MTD AC3 Professional Shop Manual
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MTD AC3 Professional Shop Manual

MTD AC3 Professional Shop Manual

4-cycle engines ac3 series
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Professional Shop Manual
AC3 Series of 4-Cycle Engines
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 2010 MTD Products Inc. All Rights Reserved

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Summary of Contents for MTD AC3

  • Page 1 Instructions, photographs and illustrations used in this publication are for reference use only and may not depict actual model and component parts. © Copyright 2010 MTD Products Inc. All Rights Reserved...

  • Page 3: Table Of Contents

    Chapter 1: Introduction ...1 Professional shop manual intent ........1 Fasteners .
  • Page 4 Chapter 6: Starters ...43 Recoil Starter Removal ......... . . 43 The starter rope, pulley and springs .

  • Page 5: Chapter 1: Introduction

    Professional Shop Manual intent This Manual is intended to provide service dealers with an introduction to the mechanical aspects of the AC3 series of 4-cycle engines. Disclaimer: The information contained in this manual is correct at the time of writing. Both the product and the infor- mation about the product are subject to change without notice.

  • Page 6: Assembly

    AC3.2 The AC3 version has an 8 to 1 compression ratio. The AC3.1 and the AC3.2 have a 9 to 1 compression ratio which increased power approximately 20%. The AC3.1 and 3.2 engines are also available with electric start versions.
  • Page 7 Engine cover splits front to back Figure 1.3 Aluminum clutch housing Figure 1.4 Plastic clutch housing Figure 1.5 • 29cc’s of displacement. • Engine covers split front and back. • Metal sump • 32 cc’s of displacement. • Aluminum clutch housing. •...

  • Page 8: Model And Serial Numbers

    ..F ... Month of production (F = June) ...29 ... Day of the month ...6 ... Last digit of the year ...D... Plant it was built in (MTD Southwest) ...Z ... Assembly line number ...0060 ... Number of unit built...

  • Page 9: Chapter 2: Maintenance

    MAINTENANCE The information in this manual applies to the AC3 series of engines. Some basic principles may apply to engines produced by other manufacturers. As the saying goes “an ounce of prevention is worth a pound of cure”. The same can be said about preventive maintenance on outdoor power equipment.

  • Page 10: Spark Plugs

    AC3 Series of engines Spark plugs The spark plug used in the 32cc trimmer is a Cham- pion RDZ19H gapped to 0.025” (.655 mm). See Figure 2.3. Wear rate will vary with severity of use. If the edges of the center electrode are rounded-off, or any other appar- ent wear / damage occurs, replace the spark plug before operating failure (no start) occurs.

  • Page 11: Air Filter

    Air filter A dirty air filter can reduce engine power, increase fuel consumption, increase CO emissions and make starting more difficult. The air filter should be cleaned every 10 hours of use (depending on area of use, dusty areas require more frequent cleanings).

  • Page 12: Spark Arrestor

    AC3 Series of engines Spark arrestor The spark arrestor should be checked and/or cleaned every 25 hours of use. NOTE: The spark arrestor also serves to keep blockages out of the exhaust system. Typical block- ages include insect nests built during the dormant season.

  • Page 13: Fuel Filer

    Fuel filer Figure 2.10 Figure 2.11 Fuel line Figure 2.12 A dirty fuel filter can result in a lean run condition. The fuel filter should be replaced every 25 hours of use. NOTE: The weighted fuel filter (clunk) keeps the filter sub- merged in the fuel at any angle of operation.

  • Page 14: Valve Lash

    AC3 Series of engines Valve lash To adjust the valves: NOTE: Loose valve lash on these engines can mimic the symptoms of a lean fuel/air mix- ture. Remove the engine cover and spark plug, following the steps described in the previous sections of this chapter.

  • Page 15: Chapter 3: Troubleshooting

    CHAPTER 3: TROUBLESHOOTING Definitions Troubleshooting - The act of gathering information by preforming tests and direct observations. Diagnosis - A theory of what the problem is, based on the information gathered by troubleshooting. Introduction Diagnosing an engine is an art form that is built on several factors. First and most importantly is a good under- standing of how the engine works.
  • Page 16 AC3 Series of Engines There are tools that the technician can use in order to define the problem, such as: Interview the customer. • Get a good description of their complaint. • If it is an intermittent problem, verify what conditions aggravates the problem as best as possible.

