MTD 78 Series Shop Manual
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MTD 78 Series Shop Manual

277/357/420cc horizontal shaft engines
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Professional Shop Manual
78/83/90 Series Horizontal Shaft Engines
(277/357/420 cc 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 78 Series

  • 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

    Table of Contents Chapter 1: Introduction Professional Service Manual Intent ....... . 1 Safety .
  • Page 4 Chapter 4: The Fuel System and Governor Inspecting the fuel ..........37 Test fuel for alcohol .
  • Page 5 Chapter 7: Ignition System Troubleshooting the ignition system ....... 81 Stop switch .

  • Page 7: Chapter 1: Introduction

    • Common sense in operation and safety is assumed. • In no event shall MTD be liable for poor text interpretation or poor execution of the procedures 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 or MTD Technical Support.
  • Page 8 78/83/90 Series Horizontal Shaft Engines • Be prepared in case of emergency: ! CAUTION ! CAUTION Keep a fire extinguisher nearby Keep a first aid kit nearby Keep emergency contact numbers handy • Replace any missing or damaged safety labels on shop equipment. •...

  • Page 9: Fasteners

    Introduction Fasteners • Most of the fasteners used on the MTD engine are metric. Some are fractional inches. 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 releas- able thread locking compound such as Loctite®...

  • Page 10: Model And Serial Number

    See Figure 1.1. Front of engine NOTE: The serial number will always start with the model number. Model /serial number Figure 1.1 MTD Horizontal Engine Model Designators 1 6 1 - S H A Starter/Alternators Major Revision 1=Recoil start Change...

  • Page 11: Maintenance

    Cleaning the spark plug: NOTE: MTD does not recommend cleaning spark plugs. Use of a wire brush may leave metal deposits on the insulator that causes the spark plug to short out and fail to spark. Use of abrasive blast for cleaning may cause damage to ceramic insulator or leave blast media in the recesses of the spark plug.

  • Page 12: Air Filter (Summer Engines)

    Foam pre-filter ground is covered with snow. Air filters used on the MTD engine are designed to prevent particles larger than 3-5 micron from pass- ing through into the engine. Figure 1.3 The filter should be checked on a regular basis pos- sibly several times in a season.

  • Page 13: Oil Type And Capacity

    Oil type and capacity MTD engines use oil with a SF/SG API rating or better is the recommended oil for this engine. The winter engines use SAE 5W-30 oil and the summer engines use SAE 10W-30 oil. The oil capacity for all of the 78/83/90 series engines is 37 fl.oz (1.1 liters).

  • Page 14: Changing The Oil

    78/83/90 Series Horizontal Shaft Engines To check the oil with a threaded dip stick: Do not thread Twist and remove the dip stick from the engine. dipstick in to check the oil level Clean the oil off of the tip of the dipstick. Re-insert the dipstick without threading it in to get the oil level reading.

  • Page 15: Fuel

    This corrodes the metal components of the fuel system, especially the carburetor. Alco- hol also does not produce as much heat as gasoline when burnt. This results in less power for the engine. A 10% alcohol mix (E10) is acceptable for MTD engines. Anything higher than that will result in performance issues.
  • Page 16 78/83/90 Series Horizontal Shaft Engines To replace the fuel filter: Front fuel tank shroud To avoid personal injury or property ! W A R N IN G ! W A R N IN G damage, use extreme care in han- dling gasoline.

  • Page 17: Valve Lash

    Introduction Valve lash Valve lash is the clearance between the top of the valve stem and the rocker arm. The valve lash should be checked after the first 25 hours of use and every 100 hours after that. Valve lash can be checked and adjusted using the following steps: Spark plug socket If the engine has been run, allow it to cool thoroughly.
  • Page 18 78/83/90 Series Horizontal Shaft Engines Probe to confirm piston Slowly pull the starter rope until air can be heard is at top of travel coming out of the spark plug hole. Confirm that the piston is at Top-Dead-Center on Valves closed the compression stroke.

  • Page 19: Cleaning The Engine

    Introduction Double-check the clearance after tightening the jam nut, to confirm that it did not shift. Re-adjust if necessary. Rotate the engine through several compression cycles: • Observe the movement of the valve gear. • Return the piston to TDC compression stroke and re-check the valve lash to confirm consistent movement of the valve gear, including the slight bump to the exhaust valve from the automatic compression release.

  • Page 20: General Torque Specifications

    78/83/90 Series Horizontal Shaft Engines General torque specifications Maintenance Chart...

  • Page 21: Frequently Used Specifications

    Introduction Frequently used specifications Minimum Maximum Intake valve lash 0.004 0.10 0.006 0.15 0.004 0.10 0.006 0.15 0.004 0.10 0.006 0.15 Exhaust valve lash 0.006 0.15 0.008 0.20 0.006 0.15 0.008 0.20 0.006 0.15 0.008 0.20 Spark plug gap 0.026 0.65 0.030 0.75...
  • Page 22 78/83/90 Series Horizontal Shaft Engines...

