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Preliminary Shop Handbook
AC3 Edger
MTD Products Inc - Product Training and Education Department
FORM NUMBER - 769-02775
10/2006
NOTE: These materials are for use by trained technicians who are experienced in the service and repair of outdoor power
equipment of the kind described in this publication, and are not intended for use by untrained or inexperienced individuals.
These materials are intended to provide supplemental information to assist the trained technician. Untrained or inexperi-
enced individuals should seek the assistance of an experienced and trained professional. Read, understand, and follow all
instructions and use common sense when working on power equipment. This includes the contents of the product’s Oper-
ators Manual, supplied with the equipment. No liability can be accepted for any inaccuracies or omission in this publication,
although care has been taken to make it as complete and accurate as possible at the time of publication. However, due to
the variety of outdoor power equipment and continuing product changes that occur over time, updates will be made to these
instructions from time to time. Therefore, it may be necessary to obtain the latest materials before servicing or repairing a
product. The company reserves the right to make changes at any time to this publication without prior notice and without
incurring an obligation to make such changes to previously published versions. Instructions, photographs and illustrations
used in this publication are for reference use only and may not depict actual model and component parts.
© Copyright 2006 MTD Products Inc. All Rights Reserved
1
1. INTRODUCTION ........................................................................................................................1
2. PERIODIC MAINTENANCE........................................................................................................2
3. INITIAL TROUBLESHOOTING...................................................................................................5
4. TROUBLESHOOTING THE IGNITION SYSTEM .......................................................................7
5. TROUBLESHOOTING THE FUEL SYSTEM..............................................................................9
6. TROUBLESHOOTING/ REPAIRING THE CARBURETOR......................................................10
7. BLADE AND BELT...................................................................................................................14
8. STARTER..................................................................................................................................16
9. FUEL SYSTEM..........................................................................................................................17
10. IGNITION SYSTEM...................................................................................................................19
Table of Contents
2
1
1. INTRODUCTION
For the 2007 season, MTD introduced a compact
edger. This edger uses the new AC3 four cycle engine
that was introduced in the spring of 2006.
Disclaimer: This handbook was written using a proto-
type. The information contained in this handbook is
correct at the time of writing. Both the product and the
information about the product are subject to change
without notice.
About the text format:
NOTE: is used to point-out information that is
relevant to the procedure, but does not fit as a
step in the procedure.
CAUTION: is used to point-out potential danger
to the technician, operator, bystanders, or sur-
rounding property.
• Bullet points: indicate sub-steps or points.
Disclaimer: This Preliminary Shop Handbook is
intended for use by trained, professional technicians.
• Common sense in operation and safety is
assumed.
• In no event shall MTD be liable for poor text
interpretation, or poor execution of the proce-
dures described in the text.
• If the person using this manual is uncomfortable
with any procedures they encounter, they should
seek the help of a qualified technician.
Figure 1.1
Fasteners:
• Most of the fasteners used on the blower are
sized in fractional inches. Some are metric.
For this reason, wrench sizes are frequently
identified in the text, and measurements are
given in U.S. and metric scales.
• If a fastener has a locking feature that has
worn, replace the fastener or apply a small
amount of releasable thread locking compound
such as Loctite® 242 (blue).
• Some fasteners like cotter pins are single-use
items that are not to be reused.
Other fasteners such as lock washers, retaining
rings, and internal cotter pins (hairpin clips) may
be reused if the do not show signs of wear or
damage. This manual leaves that decision to
the judgement of the technician.
Assembly:
Torque specifications may be noted in the part of the
text that covers assembly, they may also be summa-
rized in tables along with special instructions regarding
locking or lubrication.
Whichever method is more appropriate will be used. In
many cases, both will be used so that the manual is
handy as a quick-reference guide as well as a step-by-
step procedure guide that does not require the user to
hunt for information.
The level of assembly instructions provided will be
determined by the complexity and of reassembly, and
by the potential for unsafe conditions to arise from mis-
takes made in assembly.
