Yamaha YZF-R125 Service Manual: Measuring the compression pressure

Yamaha YZF-R125 Service Manual / Periodic checks and adjustments / Engine / 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:

Checking the fuel sender
1. Drain the fuel from the fuel tank. 2. Check: Fuel sender resistance Out of specification → Replace the fuel sender. a. Connect the pocket tester ( × 10) to the fuel sender coupler as shown. Positive tester probe sky blue "1" Negative tester probe orange/white " ...

Checking the coolant temperature sensor
1. Remove: Coolant temperature sensor WARNING Handle the coolant temperature sensor with special care. Never subject the coolant temperature sensor to strong shocks. If the coolant temperature sensor is dropped, replace it. 2. Check: Coolant temper ...

Intake air temperature sensor
1. Check: Intake air temperature sensor resistance Out of specification Replace the throttle body. a. Connect the pocket tester ( × 1k) to the throttle body sensor assembly coupler as shown. Positive tester probe brown/white "1" Negative tester probe gray/black ...