  • Page 23: Chapter 2: Basic Troubleshooting

    Also check attachments for damage and make sure they are firmly mounted. Steps to troubleshooting NOTE: The steps and the order of the steps that follow are a suggested approach to troubleshooting the MTD engine. The technician does not necessarily have to follow them as described in this chapter...

  • Page 24: Identify Factors That Could Cause The Problem

    78/83/90 Series Horizontal Shaft Engines IV. Unusual exhaust tone There are tools that the technician can use in order to define the problem, such as: Interview the customer. 1a. Get a good description of their complaint. 1b. If it is an intermittent problem, verify what conditions aggravate the problem as best as possible. 1c.
  • Page 25 ASIC ROUBLESHOOTING D. Rope jerks back. This usually indicates that the piston is stopping before top dead center on the com- pression stroke and is being driven back down by compression or combustion. The likely suspects are: Compression that is unusually high. a.
  • Page 26 78/83/90 Series Horizontal Shaft Engines VI. Check valve timing/actuation. VII. Check exhaust. Starts, runs poorly 3a. Starts, then dies Run the engine with a spark tester in-line between the spark plug wire and the spark plug or use an oscilloscope and see if the spark goes away at the same time the engine dies. II.
  • Page 27 ASIC ROUBLESHOOTING b. Intake blockage • An intake blockage up-stream of the carburetor will cause a rich fuel/air mixture and constrict the amount of air that the engine can draw in, limiting performance. • The intake valve not fully opening. A possible cause of this is loose valve lash. V.
  • Page 28 78/83/90 Series Horizontal Shaft Engines • Low octane fuel • Over-heating engine (check for blocked cooling air flow) • Carbon build-up in cylinder: glowing carbon chunks pre-igniting air fuel mix. d. Chirp • Compression, blowing-by the fire-ring of a damaged head gasket will sometimes produce a chirping noise.

  • Page 29: Repairing The Problem

    ASIC ROUBLESHOOTING Repairing the problem The third step in the troubleshooting process is to repair the problem. This step consists of: A. Form a diagnosis by using all of the information gathered from the troubleshooting that was performed. B. Physically perform the repair. The fourth, and hopefully final, step in the troubleshooting process is the follow through.

  • Page 30: Prime Test

    78/83/90 Series Horizontal Shaft Engines Prime test To perform a prime test: Prime the engine through the carburetor throat using a squirt bottle, filled with clean fresh gasoline. Make sure the throttle is in the run position and the safety key if fully inserted. Attempt to start the engine.
  • Page 31 ASIC ROUBLESHOOTING Compare the results to the following chart. Leak-down Testing Results Symptom Possible cause Air escaping from Worn cylinder or piston rings. the breather Possible blown head gasket Air escaping from Leaking exhaust valve the exhaust Air escaping from Leaking intake valve the carburetor Gauge reading...

  • Page 32: Compression Test

    78/83/90 Series Horizontal Shaft Engines Compression test To perform a compression test: NOTE: Compression should be in the range of 40-75 PSI (2.8-5.2 Bar). • Disconnect the high-tension lead from the spark plug and ground it well away from the spark plug hole. •...

  • Page 33: Pcv Testing

    The cause of this oil burning can be mistaken for a worn-out engine, if proper diagnosis (compression, leak- down, and case pressure) is not performed. Experimentation by MTD’s Training and Education Department has revealed the following characteristics of MTD engines: •...
  • Page 34 78/83/90 Series Horizontal Shaft Engines...

  • Page 35: Chapter 3: Air Intake Systems

    CHAPTER 3: AIR INTAKE SYSTEMS MTD builds horizontal crank engines for snow blowers and chore performers. The differences between snow engines and chore engines are the muffler and the air intake system. Therefore the air intake system for the snow and chore engines will be discussed separately, as will the mufflers in a later chapter.

  • Page 36: Air Filter (Summer Engines)

    Paper-pleated element pleated-paper element or foam. A combination of the two are used on the MTD engine. • Air filters used on the MTD engine are designed to prevent particles larger than 3-5 micron from passing through into the engine. •...
  • Page 37 NTAKE YSTEMS Install the air filter by following the previous steps in reverse order. NOTE: When installing the air filter, the hole in the bottom of the paper element must fit over the riser in the air filter base. See Figure 3.6. Riser Air filter base Figure 3.6...

  • Page 38: Air Filter Base And Control Panel (Summer Engines)

    78/83/90 Series Horizontal Shaft Engines Air filter base and control panel (summer engines) To remove/replace the air filter base and control panel: Remove the air filter following the steps described in the previous section. Remove the three screws that hold the air filter base to the intake elbow.
  • Page 39 NTAKE YSTEMS NOTE: All summer engines built on or after January 1, 2011 will have a hose from the evaporative emis- sions (EVAP) system attached to the control panel above the breather hose. See Figure 3.10. Breather hose Pull the control panel straight off the front of the engine.