Some instructions may refer to other parts of the man-
ual for subsidiary procedures. This avoids repeating
the same procedure two or three times in the manual.
AC3 Edger
2
2. PERIODIC MAINTENANCE
Periodic maintenance consists of:
• Changing the oil.
• Cleaning the spark arrester
• Checking and adjusting the valves.
• Changing the spark plug.
• Cleaning the air filter.
Oil Change
The oil on this engine should be changed after the first
10 hours of use and every 25 hours there after.
1. To change the oil in the AC3 engine you must
unscrew the oil fill plug in the crank case cover.
See Figure 2.1.
2. Drain oil into an approved container. Dispose of
oil in accordance with local regulations.
3. Fill the crankcase with 3.04 oz (90cc) of SAE 30
motor oil.
• The spark arrestor should be cleaned and the
valves should be adjusted at every oil change.
NOTE: The spark arrestor also serves to keep
blockages out of the exhaust system. Typical
blockages include insect nests built during the
dormant season.
Spark Arrestor
1. To clean the spark arrestor remove the engine
cover using a T-20 driver. See Figure 2.2.
2. Remove the two screws holding the spark arres-
tor cover in place with a T-25 driver.
See Figure 2.3.
Figure 2.1
Oil fill plug
metal sump
cover
Figure 2.2
Engine cover
Screws
Figure 2.3
Screws for
spark arrester
3
3. Inspect the spark arrestor. See Figure 2.4.
4. If it is blocked with carbon, remove it. It may be:
• Replaced
• Cleaned by mechanical means
• Solvent cleaned
• Burned clean using a butane or propane torch.
5. Reinstall the Spark arrestor screen and bolt
down the spark arrestor cover.
NOTE: Apply a thread locking compound such
as Loctite
®
266
TM
to the long bolt (the one with
a hex head, it is the one closest to the other two
hex head screws) and torque it to 50-55 in. lbs.
(5.7-6.2 Nm). Hand tighten the short screw.
Figure 2.4
Spark arrestor screen
Spark Plug
To remove the spark plug:
1. Unplug the spark plug wire.
CAUTION: Do not grab the spark plug wire with
pliers. Damage to the sparkplug boot will result.
A damaged spark plug boot will weaken the
spark of the spark plug.
2. Unscrew the spark plug with a 5/8” spark plug
socket.
Valve Lash
1. To adjust the valves remove the valve cover with
a T-25 driver. See Figure 2.5.
2. Rotate the crank shaft to bring the piston to top
dead center of the compression stroke (valves
closed).
• Use the starter rope to turn the crank shaft.
• Use a probe in the spark plug hole to track the
piston position.
3. The valve clearance for this engine is .003”-
.006”(.08-.15 mm) for both valves.
Figure 2.5
Valve cover
4
4. Check the valve lash by placing a feeler gauge
between the rocker and the valve stem. To
adjust the valve loosen or tighten the fulcrum nut
with a 8mm wrench until there is a slight drag on
the feeler gauge when you pull it out.
See Figure 2.6.
5. Inspect the gasket that the valve cover sits on for
damage. If it is damaged or compressed replace
it.
6. Install the valve cover.
7. Replace the spark plug with a Champion
RDZ19H, gasket seat plug.
NOTE: Torque the spark plug to 100-110 in.
lbs.(11.3-12.4 Nm).
NOTE: The spark plug gap is .025”(.655mm).
8. Install the engine cover.
NOTE: There are six screws for the engine
cover, three that go into the engine block casting
and three that go into the starter housing. Make
sure you put the machine threaded screws into
the casting and the plastic threaded screws into
the starter housing. See Figure 2.7.
Air filter
The air filter should be cleaned and oiled after every 10
hours of use (depending on area of use, dusty areas
require more frequent cleanings).
NOTE: If the edger has a paper element air filter,
Do Not oil it.