  • Page 17: Identify Factors That Could Cause The Problem

    Identify factors that could cause the problem This is the second step in the troubleshooting process. Crankshaft will not turn. • Hard to pull rope, steady pressure - This usually indicates a mechanical bind of some sort. the likely sus- pects are: A parasitic load from a jammed attachment or drive shaft.
  • Page 18 AC3 Series of Engines • Compression or ignition problem Check the engine stop and/or ignition switch. Test the ignition system using a proper tester. III. Replace the spark plug with a new one or a known good one. IV. Check compression.
  • Page 19 The muffler itself my be blocked. III. The spark arrestor may be blocked. IV. The exhaust valve may not be opening fully, possibly because of extremely loose valve lash set- tings. The exhaust valve seat may have come loose in the cylinder head. This may cause a loss of com- pression, a flow blockage or it may randomly alternate between the two.
  • Page 20 AC3 Series of Engines • Knock * Check for loose mounting of engine or driven implement * Rotate crankshaft back-and-forth to check for loose connecting rod. • Click * Half-engine speed clatter: loose valve lash. * Half-engine speed clatter, slightly heavier: wrist-pin.

  • Page 21: Repairing The Problem

    Repairing the problem The third step in the diagnostic process is to repair the problem. This step consists of: Form a diagnosis by using all of the information gathered from the troubleshooting that was performed. Physically perform the repair. The fourth, and hopefully final, step in the troubleshooting process is the follow through. This step consists of: Thoroughly test the repaired equipment: confirming that the initial diagnosis was correct.

  • Page 22: Prime Test

    AC3 Series of Engines Prime test To perform a prime test: 1. Prime the engine through the carburetor throat using a squirt bottle, filled with clean fresh gasoline. NOTE: Inspect the air filter while priming the engine. Look for a dirty or plugged filter that could prevent air flow or a missing filter that would indicate dirt ingestion.

  • Page 23: Compression Testing

    Compression testing To perform a compression test: NOTE: If the engine will run, start the engine and let it warm up first for a better reading. Gauge is zeroed Figure 3.1 NOTE: Most good quality compression gauge sets come with a 10mm adapter is needed to thread into the spark plug hole.
  • Page 24 AC3 Series of Engines...

  • Page 25: Chapter 4: Ignition

    Make sure the engine stop switch is in the “RUN” position. NOTE: Most stop switches used on MTD handheld products are spring loaded to the “RUN” position when it is released. This prevents no-start situations caused by the customer failing to turn the switch on.

  • Page 26: Testing The Module

    AC3 Series of Engines Testing the module To test the module: Remove the starter housing by following the proce- dures described in Chapter 6: Starter. Check the air gap for the module. Set it to 0.010” by following the steps described in the module section of this chapter.

  • Page 27: Test The Engine Stop Switch

    Test the engine stop switch Switch in the engine run position Figure 4.3 Figure 4.4 To test the engine stop switch: Open the grip by removing the five screws with a T-20 torx driver. Disconnect the wires from the engine stop switch. Connect an ohm meter or continuity light to the switch.

  • Page 28: Module

    AC3 Series of Engines Module Remove the ignition module: Remove the starter by following the procedures described in Chapter 6: starter. Disconnect the two wires from the module. See Figure 4.5. NOTE: The black wire will come off with the module mounting screws.

  • Page 29: Flywheel

    Flywheel Gently pry while striking Figure 4.7 To remove the flywheel: Remove the spark plug by following the steps described in Chapter 2: Maintenance. Remove the starter by following the procedures described in Chapter 6: starter. Remove the flywheel by striking the crankshaft with a brass punch.
  • Page 30 AC3 Series of Engines...

  • Page 31: Chapter 5: Fuel System And Carburetor

    CHAPTER 5: FUEL SYSTEM AND CARBURETOR The function of the fuel system is to store fuel, mix the fuel with air and deliver it to the combustion chamber. The fuel system consists of the following components: • Fuel tank • Fuel lines •...

  • Page 32: Inspecting The Fuel

    10% that is anticipated, the further the fuel/air mixture will be from the correct ratio. A 10% ethanol (E10) mix is acceptable for MTD engines. Anything higher than that will result in perfor- mance issues.
  • Page 33 Fuel System And Carburetor Generally these kits involve mixing a measured amount of water and gas together and seeing were the boundary layer is. See Figure 5.4. The test kit should come with a chart to compare the boundary layer height to alcohol percentage. Figure 5.4...

  • Page 34: Fuel Tank

    AC3 Series of Engines Fuel tank To remove/replace the fuel tank: Remove the air filter cover. Disconnect the fuel lines from the carburetor. See Figure 5.5. Remove the engine cover by removing the six screws that hold it in place. See Figure 5.6.

