Yamaha F4B, Service Manual

Page: 94 / 202

7-1
POWR
Power unit
Power unit (check and
adjustment)
Checking the compression pressure
1. Start the engine, warm it up for 5 min-
utes, and then stop it.
2. Remove the clip from the engine shut-off
switch.
3. Disconnect the spark plug cap, and then
remove the spark plug.
NOTICE: Before
removing the spark plug, remove any
dirt or dust in the spark plug well that
may fall into the cylinder.
4. Install the special service tool
a
.
5. Fully open the throttle.
6. Pull the starter handle quickly several
times to crank until the reading on the
compression gauge stabilizes, and then
measure the compression pressure.
TIP:
When pulling the starter handle to crank the
engine, the compression pressure may vary
depending on the speed at which the starter
handle is pulled.
7. Remove the special service tool.
8. Install the spark plug. See step 4 in
“Installing the cylinder head” (7-37).
9. Connect the spark plug cap.
Checking the ignition timing
1. Install the special service tools
a
and
b
to the spark plug wire
a
.
2. Start the engine and warm it up until the
idle speed stabilizes at 1450–1550 r/min.
3. Check that the pointer
b
on the manual
starter is aligned with the specified
ignition timing mark on the flywheel
magnet.
TIP:
Set the digital tachometer to the 2-cycle 1-
cylinder mode because this engine ignites
the spark plug once per crankshaft rotation.
Checking the CDI unit air gap
Do not turn the flywheel magnet counter-
clockwise. Otherwise, the water pump
impeller could be damaged.
Compression gauge
a
: 90890-03160
Minimum compression pressure
(reference data):
700.0 kPa (7.00 kgf/cm 2 , 101.5 psi)
at 20 °C (68 °F)
a
Digital tachometer
a
: 90890-06760
Timing light
b
: 90890-03141
Ignition timing: BTDC 7–9° at 1500 r/min
b
a
a
b