Figure 2.6
Feeler gauge
Fulcrum nut
Rocker
Figure 2.7
Plastic threaded
screw
Machine threaded
screw
5
1. To reach the air filter remove the cover.
1a. Remove the air filter cover by pressing in
the tab on the side of the filter housing
and lifting the cover off. See Figure 2.8.
2. Pull the filter out. See Figure 2.9.
3. If the filter is crumbling or brittle replace the filter.
4. Wash the air filter with warm soapy water. Let
the filter air dry. DO NOT wring the filter out.
Wringing the filter can tear it. Squeeze the filter,
but do not twist it. Put a 1/4 teaspoon (1.25cc) of
oil to the filter and squeeze it through out the fil-
ter.
5. Test run the engine before returning it to service
Figure 2.8
Air filter cover
Press tab
Figure 2.9
Air filter
3. INITIAL TROUBLE SHOOTING
The first step in diagnosing an engine problem is to
perform the periodic maintenance. A majority of the
poor run or no start conditions will be solved just by
doing the required maintenance.
1. Drain and inspect the fuel:
•Look for water.
• Look for dirt.
• Look for discoloration.
• Sniff carefully to see if it smells like varnish or oil.
• Save to show to customer.
2. Put fresh fuel in fuel tank.
3. Inspect the oil:
• Is there oil in it.
• What is the oil level?
• What type of oil is in it?
• If there is too much oil that could be a leaking
needle valve allowing gas to follow into the
engine.
• If the oil dirty, that would show poor maintenance
and possibly a worn engine.
• If the oil is low inspect the PCV valve for proper
operation.
• A PCV valve stuck closed will cause the engine
to smoke.
• A disconnected PCV valve will allow dirt into the
engine. See Figure 3.1.
Figure 3.1
PCV valve
6
4. Inspect the spark plug:
• What does the spark plug look like?
• A dry fowled plug could mean the engine is run-
ning rich.
• A wet fowled plug indicates the engine is burning
oil.
• A white coating on the plug would mean the
engine is running lean.
5. Inspect the air filter:
• What does the air filter look like? Dirt-blocked,
over-oiled, missing, wrong filter?
• Look for signs of dirt ingestion.
• A dirty air filter would restrict air flow.
• A missing air filter means that the engine has
ingested dirt that could damage the piston and
cylinder.
NOTE: Most gasoline engine diagnosis involves
isolating problems in the four critical factors an
engine needs to run properly:
• Ignition
- sufficient spark to start combustion in
the cylinder, occurring at the right time.
• Compression
- enough pressure in the cylinder to
convert combustion into kinetic motion. It also
needs sufficient sealing to generate the vacuum
needed to draw in and atomize the next intake
charge.
•Fuel
- correct type and grade of fresh gasoline; in
sufficient quantity, atomized (tiny droplets) and in
correct fuel/air proportions.
•Flow
- if all of the above conditions are met, but
the flow of air is constricted on the inlet or
exhaust side it will cause the engine to run
poorly or not at all.
6. To isolate the ignition system and compression
from the carburetor system:
6a. Prime the engine through the carburetor
throat using a squirt bottle.
6b. Verify that the engine stop switch is in the
engine run position. Most switches are
spring loaded to the run (1) position, but
some are not.
6c. Attempt to start the engine.
6d. If the engine starts and runs long enough to
burn the prime, the problem is effectively
isolated to the fuel system. proceed to the
Troubleshooting the Fuel System section
of this manual.
6e. If the engine fails to burn the prime, check
the compression.
6f. If compression is OK, check ignition system
as described in the Troubleshooting the
Ignition System section of this manual.
7. To perform a compression test:
NOTE: If the engine will run, start the engine and
let it warm up first for a better reading.
7a. Remove the spark plug.
7b. Screw the compression tester into the spark
plug hole.
7c. Pull the starter rope until the needle on the
compression gauge does not move any-
more.