  • Page 40: Carburetor And Insulator

    78/83/90 Series Horizontal Shaft Engines Carburetor and Insulator Siphon the fuel out of the fuel tank. Remove these screws To avoid personal injury or prop- ! WARNING ! WARNING erty damage, use extreme care in handling gasoline. Gasoline is extremely flammable and the vapors are explo- sive.
  • Page 41 NTAKE YSTEMS Remove the Heat box (snow engines) or the control panel (summer engines) by following the steps described in appropriate section of this chapter. Disconnect the throttle linkage and spring. See Figure 3.13. Slide the carburetor off of the mounting studs. Remove the carburetor.
  • Page 42 78/83/90 Series Horizontal Shaft Engines NOTE: An insulator block separates the carburetor from the cylinder head. There is a gasket on each side of the insulator. See Figure 3.16. NOTE: The gaskets are different, and there is an orientation to the insulator. •...

  • Page 43: Chapter 4: The Fuel System And Governor

    YSTEM AND OVERNOR CHAPTER 4: THE FUEL SYSTEM AND GOVERNOR The function of the fuel system is to store fuel, mix the fuel with air in the correct ratio and deliver it to the intake port. The fuel system consists of the following components: •...

  • Page 44: Test Fuel For Alcohol

    This corrodes the metal components of the fuel system, especially the carburetor. Alcohol also does not produce as much heat as gasoline when burnt. This results in less power for the engine. A 10% ethanol (E10) mix is acceptable for MTD engines. Anything higher than that will result in performance issues.

  • Page 45: Fuel Tank Vent

    YSTEM AND OVERNOR Fuel tank vent The fuel tank vent performs the important task of allowing air into the fuel tank. As fuel is being used by the engine, the fuel level in the tank drops. The dropping fuel level then creates a vacuum in the tank. If the fuel tank could not draw air through the vent, the vacuum would prevent the fuel from getting to the carburetor.

  • Page 46: The Fuel Filter

    78/83/90 Series Horizontal Shaft Engines The fuel filter The fuel filter is located in the fuel tank nipple. It can be removed and cleaned with a can of carb cleaner or it can be replaced. See Figure 4.4. NOTE: If cleaning a filter, back-flush it by spraying the carb cleaner through the barb end and out of the screen.

  • Page 47: The Fuel Tank

    YSTEM AND OVERNOR The fuel tank To remove the fuel tank: Drain the tank. Remove the four screws that secure the fuel tank using a 12 mm wrench. See Figure 4.5. Remove these screws Figure 4.5 Lift up on the fuel tank and disconnect the fuel line. See Figure 4.6.

  • Page 48: Evaporative (Evap) Emissions System

    Effective January 1, 2011; the EPA has mandated that all summer engines with 225cc’s of displacement or more must be equipped with an evaporative emissions system. The EVAP system used by MTD consists of: • A charcoal canister Roll over valve •...
  • Page 49 YSTEM AND OVERNOR This system operates as follows: Unvented The gasoline evaporate and lets off vapors. The vapors exit the fuel tank through the roll over valve vent. NOTE: The fuel cap used with the charcoal canister sys- tem is not vented. If a vented cap is used, the EVAP system will not work.

  • Page 50: Troubleshooting The Evap System

    78/83/90 Series Horizontal Shaft Engines Troubleshooting the EVAP System There are four failure modes for the EVAP system: • A leak in the system - will allow dirt ingestion. • A blockage in the system (between the canister and the fuel tank) - acts a plugged vent. •...
  • Page 51 YSTEM AND OVERNOR Testing the roll over valve The roll over valve vent has two functions. The first function is to vent the tank and the second function is to close off the vent if the tank is inverted. Test the roll over valve by: Hose to roll over valve Disconnect the hose that runs from the charcoal canister to the roll over valve.

  • Page 52: Charcoal Canister

    78/83/90 Series Horizontal Shaft Engines Charcoal canister To remove/replace the charcoal canister: Disconnect the hoses from the canister. See Figure 4.15. Hoses Figure 4.15 Remove the screw from the canister mounting strap using a 10 mm wrench. See Figure 4.16. Screw Figure 4.16 To Install the canister:...

  • Page 53: Roll Over Valve

    YSTEM AND OVERNOR Roll over valve To remove the roll over valve: Gently pry the grommet from in between the roll over valve and the fuel tank. See Figure 4.18. Grommet NOTE: Leaving the hose connected to the roll over valve will help prevent it from falling inside the tank once the grommet is removed.

