Yamaha YZF-R125 Service Manual: Measuring the compression pressure

NOTE:

Insufficient compression pressure will result in a loss of performance.

1. Measure:

Valve clearance Out of specification →Adjust.

Refer to "ADJUSTING THE VALVE CLEARANCE" on page 3-3.

2. Start the engine, warm it up for several minutes, and then turn it off.

3. Remove:

  • Rider seat
  • Right upper side cowling Refer to "GENERAL CHASSIS" on page 4-1.

4. Remove:

  • Fuel tank Refer to "FUEL TANK" on page 7-1.

5. Disconnect:

  • Coolant temperature sensor coupler "1"
  • Spark plug cap "2"

Measuring the compression pressure


6. Remove:

  • Spark plug
CAUTION:

Before removing the spark plug, use compressed air to blow away any dirt accumulated in the spark plug well to prevent it from falling into the cylinder.

7. Install:

  • Extension "1"
  • Compression gauge "2"

Measuring the compression pressure


8. Measure:

  • Compression pressure Out of specification Refer to steps (c) and (d).

a. Set the main switch to "ON".

b. With the throttle wide open, crank the engine until the reading on the compression gauge stabilizes.

c. If the compression pressure is above the maximum specification, check the cylinder head, valve surfaces and piston crown for carbon deposits.

Carbon deposits Eliminate.

d. If the compression pressure is below the minimum specification, pour a teaspoonful of engine oil into the spark plug bore and measure again.

Refer to the following table.


9. Remove:

  • Extension
  • Compression gauge

10.Install:

  • Spark plug

11.Connect:

  • Spark plug cap
  • Coolant temperature sensor coupler

12.Install:

  • Fuel tank Refer to "FUEL TANK" on page 7-1.

13.Install:

  • Right upper side cowling
  • Rider seat Refer to "GENERAL CHASSIS" on page 4-1.
    Checking the ignition timing
    NOTE: Prior to checking the ignition timing, check the wiring connections of the entire ignition system. Make sure all connections are tight and free of corrosion. 1. Remove: Rider seat L ...

    Checking the engine oil level
    1. Stand the vehicle on a level surface. NOTE: Place the vehicle on a suitable stand. Make sure the vehicle is upright. 2. Start the engine, warm it up for several minutes, and then turn it ...

    Other materials:

    Cleaning the air filter element
    NOTE: There is a check hose "1" at the bottom of the air filter case. If dust and/or water collects in this hose, clean the air filter element and air filter case. 1. Remove: Rider seat Refer to "GENERAL CHASSIS" on page 4-1. 2. Remove: Fuel tank Refer to "FUEL TANK" on page ...

    Checking the steering head
    1. Wash: Bearings Bearing races 2. Check: Bearings Bearing races Damage/pitting →Replace. 3. Replace: Bearings Bearing races a. Remove the bearing races from the steering head pipe with a long rod "1" and hammer. b. Remove the bearing race from the lower bracket ...

    Installing the throttle body
    1. Install: Throttle body joint clamps NOTE: Align the projections "a" on the throttle body joint with the slot "b" in each throttle body joint clamp. 2. Install: Throttle body joint NOTE: Align the projection "a" on the throttle body joint with the slot "b" in the intake mani ...

    2026 Copyright www.yam-r125.net |