7d. Read the gauge. See Figure 3.2.
NOTE: If the engine has less than 90 psi (6.2
bar) compression, the carburetor will not have
enough vacuum to draw fuel into the engine.
• The engine will have to be torn down to deter-
mine the cause of the low compression.
NOTE: If the engine has more than 125 psi (8.6
bar), the engine will not breath properly.
• Check the valve adjustment.
• Check for restrictions in the exhaust.
Figure 3.2
7
8. Check and adjust the valve lash by following the
procedures described in the section on periodic
maintenance.
9. Test run the engine before returning it to service.
4. TROUBLESHOOTING THE IGNITION SYSTEM
To test the ignition system:
1. Disconnect the spark plug wire.
2. Connect a spark tester to the spark plug wire.
3. Connect the other end of the spark tester to the
engine block. See Figure 4.1.
CAUTION: Never remove the spark plug and
hold it against the engine block to test for spark.
The fuel/air mix coming out of the spark plug
hole will catch on fire.
NOTE: The spark should be a minimum of 10 Kv
(10,000 volts) at pull over speed.
4. Pull the starter rope and watch the spark tester.
If sparks are seen in the spark tester, the ignition
system is working. Replace the spark plug.
• If no sparks are seen in the spark tester further
testing is required.
To test the module:
1. Remove the starter housing as described in the
“starter” section of this manual to expose the
module.
2. Check the air gap for the module. set it to .010
by following the steps described in the section
on the ignition system.
Figure 4.1
Spark tester
8
3. Disconnect the red wire. See Figure 4.2.
4. Install the starter as described in the “starter”
section of this manual.
5. Install the clutch as described in the ““clutch”
section of this manual.
6. Try to start the engine with the spark tester still
hooked up.
NOTE: If there is spark now, test the engine kill
switch and check the red wire for as short to
ground.
NOTE: If there still in no spark, hold a screw-
driver against the magnets on the flywheel to
feel if they are magnetic.
• If the magnets are good, replace the module. If
not replace the flywheel.
Test the engine stop switch:
1. Remove the grip by removing the six
screws with
a T-20 driver.
2. Disconnect the wires from the engine kill switch.
3. Connect an ohm meter or continuity light to the
switch.
4. With the switch in the engine run position (l), the
meter should indicate no continuity.
See Figure 4.3.
NOTE: Most stop switches are spring loaded to
the run position when it is released. this prevents
no-start situations caused by the customer fail-
ing to turn the switch on.
5. Hold the switch in the stop position (0). The
meter should indicate continuity. See Figure 4.4.
Figure 4.2
Disconnect the red wire
Figure 4.3
Switch in the engine
run position
Figure 4.4
9
NOTE: If the results are not as described, the
switch is bad and should be replaced.
6. If there is reason to suspect that the ignition tim-
ing is off:
6a. Remove the starter by following the steps
described in the section on starter
removal.
6b. Make sure the module air gap is correct by
following the steps described in the sec-
tion on the Ignition Repair.
6c. Inspect the pawls on the flywheel for dam-
age. Make sure they move freely.
6d. If the flywheel is damaged or the starter
pawls or sticking, replace the flywheel.
6e. Remove the flywheel.
6f. Inspect the flywheel key, If damaged
replace the key.
6g. Inspect the key way on the flywheel, if dam-
aged replace the flywheel.
6h. Inspect the key way on the crank shaft for
damage, if damaged short block the
engine.
7. Assemble and test run the engine before return-
ing it to service.
5. TROUBLESHOOTING THE FUEL SYSTEM
1. Drain and inspect the fuel:
•Look for water.
• Look for dirt.
• Look for discoloration.
• Sniff carefully to see if it smells like varnish or
oil.
• Save to show to customer.
2. Inspect the fuel filter. If it is dirty, replace it follow-
ing the steps described in the section on Fuel
System Repair in this manual. See Figure 5.1.
3. Inspect the fuel lines:
• Are they cracked?