  • Page 54: Choke

    78/83/90 Series Horizontal Shaft Engines Choke 78/83/90 series MTD engines are equipped with a choke. The snow engines are also equipped with a primer, both of which must be used to start the engine. NOTE: The choke should be opened after the engine starts. This can be a source of starting issues with cus- tomers who are not familiar with manual chokes.
  • Page 55 YSTEM AND OVERNOR The choke rod is part of the heat box assembly mounted on the front of the carburetor. See Figure 4.22. Choke rod Heat box Figure 4.22 NOTE: The choke rod can be bent slightly to facilitate adjustment. To adjust the choke rod: Remove the choke knob and the engine shroud by following the steps described in Chapter 3: Air Intake...

  • Page 56: Primers (Snow Engines)

    Primers (snow engines) MTD engines use a dry bulb primer. This means that there is no fuel in the primer bulb. The primer works by pushing air into the float chamber of the carburetor when the primer bulb is depressed. This will force fuel to be sprayed out of the main nozzle into the throat of the carburetor.

  • Page 57: Carburetors

    YSTEM AND OVERNOR Carburetors If diagnosis indicates a fuel problem, inspect the carburetor. This is important even if problems are identified elsewhere in the fuel system. IMPORTANT: the fuel must be tested for alcohol content before diagnosing anything else on the engine. NOTE: It is important to perform a compression or leak down test before condemning a carburetor.

  • Page 58: Disassembly And Rebuilding Of The Carburetor

    78/83/90 Series Horizontal Shaft Engines Disassembly and rebuilding of the carburetor Clamp-off the fuel line to prevent fuel spillage, then disconnect the fuel line from the carburetor. Disconnect the primer hose. Remove the carburetor by following the steps described in Chapter 3: Air Intake and Filter. NOTE: An insulator separates the carburetor from the cylinder head.
  • Page 59 YSTEM AND OVERNOR Float bowl Remove the pin that the float hinges on to remove the float. Drain bolt NOTE: The float is not adjustable. Spring tension against the float valve begins to build from the horizontal position, putting progressively more pressure between the tip of the valve and the seat.
  • Page 60 78/83/90 Series Horizontal Shaft Engines Emulsion air port: pilot jet NOTE: The main jet secures the emulsion tube in the central column of the carburetor. Emulsion tube See Figure 4.30. Main jet Emulsion air port: main jet Figure 4.30 Throttle stop screw The throttle stop screw has a large pliable lip around Welch plug...
  • Page 61 YSTEM AND OVERNOR End view 12. Examine the metering plug: See Figure 4.33. • Fuel, drawn from the central column via the long O-rings fuel feed leg, is metered by the brass orifice in the tip of the metering plug. •...

  • Page 62: Fuel Shut-Off Valve

    78/83/90 Series Horizontal Shaft Engines Fuel shut-off valve To prevent the needle valve from un-seating and flooding the engine while being towed, engines designed to go onto log splitters have a fuel shut-off valve built into the carburetor. To service the shut-off valve: Remove the carburetor by following the procedures described in Chapter 3: Air Intake Systems.
  • Page 63 YSTEM AND OVERNOR 6.Remove the fuel shut-off lever. See Figure 4.38. Fuel shut-off lever Figure 4.38 NOTE: There is a passage cast into the shut-off lever. Fuel passage When this passage lines up with the two posts in the carburetor, fuel will flow to the needle valve. Figure 4.39 Remove the rubber gasket.

  • Page 64: Governor

    78/83/90 Series Horizontal Shaft Engines Governor The engine speed is controlled by a balance between the force applied by a spring (pulling the throttle open) and Spring tension a flyweight mechanism within the engine applying force to the governor arm (pushing the throttle closed). See Figure 4.41.

  • Page 65: Governor Shaft

    YSTEM AND OVERNOR Governor shaft To remove or replace the governor shaft: Remove the engine from the equipment that it pow- ers. Remove the governor arm by following the previously described steps. Remove the flywheel by following the steps described in Chapter 7: Ignition Systems. Remove the crank case cover and crankshaft from the engine by following the steps described in Chap- ter 10: Cam, Crankshaft and Piston.

  • Page 66: Governor Cup And The Governor Gear

    78/83/90 Series Horizontal Shaft Engines Governor cup and the governor gear The Governor gear and cup are located inside the crankcase cover. The flyweights and the governor cup are Pin punch inter locked on this family of engines. The governor gear and cup are serviced as a complete assembly.

  • Page 67: Governor Adjustment

    YSTEM AND OVERNOR Governor adjustment To adjust the governor: Remove the fuel tank by following the procedures described in the fuel tank section of this chapter. Loosen the governor arm nut but do not remove the nut completely. Loosen nut Pry open the governor arm crimp with a flat head screwdriver.
  • Page 68 78/83/90 Series Horizontal Shaft Engines...

