MTD 420 Series Service Manual

Professional Shop Manual78/83/90 Series Horizontal Shaft Engines (277/357/420 cc Engines)NOTE: These materials are for use by trained technicians who

78/83/90 Series Horizontal Shaft Engines4Model and serial number The model and serial number can be found on a white sticker with a bar code. The

78/83/90 Series Horizontal Shaft Engines94Muffler removal/replacement (snow engines)To remove/replace the muffler:1. Remove the control panel by follo

Exhaust956. Remove the two muffler nuts using a 13mm wrench and lift the muffler off of the engine. See Figure 8.10.7. Clean all of the gasket materi

78/83/90 Series Horizontal Shaft Engines96For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

Cylinder head97Cylinder head removal The Cylinder head of the MTD engine can be removed without removing the engine from the piece of equipment.To

78/83/90 Series Horizontal Shaft Engines988. Remove the screw that fastens the throttle lever to the rear of the cylinder head using a 10 mm wrench. S

Cylinder head99NOTE: 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 a

78/83/90 Series Horizontal Shaft Engines100Cylinder head installation1. Place a new head gasket on the cylinder, allowing the alignment dowels to hold

Cylinder head101ValvesThe valves and valve seats can be serviced by grinding and lapping or the head can be replaced. Depending on local machine and l

78/83/90 Series Horizontal Shaft Engines1024. Lift the springs off of the valve stems.5. Slide the valves out of the cylinder head.NOTE: Only the inta

Cylinder head10310. 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 a

Introduction5Maintenance The information in this manual applies to the MTD engine. Some basic principles may apply to engines pro-duced by other m

78/83/90 Series Horizontal Shaft Engines104For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

Crankshaft, piston and connecting rod105The exact procedure a technician uses to disassemble an engine depends on the type of repairs needed. This cha

78/83/90 Series Horizontal Shaft Engines10612. Remove the camshaft.13. Remove the balance shaft. See Figure 10.2.NOTE: The valve tappet should be kept

Crankshaft, piston and connecting rod10716. Push the piston out of the cylinder.NOTE: Sometimes a ridge of carbon builds up where the cylinder meets t

78/83/90 Series Horizontal Shaft Engines108Crankshaft inspection1. Inspect the crankshaft journals and the crank pin for galling, scoring, pitting or

Crankshaft, piston and connecting rod109Piston Inspection1. Clean the piston and remove all carbon from the rings and ring groves.2. Clean the cyl

78/83/90 Series Horizontal Shaft Engines1107. Measure the gap between the ring and the ring land using a feeler gauge and compare the measure-ment to

Crankshaft, piston and connecting rod111Connecting rod inspection1. Inspect the connecting rod for cracks or any signs of damage.2. Install the ro

78/83/90 Series Horizontal Shaft Engines112Balance Shaft (483 & 490)There are two primary motions that generate most of the vibrations in single-c

Crankshaft, piston and connecting rod113Reassembly1. Install the governor shaft.NOTE: The governor shaft MUST be installed before the crankshaft is in

78/83/90 Series Horizontal Shaft Engines6Air filter (summer engines)Generally air filters come in two different types, a pleated-paper element or foam

78/83/90 Series Horizontal Shaft Engines1146. Install the balance shaft by:6a. Pre-lube the balance shaft with clean 10W-30 motor oil6b. Rotate the cr

Crankshaft, piston and connecting rod115Engine specifications chartSpecification New Min New Max Service Limitin. mm in. mm in. mmBore (digit 2&3

78/83/90 Series Horizontal Shaft Engines116in. mm in. mm in. mmIntake lobe height78 1.248 31.70 1.252 31.80 1.237 31.4283 1.281 32.54 1.285 32.64 1.26

Crankshaft, piston and connecting rod117in. mm in. mm in. mmBalance shaft bearing max. ID (cylinder block)78 N/A N/A N/A N/A N/A N/A83 0.591 15