• Are they clogged?
• Are they brittle?
• If the answer to any of the above is yes, replace
the fuel tank following the procedure described
in the section on the Fuel System Repair.
4. Test and inspect the primer bulb:
• Is it leaking or is it brittle?
• If so, replace the primmer bulb.
• Does it circulate fuel when pumped?
• If not, replace the primer.
5. If compression, ignition and fuel supply are OK,
but it does not run with fresh fuel repair/replace
the carburetor as described in the Carburetor
Repair section.
6. Test run the engine before returning it to service.
Figure 5.1
Fuel filter
10
6. TROUBLESHOOTING/REPAIRING THE CAR-
BURETOR
Typically, troubleshooting the carburetor is the last step
in the diagnostic process. The other factors are more
readily identified; spark vs. no spark, Specific pressure
readings on a compression gauge, or a visible block-
age in the muffler. Carburetor function is more subtle.
While specific problems with a carburetor can be identi-
fied on tear-down, identification of the carburetor as the
location of the problem is usually done by process of
elimination.
To troubleshoot the carburetor:
1. Inspect and clean the vent under the primer
body for debris. See Figure 6.1.
2. If the carburetor is running lean:
• Inspect the carburetor spacer for cracks.
• Inspect the spacer gasket.
NOTE: A cracked spacer or a leaking gasket
between the spacer and the cylinder could result
in a lean run or prevent the impulses from the
engine from driving the fuel pump.
See Figure 6.2.
3. Disassemble the carburetor. See Figure 6.3.
Figure 6.1
Primer body
Vent hole
Primer check
valve
Primer bulb
Figure 6.2
Gasket
Spacer
Figure 6.3
11
4. Inspect the throttle valve assemble for debris
and freedom of movement. See Figure 6.4..
5. If there is a problem with the throttle valve
assembly, check on availability and price of parts
to determine if the carburetor should be repaired
or replaced.
Disassemble the carburetor:
1. Inspect the diaphragms. If torn, damaged or brit-
tle install a diaphragm kit.
2. Inspect for dirt or varnish build up inside the car-
buretor. If there is a lot of dirt/varnish in the car-
buretor, replace it.
3. Check that the needle valve is set to the right
height. See the carburetor manufacturer for the
proper procedure.
4. Inspect the needle valve and seat.
5. If the seat is damaged, replace the carburetor.
6. If there is a minor amount of dirt/varnish in the
carburetor or debris and/or damage to the nee-
dle valve, install a rebuild kit.
To rebuild/repair a carburetor:
1. Remove the air filter housing as described in the
section on the fuel system.
2. Carefully remove the two fuel lines.
Figure 6.4
Throttle valve assembly
3. Disconnect the throttle cable by unhooking the
Z-fitting. See Figure 6.5.
4. Place the carburetor in a clean area on the work
bench.
5. Remove the four screws going through the
primer bulb housing with a #0 phillips screw-
driver.
6. Separate the carburetor, placing each part on
the bench in the order they came apart.
See Figure 6.6.
Figure 6.5
Unhook the
Z-fitting
Figure 6.6
12
7. Inspect the metering valve and the metering
valve seat for dirt and/or pitting. See Figure 6.7.
NOTE: If the metering valve seat is damaged
replace the carburetor. See Figure 6.8.
8. Inspect the Metering valve body, primer pump
body and the ventri housing for dirt and/or var-
nish.
NOTE: If there is a minor amount of dirt/varnish
in the carburetor it would be worth while to clean
and rebuild the carburetor by following the pro-
cedures recommended by the carburetor manu-
facturer.
NOTE: If there is a lot of dirt/varnish in the car-
buretor, replace the carburetor.
9. Clean the ventri and the metering valve hous-
ings.
10. Inspect the diaphragms for brittleness and/or
damage.