  • Page 35: Fuel Lines

    Blue line = return from primer to tank NOTE: MTD started using low permeation fuel lines on some of the AC3 series of engines in 2009. In 2011, all MTD hand held engines will have the low permeation fuel line to comply with EPA guidelines. The low permeation fuel lines can be identified by their black color.

  • Page 36: Carburetor

    AC3 Series of Engines Carburetor The AC3 series of engines uses the Walbro WYL series carburetors, or a carburetor of similar design from another manufacturer. The main distinguishing features of this carburetor design are: • It has a rotary throttle valve, not a butterfly valve.
  • Page 37 Throttle opening Figure 5.10 Throttle opening Figure 5.11 Throttle fully open Figure 5.12 In the second frame, the throttle has been rotated slightly (tip-in), admitting some air through the rotary valve. See Figure 5.10. At mid-range throttle, the throttle is rotated so that the bore is nearly aligned with the throat of the carbure- tor.
  • Page 38 AC3 Series of Engines In idle position, the throttle is closed, the hole through the drum is cross-wise to the throat of car- buretor and the drum is seated at the bottom of its travel. See Figure 5.13. • Throttle closed.

  • Page 39: Troubleshooting The Carburetor

    Troubleshooting the carburetor 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 blockage in the muffler. Carburetor function is more subtle. While specific problems with a carburetor can be identified on tear-down, identification of the carburetor as the location of the problem is usually done by process of elimination.
  • Page 40 AC3 Series of Engines 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 5.17. Inspect the throttle valve assemble for debris and freedom of movement.

  • Page 41: Disassembly Of The Carburetor

    Disassembly of the carburetor Remove these screws Figure 5.19 Inspect for dirt or damage Figure 5.20 Rust Figure 5.21 Fuel System And Carburetor Remove the four screws that hold the carburetor together. See Figure 5.19. Separate the carburetor, placing each part on the bench in the order they came apart.
  • Page 42 AC3 Series of Engines Remove the throttle valve assembly. See Figure 5.22. The orifice can also be seen from the pump side of the throttle housing. See Figure 5.23. The fuel pump in the carburetor is driven by vacuum pulses in the intake port. The impulse port from the spacer lets the vacuum pulses into the pump cam- ber.

  • Page 43: Re-Assembly Of The Carburetor

    Re-assembly of the carburetor Needle valve seat Figure 5.25 The lines show the paths of the different ports Filter screen Figure 5.26 Primer bulb Primer pump body Figure 5.27 Fuel System And Carburetor Place the carburetor in a clean area on the work bench.
  • Page 44 AC3 Series of Engines Clean the metering orifice with carburetor cleaner. Do not insert anything into the ! CAUTION ! CAUTION orifice to clean it. That will dam- age the orifice resulting in the carburetor being unrepairable. See Figure 5.28.

  • Page 45: Carburetor Insulator

    Carburetor insulator Press in the tabs Figure 5.30 Insulator block Figure 5.31 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.
  • Page 46 AC3 Series of Engines...

  • Page 47: Chapter 6: Starters

    Recoil Starter Removal Remove these screws Figure 6.1 Starter housing screws Figure 6.2 CHAPTER 6: STARTERS To remove the starter assembly: Remove the clutch following the steps described in the section on clutch removal. NOTE: The drive shaft will come off with the clutch hous- ing.

  • Page 48: The Starter Rope, Pulley And Springs

    The starter rope, pulley and springs The AC3 series of engines are equipped with the Assist Spring Technology (AST) starters.The AST system allows the customer to pull the recoil rope at any pace they please, yet still generate enough force to start the engine.
  • Page 49 Figure 6.5 Enclosed spring NOTE: the hook on the end of the spring seats in this notch Figure 6.6 NOTE: The recoil pulley assembly consists of three parts, but it is serviced as one assembly. The three parts are: the inner pulley, the starter assist (accumula- tor) spring, and the outer pulley.
  • Page 50 AC3 Series of Engines To assembly the starter pulley assembly: Install a new starter rope in the recoil pulley assembly. Remove the old rope from the pulley. Cut a piece of #3 1/2 (7/64”) starter rope 43” (109 cm) long.
  • Page 51 2 half hitch knots Figure 6.10 Deep well socket Figure 6.11 Feller gauge Figure 6.12 Secure the recoil handle to the rope using two half hitch knots. See Figure 6.10. Release the slip knot. 10. Install the pulley retainer using a deep socket to apply even pressure to the retainer while driving it into place.