78/83/90 Series Horizontal Shaft Engines118Minimum Maximumin. mm in. mmIntake valve lash78 0.004 0.10 0.006 0.1583 0.004 0.10 0.006 0.1590 0.004 0.10

Crankshaft, piston and connecting rod119Engine torque values chartFastener Torque 78 83 90Blower housing 80-106 in-lbs (9-12 Nm) 80-106 in-lbs (9-12 N

78/83/90 Series Horizontal Shaft Engines120For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

Failure Analysis121A properly maintained engine will provide years of service. Occasionally an engine will fail. An important part of working on engin

78/83/90 Series Horizontal Shaft Engines1225. When particles enter the combustion chamber, the up and down motion of the piston grinds the parti-cles

Failure Analysis1238. Because the oil suspends the abrasive particles, the engine components that are immersed in oil will show definite signs of abra

Introduction7Oil type and capacityMTD engines use oil with a SF/SG API rating or better is the recommended oil for this engine. The winter engines use

78/83/90 Series Horizontal Shaft Engines124Insufficient lubricationThe bearing surfaces in an engine are not smooth. As a result of the machining proc

Failure Analysis125Engine OverspeedThe MTD engine is designed for a maximum speed of 3600 rpm. When the governor is unable to control the engine rpm,

78/83/90 Series Horizontal Shaft Engines126OverheatedThe MTD engines are air cooled engines. Because of this, cleanliness of the engine is very import

Failure Analysis127Mechanical Breakage/ Wear Sometimes an engine fails because a part breaks. There are generally three causes of a broken part, o

78/83/90 Series Horizontal Shaft Engines128For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

MTD Products Inc - Product Training and Education DepartmentFORM NUMBER - 769-04951A10/2010For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts

78/83/90 Series Horizontal Shaft Engines8To check the oil with a threaded dip stick:1. Twist and remove the dip stick from the engine. 2. Clean the o

Introduction9Fuel Gasolines currently on the market are not pure gasoline. Today’s fuels have alcohol and other additives in them to reduce emissi

78/83/90 Series Horizontal Shaft Engines10To replace the fuel filter:1. Siphon the fuel out of the fuel tank.2. Remove the front fuel tank shroud usin

Introduction11Valve lash Valve lash is the clearance between the top of the valve stem and the rocker arm. The valve lash should be checked after

78/83/90 Series Horizontal Shaft Engines126. Slowly pull the starter rope until air can be heard coming out of the spark plug hole.7. Confirm that the

Introduction1313. Double-check the clearance after tightening the jam nut, to confirm that it did not shift. Re-adjust if necessary. 14. Rotate the en

For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

78/83/90 Series Horizontal Shaft Engines14General torque specifications Maintenance Chart 000H]LV0000H]LVVEOWIVEOQ

Introduction15Frequently used specificationsMinimum Maximumin. mm in. mmIntake valve lash78 0.004 0.10 0.006 0.1583 0.004 0.10 0.006 0.1590 0.004 0.10

78/83/90 Series Horizontal Shaft Engines16For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

BASIC TROUBLESHOOTING17DefinitionsTroubleshooting - The act of gathering information by preforming tests and direct observations.Diagnosis - Developin

78/83/90 Series Horizontal Shaft Engines18IV. Unusual exhaust toneThere are tools that the technician can use in order to define the problem, such as:

BASIC TROUBLESHOOTING19D. Rope jerks back. This usually indicates that the piston is stopping before top dead center on the com-pression stroke and is

78/83/90 Series Horizontal Shaft Engines20VI. Check valve timing/actuation.VII. Check exhaust.3. Starts, runs poorly3a. Starts, then diesI. Run the e

BASIC TROUBLESHOOTING21b. Intake blockage• An intake blockage up-stream of the carburetor will cause a rich fuel/air mixture and constrict the amount

78/83/90 Series Horizontal Shaft Engines22• Low octane fuel • Over-heating engine (check for blocked cooling air flow)• Carbon build-up in cylinder: g

BASIC TROUBLESHOOTING23Repairing the problemThe third step in the troubleshooting process is to repair the problem. This step consists of:A. Form a di

IChapter 1: IntroductionProfessional Service Manual Intent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Safety . . . . . . . . .

