Cub Cadet ST426s Operator's Manual page 9

NOTE: Careless adjustments can seriously damage your unit. An
authorized service dealer should make carburetor adjustments.
If, after checking the fuel and cleaning the air filter, the engine still will not
idle, adjust the idle speed screw as follows:
1. Start the engine and let it run at a high idle for a minute to warm up. Refer
to Starting/Stopping Instructions.
2. Release the throttle trigger and let the engine idle. If the engine stops,
insert a small phillips or flat blade screwdriver into the hole in the air
filter/muffler cover (Fig. 30). Turn the idle speed screw in, clockwise, 1/8
of a turn at a time (as needed) until the engine idles smoothly.
NOTE: The cutting attachment should not rotate when the engine idles.
3. If the cutting attachment rotates when the engine idles, turn the idle
speed screw counterclockwise 1/8 of a turn at a time (as needed), to
reduce idle speed.
Checking the fuel, cleaning the air filter, and adjusting the idle speed should
solve most engine problems. If not and all of the following are true:
• the engine will not idle
• the engine hesitates or stalls on acceleration
• there is a loss of engine power
Have the carburetor adjusted by an authorized service dealer.
ROCKER ARM CLEARANCE
WARNING:
To avoid serious
personal injury, always turn the unit
off and allow it to cool before
cleaning or servicing it.
This requires disassembly of the engine. If
feeling unsure or unqualified to perform this,
take the unit to an authorized service center.
• The engine must be cold when checking or
adjusting the valve clearance.
• This task should be performed inside in a
clean, dust free area.
1. Remove the two (2) screws on back of the
engine cover and the one on the front of the
engine cover with a Flat-head or T-25 Torx
screwdriver (Fig. 31).
2. Remove the engine cover (Fig. 32).
3. Disconnect the spark plug wire.
4. Clean dirt from around the spark plug.
Remove the spark plug from the cylinder
head by turning a 5/8 in. socket
counterclockwise.
5. Clean dirt from around the rocker arm cover.
Remove the screw holding the rocker arm
cover with a large flat blade screwdriver or
Torx T-25 bit (Fig. 33). Remove the rocker
arm cover and gasket.
6. Pull the starter rope slowly to bring the
piston to the top of its travel, (known as top
dead center). Check that:
• The piston is at the top of its travel while
looking in the spark plug hole (Fig. 33)
• Both rocker arms move freely, and both
valves are closed
If these statements are not true, repeat this
step.
7. Slide the feeler gauge between the rocker
arm and the valve return spring. Measure the
clearance between the valve stem and
rocker arm (Fig. 35). Measure both the intake
and exhaust valves.
The recommended clearance for both intake
and exhaust is .003 – .006 in. (.076 – 0.152
mm). Use a standard automotive .005 in. (0.127 mm) feeler gauge. The
feeler gauge should slide between the r ocker arm and valve stem with a
slight amount of resistance, without binding. Figure 34 and 35 show how to
measure the clearance.
8. If the clearance is not within specification:
a. Turn the adjusting nut using a 5/16 inch (8 mm) wrench or nut driver
(Fig. 35).
• To increase clearance, turn the adjusting nut counterclockwise.
• To decrease clearance, turn the adjusting nut clockwise.
b. Recheck both clearances, and adjust as necessary.
MAINTENANCE AND REPAIR INSTRUCTIONS
Remove Screws
Fig. 31
Engine
Cover
Muffler
Fig. 32
Screw
Rocker Arm
Cover
Spark
Plug Hole
Fig. 33
Adjusting Nuts
INTAKE
Rocker
EXHAUST
Arms
Fig. 34
Exhaust Rocker Arm
Adjusting
Feeler
Nut
Gauge
0.003–0.006
in.
(0.076–0.152
mm)
Valve Stem
Fig. 35
9. Reinstall the rocker arm cover using a new gasket. Torque the screw to
20–30 in•lb (2.2–3.4 N•m).
10. Check the spark plug and reinstall. See Replacing the Spark Plug.
11. Replace the spark plug wire.
12. Reinstall the engine cover. Check alignment of the cover before
tightening the screws. Tighten screws.
REPLACING THE SPARK PLUG
Use a replacement part number 753-05784 or Champion® RDZ4H spark
plug. The correct air gap is 0.025 in. (0.635 mm).
1. Stop the engine and allow it to cool. Remove the two (2) screws on back
of the engine cover and the one on the front of the engine cover with a
Flat-head or T-25 Torx screwdriver (Fig. 31).
2. Grasp the plug wire firmly and pull the cap from the spark plug.
3. Clean dirt from around the spark plug. Remove the spark plug from the
cylinder head by turning a 5/8 in. socket counterclockwise.
WARNING:
Do not sand blast,
scrape or clean electrodes. Grit in the
engine could damage the cylinder.
3. Replace cracked, fouled or dirty spark plug.
Set the air gap at 0.025 in. (0.635 mm) using
a feeler gauge (Fig. 36).
4. Install a correctly-gapped spark plug in the
cylinder head. Turn the 5/8 in. socket
clockwise until snug.
If using a torque wrench torque to: 110-120 in.•lb. (12.3-13.5 N•m)
Do not over tighten.
CLEANING
WARNING:
To avoid serious personal injury, always turn the
unit off and allow it to cool before cleaning or servicing it.
Use a small brush to clean off the outside of the unit. Do not use strong
detergents. Household cleaners that contain aromatic oils such as pine and
lemon, and solvents such as kerosene, can damage plastic housing or
handle. Wipe off any moisture with a soft cloth.
STORAGE
• Never store the unit with fuel in the tank where fumes may reach an
open flame or spark.
• Allow the engine to cool before storing.
• Lock up the unit to prevent unauthorized use or damage.
• Store the unit in a dry, well-ventilated area.
• Store the unit out of the reach of children.
Short Term Storage (1-2 weeks)
1. Store the unit in a horizontal position. If this is not possible, store the unit
vertically with the engine at the top.
Long Term Storage
1. Drain all gasoline from the gas tank into a container. Do not use gas that
has been stored for more than 30 days. Dispose of the old gasoline in
accordance with federal, state and local regulations.
2. Start the engine and allow it to run until it stalls. This ensures that all
gasoline has been drained from the carburetor.
3. Allow the engine to cool. Remove the spark plug and put 5 drops of high
quality motor oil into the cylinder. Pull the starter rope slowly to distribute
the oil. Reinstall the spark plug.
NOTE: Remove the spark plug and drain all of the oil from the cylinder
before attempting to start the trimmer after storage.
4. Change the oil, referring to Changing the Oil. Dispose of the old oil in
accordance with federal, state and local regulations.
5. Thoroughly clean the unit and inspect for any loose or damaged parts.
Repair or replace damaged parts and tighten loose screws, nuts or bolts.
The unit is ready for storage.
0.025 in.
(0.635 mm)
Fig. 36
9