mechanism maintain a constant fuel level in the float bowl. The pilot system supplies fuel at low speeds. The main fuel system supplies fuel at medium and high speeds. Finally, a starter (choke) system supplies the very rich mixture needed to start a cold engine.

FRONT SUSPENSION AND STEERING

This chapter discusses service operations on suspension components, steering, wheels and related items. Specifications are listed in Table 1. Tables 1-3 are at the end of the chapter. Complete suspension tuning information is found in Chapter Twelve. Because the YZ models are competition bikes, care and attention given to the suspension components are vital to the overall safe handling and operation of the bike.
FRONT WHEEL Refer to Figure 1 when performing these procedures. REMOVAL

1. Place a milk crate or wood block(s) under the crankcase to lift the front wheel off the ground.
2. Loosen the front axle holders.
3. Unscrew the axle and remove it.
4. Remove the wheel.
5. Remove the left (Figure 2) and right (Figure 3) side axle spacers.
NOTE: Insert a piece of wood in the caliper (Figure 4) in place of the disc. That way, if the brake lever is inadvertently squeezed, the piston will not be forced out of the cylinder. If this does happen, the caliper might have to be disassembled to reseat the piston and the system will have to be bled. By using the wood, bleeding the brake should not be necessary when installing the wheel.
6. Install the axle spacer, washers and axle nut on the axle to prevent their loss when servicing the wheel or brake disc.

Subject: 1985, 1986, 1987, 1988, 1989, 1990 Yamaha YZ125, YZ250, YZ490 service, maintenance, repair. ISBN-10: 0892876093 | ISBN-13: 9780892876099 | Clymer M390