78/83/90 Series Horizontal Shaft Engines24Prime testTo perform a prime test:1. Prime the engine through the carburetor throat using a squirt bottle, f

BASIC TROUBLESHOOTING257. Compare the results to the following chart. Leak-down Testing ResultsSymptom Possible causeAir escaping from the breatherWor

78/83/90 Series Horizontal Shaft Engines26Compression testTo perform a compression test:NOTE: Compression should be in the range of 40-75 PSI (2.8-5.2

BASIC TROUBLESHOOTING27PCV testing The PCV (Positive Crankcase Ventilation) valve is located in the valve cover and allows the crankcase pressure

78/83/90 Series Horizontal Shaft Engines28For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

AIR INTAKE SYSTEMS29MTD builds horizontal crank engines for snow blowers and chore performers. The differences between snow engines and chore engines

78/83/90 Series Horizontal Shaft Engines30Air filter (summer engines)Generally air filters come in two different types, a pleated-paper element or foa

AIR INTAKE SYSTEMS315. Install the air filter by following the previous steps in reverse order.NOTE: When installing the air filter, the hole in the b

78/83/90 Series Horizontal Shaft Engines32Air filter base and control panel (summer engines) To remove/replace the air filter base and control panel:

AIR INTAKE SYSTEMS33NOTE: All summer engines built on or after January 1, 2011 will have a hose from the evaporative emis-sions (EVAP) system attached

IIChapter 4: The Fuel System and GovernorInspecting the fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

78/83/90 Series Horizontal Shaft Engines34Carburetor and Insulator1. Siphon the fuel out of the fuel tank.NOTE: Dispose of drained fuel in a safe and

AIR INTAKE SYSTEMS354. Remove the Heat box (snow engines) or the control panel (summer engines) by following the steps described in appropriate sectio

78/83/90 Series Horizontal Shaft Engines36NOTE: An insulator block separates the carburetor from the cylinder head. There is a gasket on each side of

FUEL SYSTEM AND GOVERNOR37The function of the fuel system is to store fuel, mix the fuel with air in the correct ratio and deliver it to the intake po

78/83/90 Series Horizontal Shaft Engines38Test fuel for alcoholFuels currently on the market contain a wide array of additives. Some of these additive

FUEL SYSTEM AND GOVERNOR39Fuel tank ventThe fuel tank vent performs the important task of allowing air into the fuel tank. As fuel is being used by th

78/83/90 Series Horizontal Shaft Engines40The fuel filter The fuel filter is located in the fuel tank nipple. It can be removed and cleaned with a can

FUEL SYSTEM AND GOVERNOR41The fuel tankTo remove the fuel tank:1. Drain the tank.2. Remove the four screws that secure the fuel tank using a 12 mm wr

78/83/90 Series Horizontal Shaft Engines42Evaporative (EVAP) emissions systemGasoline is made from the graduated distillation of crude oil. It consist

FUEL SYSTEM AND GOVERNOR43This system operates as follows:1. The gasoline evaporate and lets off vapors.2. The vapors exit the fuel tank through the

IIIChapter 7: Ignition SystemTroubleshooting the ignition system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81Stop switch . . . .

78/83/90 Series Horizontal Shaft Engines44Troubleshooting the EVAP SystemThere are four failure modes for the EVAP system:• A leak in the system - wil

FUEL SYSTEM AND GOVERNOR45Testing the roll over valveThe roll over valve vent has two functions. The first function is to vent the tank and the second

78/83/90 Series Horizontal Shaft Engines46Charcoal canister To remove/replace the charcoal canister:1. Disconnect the hoses from the canister.