  • Page 69: Chapter 5: Lubrication

    • Synthetic oil is a suitable alternative, but it does not extend service intervals. NOTE: MTD recommends the use of petroleum oil during the break in period to ensure the piston rings cor- rectly break in. Synthetic vs. Petroleum based oil: To simply look at synthetic oil and to compare it with Petroleum based oil there is very little difference.

  • Page 70: Oil Dip Stick

    Oil dip stick NOTE: There are two types of dip sticks that can be found on MTD engines; a threaded dip stick that was used on older engines and a quarter turn dip stick that is used on engines currently being produced. See Figure 5.1.

  • Page 71: Dip Stick Tube Removal

    Lubrication Dip stick tube removal To remove/replace the dip stick tube: NOTE: The procedure to remove the dip stick tube is the same for both styles of dip stick. Remove these NOTE: A threaded dip stick and tube assembly is inter- screws changeable with a quarter turn dip stick and tube assembly.

  • Page 72: Lubrication System

    78/83/90 Series Horizontal Shaft Engines Lubrication system MTD uses a splash lube system for its horizontal shaft engines. The connecting rod has a dipper on it that “splashes” oil around the inside of the engine. See Figure 5.5. NOTE: The cam and the balance shaft were removed for a better view of the lubrication system.

  • Page 73: Positive Crankcase Ventilation Valve

    Lubrication Positive crankcase ventilation valve To remove the valve cover and PCV valve: Breather hose NOTE: The PCV valve is located inside the valve cover. The function and test procedures for the PCV valve is covered in Chapter 2: Basic Troubleshoot- ing.
  • Page 74 78/83/90 Series Horizontal Shaft Engines...

  • Page 75: Chapter 6: Starter And Charging Systems

    CHAPTER 6: STARTER AND CHARGING SYSTEMS There are two starter systems available for the MTD engine. The first one is a recoil starter and the second is a 110 volt electric starter. All of the 78/83/90 series MTD engines equipped with an electric starter, are also equipped with a recoil starter.

  • Page 76: Starter Cup

    78/83/90 Series Horizontal Shaft Engines Starter Cup The starter cup is a steel cup that is bolted to the flywheel. Starter cup Inspect the inside of the starter cup. See Figure 6.2. NOTE: If the starter was failing to engage the fly- wheel, and the edges of the slots inside the cup are burred or damaged, replace the starter cup.

  • Page 77: Starter Rope

    Starter and Charging System Starter Rope The most common failure mode for most recoil assemblies is a broken rope. NOTE: If the spring was not damaged when the recoil sprung back, it is possible to simply remove the rem- nants of the old rope and install a new rope. Rope inserted from the inside Remove the starter by following the steps described...
  • Page 78 78/83/90 Series Horizontal Shaft Engines Rope-return tension may be increased by winding the rope and pulley counter clockwise. Give the starter a couple of test pulls to verify the right amount of tension on the starter rope and check the action of the pawls. NOTE: If starter rope tension needs to be adjusted, hook the rope into the notch in the pulley and wind the pulley a couple of turns to add...

  • Page 79: Starter Pulley And Recoil Spring

    Starter and Charging System Starter pulley and recoil spring If damage is suspected, the recoil may be disassembled NOTE: The recoil spring is nested within the starter pulley and both parts are sold as a single part number. Eye protection should be worn if ! CAUTION ! CAUTION the starter pulley is to be removed.
  • Page 80 78/83/90 Series Horizontal Shaft Engines Carefully lift the spring and pulley out of the recoil Pulley housing. See Figure 6.9. Housing The recoil spring is under tension and can release as the pulley is ! CAUTION ! CAUTION removed. Eye protection should be worn while removing the starter pulley.

  • Page 81: Electric Starter

    Starter and Charging System Electric starter The electric starter is only available on the snow engine. It is powered by an extension cord that is plugged into household 120 volt AC current. The starter and switch assembly are one piece and are not serviceable. To replace the starter assembly: Remove these screws Disconnect the extension cord.
  • Page 82 78/83/90 Series Horizontal Shaft Engines Pull back and angle the Pull back on the starter approximately 1/2”. Then starter away while angle it away from the engine while sliding it out of sliding it out the engine. See Figure 6.13. NOTE: Before condemning a starter make sure to bench test it.

  • Page 83: Charging System

    Starter and Charging System Charging system Some engines are equipped with a charging system. The charging system used on MTD engines consists of three components, the rotor, stator and the rectifier. • Alternator rotor: The rotor consists of five magnets on the inside of the flywheel that rotate around a stator that is mounted to the cylinder block.

  • Page 84: Charging System Testing

    78/83/90 Series Horizontal Shaft Engines Charging system testing To test the charging system: Locate the charger harness. It will be by the right handle bar of the snow thrower when the engine is installed. See Figure 6.16. Start the engine and run it at full throttle. Charger harness Figure 6.16...