  • Page 52: Electric Starter System

    AC3 Series of Engines Electric starter system For the 2009 model year, MTD introduced an electric start option for some of its hand held engines. The engines equipped with the electric start feature can be identified by the coupler in the sump cover.

  • Page 53: Chapter 7: Clutch And Upper Drive Shaft

    CHAPTER 7: CLUTCH AND UPPER DRIVE SHAFT Upper drive shaft assembly Press button while pulling lower attachment Loosen knob first Figure 7.1 If rounded or worn, replace the upper drive shaft housing Figure 7.2 Clutch and Upper Drive Shaft The drive shaft assembly consists of the drive shaft housing and a flexible inner drive shaft.
  • Page 54 AC3 Series of Engines Remove the six screws in the grip with a T-20 Torx ® driver. See Figure 7.3. Let the grip hang off to the side. NOTE: There are a half dozen parts inside the grip assembly. Be careful that they don’t fall out of the grip.

  • Page 55: Clutch Removal/Replacement

    Clutch Removal/replacement Clutch housing screws Figure 7.6 Insert at least 18” of rope in the hole Figure 7.7 Figure 3.0 NOTE: The upper drive shaft may be left connected to the clutch housing. Removing the three screws in the clutch housing will allow the clutch housing and the upper drive shaft to be removed as one.
  • Page 56 AC3 Series of Engines Remove the clutch rotor with a clutch removal tool. NOTE: Older engines will have a larger clutch that has roughly a 2 7/16” diameter. They can be removed with part number 791-180918 or the newer clutch tool part number 797- 00062.

  • Page 57: Chapter 8: Engine Assembly

    CHAPTER 8: ENGINE ASSEMBLY Economics and parts availability will determine how much engine disassembly is feasible. There are three main levels of engine disassembly. Short block installation. Short block installation consists of removing the engine, transferring the external parts to the replacement short block, and installing the engine.

  • Page 58: Engine Disassembly

    AC3 Series of Engines Engine disassembly NOTE: There are a few different paths that can be followed when disassembling an engine. This chapter will cover the removal of com- ponents in one order, but it is written so that the technician can go to the component being removed.
  • Page 59 Carefully remove the breather/PCV hose Figure 8.4 Throttle cable Figure 8.5 Press in the tabs Gasket Figure 8.6 Carefully disconnect the breather hose from the air filter housing. Disconnect the throttle cable. See Figure 8.5. 10. Remove the carburetor, fuel tank and lines as one assembly.
  • Page 60 AC3 Series of Engines Remove the carburetor insulator block using a T-25 torx driver. See Figure 8.7. Rotate the flywheel so that the magnets are away from the module. Remove the two screws that secures the ignition module to the block using a T-25 torx driver.
  • Page 61 Crankcase cover Figure 8.10 Fulcrum nut Figure 8.11 Press down and slide back Slot in keeper Grove on valve stem Figure 23.0 IMPORTANT: Check parts price and availability before doing any internal engine repairs. 18. Remove the four screws that secure the crankcase cover to the block using a T-20 torx driver.
  • Page 62 AC3 Series of Engines Slide the connecting rod off of the crank pin. NOTE: On engines equipped with the electric start feature, the crank pin will be longer so that it can fit into the coupler in the crank case cover.

  • Page 63: Engine Reassembly

    Engine Reassembly Crank pin Crank shaft arm Figure 8.15 Apply lithium grease Timing mark Figure 8.16 Tighten Figure 8.17 Rotate the crankshaft to top dead center (crank pin at the top). See Figure 8.15. NOTE: There are no timing marks on the crankcase. Install the cam onto the cam bracket.
  • Page 64 AC3 Series of Engines Install the valve springs and keepers: Place the spring over the valve stem. Set the valve keeper on top of the spring. While holding the valve in place, press down on the keeper with two fingers and slide it into the notch on the valve stem.
  • Page 65 Slide the connecting rod on first Figure 8.21 Figure 8.22 Push rods Figure 8.23 12. Rotate the piston so that the lettering on the connect- ing rod bearing faces the crank shaft journal when installed. NOTE: Apply a coating of Slick 50 the connecting rod bearing as a prelube.
  • Page 66 AC3 Series of Engines Install the crankcase cover so that the two dimples are toward the cylinder. NOTE: On engines equipped with the electric start feature, make sure the crank pin slides into one of the holes in the coupler attached to the crank case cover.
  • Page 68 MTD Products Inc - Product Training and Education Department FORM NUMBER - 769-02614A 07/202010...

This manual is also suitable for:

Ac3.1Ac3.2

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