NOTE: The carburetor used in this manual is a
Walbro. Depending on the application the
engine may have a different carburetor. All car-
buretors have a manufacturer name cast on
them. It is advisable to contact the carburetor
manufacturer for the proper rebuild procedure.
11. Remove the throttle valve assembly.
See Figure 6.9.
12. Clean the metering orifice with carb cleaner.
CAUTION: Do not insert anything into the orifice
to clean it. That will damage the orifice resulting
in the carburetor being unrepairable.
See Figure 6.10.
Figure 6.7
Inspect for dirt or damage
Figure 6.8
Needle valve
seat
Figure 6.9
Throttle valve assembly
Idle stop
screw
Figure 6.10
Orifice from the
Venturi side
13
13. The orifice can also be seen from the pump side
of the throttle housing. See Figure 6.11.
14. The fuel pump in the carburetor is driven by vac-
uum pulses in the intake port. The impulse port
from the spacer lets the vacuum pulses into the
pump camber. See Figure 6.12.
15. Make sure this port is clean and free of debris.
16. Reinstall the Throttle valve assembly.
Figure 6.11
Orifice
Pump spring
Figure 6.12
Impulse port
Vent through metered
orifice to atmosphere
17. Set the needle valve lever as per the carburetor
manufacturer’s recommendations using a W-
tool. See Figure 6.13.
18. There are ports in the metering valve body. it is
important to make sure they are clean and free
of debris. See Figure 6.14.
19. Follow steps in reverse order to rebuild the car-
buretor.
20. Set the needle valve to the manufacturers’
specifications
21. Preform a needle valve pop off test by following
the carburetor manufacturer’s recommenda-
tions.
22. Install the carburetor on the engine.
23. Test run the engine before returning it to service.
Figure 6.13
Needle valve lever
Figure 6.14
The lines show the
paths of the different
ports
Filter screen
14
7. BLADE AND BELT
Blade removal/replacement
1. Remove the blade retaining nut with a 15/16”
socket and an impact wrench. See Figure 7.1.
NOTE: Production units may have a hex milled
into the spindle shaft to allow use of an allen
wrench to hold the shaft while removing the
blade.
2. Install the blade by following the above step in
reverse order.
Belt removal/replacement
1. Remove the blade as described above.
2. Remove the three screws holding the blade
guard in place using a T-40 Torx driver.
See Figure 7.2.
NOTE: The spindle hub will Come off with the
blade guard.
3. Remove the spindle pulley cover by removing
the three screws using a 3/8” wrench.
See Figure 7.3.
Figure 7.1
Blade retaining nut
Figure 7.2
Remove these three screws
Figure 7.3
Remove these three screws
15
4. Remove the belt cover by removing the three
screws using a T-20 Torx driver. See Figure 7.4.
5. Remove the belt guide using a 3/8” wrench.
See Figure 7.5.
Figure 7.4
Remove these three
screws
Figure 7.5
Belt guide
6. Remove the spindle pulley. See Figure 7.6.
7. The belt will now slide off.
8. Install belt by following the above steps in
reverse order.
Figure 7.6
Spindle pulley
16
8. STARTER REMOVAL
1. Remove the belt cover and belt following the
steps described in the Belt and Blade section.
2. Remove the spark plug.
3. Insert a long length of rope into the cylinder to
block the engine.
4. Remove the engine pulley using a 1” wrench.
See Figure 8.1.
5. Remove the three screws that go through the
frame in to the starter housing. See Figure 8.2.
6. The entire engine will now come off of the edger.
7. Remove the screws that hold the engine cover
to the starter housing. See Figure 8.3.
8. Remove the four screws in the front of the starter
housing. See Figure 8.4.
9. The starter will now slide off.
NOTE: If just the starter just needs a new starter
rope you can replace the rope; otherwise it is
more economical to replace the whole starter
housing assembly.
Figure 8.1
Engine pulley
Figure 8.2
Remove these three screws
Figure 8.3
Remove these
screws
Figure 8.4
screws
Starter housing
/