  • Page 85: Stator

    Starter and Charging System Stator To remove/replace the stator: Baffle Remove and ground the spark plug wire. Remove the flywheel by following the steps described in Chapter 7: Ignition System. Remove the baffle that covers the charger harness using a 10mm wrench. See Figure 6.19. Slide the grommet out of the engine block.
  • Page 86 78/83/90 Series Horizontal Shaft Engines...

  • Page 87: Chapter 7: Ignition System

    Ignition System CHAPTER 7: IGNITION SYSTEM Troubleshooting the ignition system The purpose of the ignition system is to provide a spark in the combustion chamber at the proper time to efficiently ignite the fuel/air mixture. The steps in troubleshooting the ignition system are: Examine the spark plug(s) by following the steps described in the spark plug section of this chapter.

  • Page 88: Stop Switch

    78/83/90 Series Horizontal Shaft Engines Stop switch All MTD horizontal engines that are in use in North America have a stop switch built into the throttle lever Disconnect switch assembly. MTD engines used on snow blowers have an additional stop (ignition) switch in the engine shroud.

  • Page 89: Remote (Ignition) Stop Switch (Snow Engines)

    Ignition System Remote (ignition) stop switch (snow engines) To test the remote stop switch: Remove the engine shroud by following the proce- dures described in Chapter 3: Air Intake System. Disconnect the two wires from the remote switch. See Figure 7.5. Wires Remote switch Figure 7.5...

  • Page 90: Test For Ignition That Won't Turn Off

    78/83/90 Series Horizontal Shaft Engines Test for ignition that won’t turn off MTD engines are turned off by removing the spark from the engine. This is accomplished by shorting the primary windings of the coil to ground. If the engine does not stop when the key is removed...

  • Page 91: The Module

    Ignition System The module The coil in this ignition system is an inductive discharge magneto, contained in a single module. • The inductive discharge magneto has a two leg design. • The magneto is energized by the passing of a magnet mounted in the flywheel. •...

  • Page 92: Module Removal

    78/83/90 Series Horizontal Shaft Engines Module removal Front fuel tank shroud Unplug the spark plug. Remove the engine shroud by following the steps procedures in Chapter 3: Air Intake Systems. Remove the front fuel tank shroud using a 10 mm wrench.

  • Page 93: Installing The Module And Setting The Air Gap

    Ignition System Installing the module and setting the air gap NOTE: If just setting the air gap, loosen the module mounting screws first then follow the same steps as described below. Rotate the flywheel so that the magnets are away from where the module is mounted.

  • Page 94: Flywheel

    A sheared flywheel key will throw off the ignition timing. Sheared keys are uncommon on MTD engines. If one is found, check the crankshaft and flywheel for damage. To Remove and/or inspect the flywheel and key: Remove the blower housing.

  • Page 95: About The Spark Plug

    • The spark plug is a F6RTC, gapped to 0.026” - 0.030” (0.65 - 0.75 mm). NOTE: The F6RTC plug is the only plug that is EPA certified for the MTD engine. • Wear rate will vary somewhat with severity of use. If the edges of the center electrode are rounded-off, or any other apparent wear / damage occurs, replace the spark plug before operating failure (no start) occurs.
  • Page 96 78/83/90 Series Horizontal Shaft Engines...

  • Page 97: Chapter 8: Exhaust

    Exhaust CHAPTER 8: EXHAUST The exhaust system is a frequently overlooked component of an engine. It is important to make sure the muffler is in good condition and free of debris and/or insects. NOTE: A blocked muffler will result in poor performance. If a muffler is completely blocked the engine may not start.

  • Page 98: Muffler Removal/Replacement

    Clean all of the gasket material off of the cylinder head and the muffler. See Figure 8.4. NOTE: The MTD engine uses a graphite exhaust gasket. It is not reusable and must be replaced every time the muffler nuts are loosened.
  • Page 99 Exhaust Remove the four screws that secure the outer heat shield to the muffler. See Figure 8.6. Slide the outer heat shield off of the muffler. NOTE: The inner heat shield is serviced with the muffler. Install the muffler by following the previous steps in reverse order.

  • Page 100: Muffler Removal/Replacement (Snow Engines)

    78/83/90 Series Horizontal Shaft Engines Muffler removal/replacement (snow engines) To remove/replace the muffler: Remove the control panel by following the proce- dures described in Chapter 3: Air Intake System. Remove the muffler heat shield by: Remove the two rear heat shield screws using a 10 mm wrench.
  • Page 101 Clean all of the gasket material off of the cylinder head and the muffler (if reusing the muffler) NOTE: The MTD engine uses a graphite exhaust gasket. It is not reusable and must be replaced every time the muffler nuts are loosened.
  • Page 102 78/83/90 Series Horizontal Shaft Engines...

  • Page 103: Chapter 9: Cylinder Head

    Cylinder head CHAPTER 9: CYLINDER HEAD Cylinder head removal The Cylinder head of the MTD engine can be removed without removing the engine from the piece of equipment. To remove the cylinder head: Disconnect and ground the spark plug high tension lead.
  • Page 104 78/83/90 Series Horizontal Shaft Engines Remove the screw that fastens the throttle lever to the rear of the cylinder head using a 10 mm wrench. See Figure 9.3. NOTE: The throttle lever can be disconnected and removed from the engine or moved to the side.
  • Page 105 Cylinder head Oversized bolt NOTE: The 490 engine has an over sized bolt with a Belleville washer in the number 4 position. The wrench size and torque is the same as the other three bolts. See Figure 9.6. Figure 9.6 14.

  • Page 106: Cylinder Head Installation

    Graphite head the alignment dowels to hold it in place. gasket See Figure 9.8. NOTE: MTD uses graphite head gaskets that have a bead of silicon on them. They are not reus- able. Figure 9.8 Position the cylinder head on the engine block.

  • Page 107: Valves

    Cylinder head Valves The valves and valve seats can be serviced by grinding and lapping or the head can be replaced. Depending on local machine and labor costs, it is probably more economical to replace the cylinder head versus servicing the valves.
  • Page 108 78/83/90 Series Horizontal Shaft Engines Lift the springs off of the valve stems. Slide the valves out of the cylinder head. NOTE: Only the intake valve has a valve guide seal. Seal See Figure 9.12. Figure 9.12 Inspect the valve seat. See Figure 9.13. •...
  • Page 109 Cylinder head Test the valves for leaks by: 10a. Place the cylinder head on a couple of wood blocks with the valves facing up. 10b. Pour a small amount of gasoline or parts cleaning solvent into the combustion chamber (just enough to cover the valves).
  • Page 110 78/83/90 Series Horizontal Shaft Engines...

  • Page 111: Chapter 10: Crankshaft, Piston And Connecting Rod

    Crankshaft, piston and connecting rod CHAPTER 10: CRANKSHAFT, PISTON AND CONNECTING ROD The exact procedure a technician uses to disassemble an engine depends on the type of repairs needed. This chapter is written as a set of procedures that should provide the user with sufficient information to complete any fea- sible repair to the engine short block assembly.
  • Page 112 78/83/90 Series Horizontal Shaft Engines Remove the balance shaft Remove the camshaft. Remove the balance shaft. See Figure 10.2. Cam shaft Compression relief Figure 10.2 NOTE: The valve tappet should be kept riding against their original lobes. Once broken in, switching the tappets to run on different cam lobes will cause rapid tappet and cam wear.
  • Page 113 Crankshaft, piston and connecting rod 16. Push the piston out of the cylinder. NOTE: Sometimes a ridge of carbon builds up where the cylinder meets the head. If this happens, the piston can be removed from inside of the cylinder block. 17.

  • Page 114: Crankshaft Inspection

    78/83/90 Series Horizontal Shaft Engines Crankshaft inspection Inspect the crankshaft journals and the crank pin for galling, scoring, pitting or any other form of damage. NOTE: This is mostly a visual check. Measurement is to determine if it is within the specifications after it is found to be OK visually.

  • Page 115: Piston Inspection

    Figure 10.8 Top piston ring Install rings back onto the piston. NOTE: The compression rings on the MTD engine have Middle piston different profiles. It is important that the proper pro- ring filed ring is on the right grove. See Figure 10.9.
  • Page 116 78/83/90 Series Horizontal Shaft Engines Measure the gap between the ring and the ring land using a feeler gauge and compare the measure- ment to the chart at the end of this chapter. See Figure 10.11. Feeler gauge Figure 10.11 Measure the piston pin bore on both sides of the piston using telescoping gauges or vernier caliper.

  • Page 117: Connecting Rod Inspection

    Crankshaft, piston and connecting rod Connecting rod inspection Measure at Inspect the connecting rod for cracks or any signs of right angles damage. Measure at right angles Install the rod cap and tighten to a torque of 106 - 124 in-lbs (12 - 14Nm).

  • Page 118: Balance Shaft (483 & 490)

    (the piston, piston pin and one-third of the connecting rod). The crankshafts on MTD engines are not balanced. Extra weight is added to the crankshaft counter-weight to reduce Reciprocating the total reciprocating and rotational forces.

  • Page 119: Reassembly

    Crankshaft, piston and connecting rod Reassembly Install the governor shaft. NOTE: The governor shaft MUST be installed before the crankshaft is installed. Clean the cylinder 2a. Remove all gasket material from all mating surfaces. 2b. Clean the cylinder and crank case cover. Oil seals 3a.
  • Page 120 78/83/90 Series Horizontal Shaft Engines Timing marks Install the balance shaft by: Pre-lube the balance shaft with clean 10W-30 motor oil Rotate the crankshaft until the timing mark on the larger gear points to the 7:30 position. Insert the balance shaft while aligning the tim- ing marks.

  • Page 121: Engine Specifications Chart

    Crankshaft, piston and connecting rod Engine specifications chart Specification New Min New Max Service Limit Bore (digit 2&3 of the model number) 3.071 78.02 3.072 78.03 3.080 78.23 3.268 83.02 3.269 83.03 3.277 83.24 3.544 90.02 3.544 90.03 3.553 90.26 Crank pin min.
  • Page 122 78/83/90 Series Horizontal Shaft Engines Specification New Min New Max Service Limit Intake lobe height 1.248 31.70 1.252 31.80 1.237 31.42 1.281 32.54 1.285 32.64 1.269 32.24 1.281 32.54 1.285 32.64 1.269 32.24 Exhaust lobe height 1.248 31.70 1.252 31.80 1.237 31.52 1.261...
  • Page 123 Crankshaft, piston and connecting rod Specification New Min New Max Service Limit Balance shaft bearing max. ID (cylinder block) 0.591 15.00 0.591 15.00 0.592 15.03 0.591 15.00 0.591 15.00 0.592 15.03 Compression ring to land max. clearance 0.001 0.02 0.002 0.06 0.008 0.20...
  • Page 124 78/83/90 Series Horizontal Shaft Engines Minimum Maximum Intake valve lash 0.004 0.10 0.006 0.15 0.004 0.10 0.006 0.15 0.004 0.10 0.006 0.15 Exhaust valve lash 0.006 0.15 0.008 0.20 0.006 0.15 0.008 0.20 0.006 0.15 0.008 0.20 Spark plug gap 0.026 0.65 0.030...

  • Page 125: Engine Torque Values Chart

    Crankshaft, piston and connecting rod Engine torque values chart Fastener Torque Blower housing 80-106 in-lbs (9-12 Nm) 80-106 in-lbs (9-12 Nm) 80-106 in-lbs (9-12 Nm) Carburetor drain bolt 80-106 in-lbs (9-12 Nm) 80-106 in-lbs (9-12 Nm) 80-106 in-lbs (9-12 Nm) Carburetor mounting nuts 80-106 in-lbs (9-12 Nm) 80-106 in-lbs (9-12 Nm)
  • Page 126 78/83/90 Series Horizontal Shaft Engines...

  • Page 127: Chapter 11: Failure Analysis

    Failure Analysis CHAPTER 11: FAILURE ANALYSIS A properly maintained engine will provide years of service. Occasionally an engine will fail. An important part of working on engines is finding out why they failed. Was it something the customer did? Was it a manufacturing defect? Did the engine just wear out? All of these questions need to be answered when a failed engine is found.
  • Page 128 78/83/90 Series Horizontal Shaft Engines When particles enter the combustion chamber, the Cross hatch polished off up and down motion of the piston grinds the parti- cles into the side of the cylinder walls and damages the cylinder wall, piston and piston rings This can be identified by the scoring along the verti- cal axis of the piston and cylinder wall or the cross hatch on the cylinder wall being worn off.
  • Page 129 Failure Analysis Because the oil suspends the abrasive particles, the engine components that are immersed in oil will show definite signs of abrasive ingestion especially around the connecting rod and main bearing journals. See Figure 11.5. NOTE: Abrasives that are trapped in the oil will cause the lower portion of the combustion chamber to wear- ing more than the upper portion.

  • Page 130: Insufficient Lubrication

    78/83/90 Series Horizontal Shaft Engines Insufficient lubrication The bearing surfaces in an engine are not smooth. As a result of the machining processes to make the engine parts, there are little peaks and valleys that are only visible on a microscopic scale. These peaks are called asperi- ties.

  • Page 131: Engine Overspeed

    Generally speaking this is at the narrowest part of the connecting rods. On most engines that would be about an inch below the wrist pin, but on the MTD engine it is at the wrist pin.

  • Page 132: Overheated

    Overheated The MTD engines are air cooled engines. Because of this, cleanliness of the engine is very important to the life of the engine. Dirt, grass and sludge all form an insulating layer on the engine. This will trap the heat in the engine and cause it to over heat.

  • Page 133: Mechanical Breakage/ Wear

    Failure Analysis Mechanical Breakage/ Wear Sometimes an engine fails because a part breaks. There are generally three causes of a broken part, outside of the previously discussed engine failures. They are abuse, wear, and manufacturing defects. Bent blade A very common way to abuse an engine is a bent crank shaft.
  • Page 134 78/83/90 Series Horizontal Shaft Engines...
  • Page 136 MTD Products Inc - Product Training and Education Department FORM NUMBER - 769-04951A 10/2010...

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