FUEL SYSTEM AND GOVERNOR47Roll over valve To remove the roll over valve:1. Gently pry the grommet from in between the roll over valve and the fuel ta

78/83/90 Series Horizontal Shaft Engines48Choke 78/83/90 series MTD engines are equipped with a choke. The snow engines are also equipped with a prime

FUEL SYSTEM AND GOVERNOR49 The choke rod is part of the heat box assembly mounted on the front of the carburetor. See Figure 4.22.NOTE: The choke

78/83/90 Series Horizontal Shaft Engines50Primers (snow engines)MTD engines use a dry bulb primer. This means that there is no fuel in the primer bulb

FUEL SYSTEM AND GOVERNOR51CarburetorsIf diagnosis indicates a fuel problem, inspect the carburetor. This is important even if problems are identified

78/83/90 Series Horizontal Shaft Engines52Disassembly and rebuilding of the carburetor1. Clamp-off the fuel line to prevent fuel spillage, then discon

FUEL SYSTEM AND GOVERNOR538. Remove the pin that the float hinges on to remove the float.NOTE: The float is not adjustable. Spring tension against the

IVFor Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

78/83/90 Series Horizontal Shaft Engines54NOTE: The main jet secures the emulsion tube in the central column of the carburetor. See F

FUEL SYSTEM AND GOVERNOR5512. Examine the metering plug: See Figure 4.33.• Fuel, drawn from the central column via the long fuel feed leg, is metered

78/83/90 Series Horizontal Shaft Engines56Fuel shut-off valveTo prevent the needle valve from un-seating and flooding the engine while being towed, en

FUEL SYSTEM AND GOVERNOR576.Remove the fuel shut-off lever. See Figure 4.38.NOTE: There is a passage cast into the shut-off lever. When this passage l

78/83/90 Series Horizontal Shaft Engines58GovernorThe engine speed is controlled by a balance between the force applied by a spring (pulling the throt

FUEL SYSTEM AND GOVERNOR59Governor shaftTo remove or replace the governor shaft:1. Remove the engine from the equipment that it pow-ers.2. Remove the

78/83/90 Series Horizontal Shaft Engines60Governor cup and the governor gear The Governor gear and cup are located inside the crankcase cover. The

FUEL SYSTEM AND GOVERNOR61Governor adjustmentTo adjust the governor:1. Remove the fuel tank by following the procedures described in the fuel tank se

78/83/90 Series Horizontal Shaft Engines62For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

Lubrication63Oil type and quantityMTD engines use oil with a SF/SG API rating or better is the recommended oil for this engine. • The winter engines u

Introduction1Professional Service Manual Intent This manual is intended to provide service dealers with an introduction to proven diagnostic and repai

78/83/90 Series Horizontal Shaft Engines64Oil dip stickNOTE: There are two types of dip sticks that can be found on MTD engines; a threaded dip s

Lubrication65Dip stick tube removalTo remove/replace the dip stick tube:NOTE: The procedure to remove the dip stick tube is the same for both styles o

78/83/90 Series Horizontal Shaft Engines66Lubrication systemMTD uses a splash lube system for its horizontal shaft engines. The connecting rod has a d

Lubrication67Positive crankcase ventilation valve To remove the valve cover and PCV valve:NOTE: The PCV valve is located inside the valve cover. The f

78/83/90 Series Horizontal Shaft Engines68For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

Starter and Charging System69There are two starter systems available for the MTD engine. The first one is a recoil starter and the second is a 110 vol

78/83/90 Series Horizontal Shaft Engines70Starter CupThe starter cup is a steel cup that is bolted to the flywheel.1. Inspect the inside of the starte

Starter and Charging System71Starter Rope The most common failure mode for most recoil assemblies is a broken rope. NOTE: If the spring was not da

78/83/90 Series Horizontal Shaft Engines729. Give the starter a couple of test pulls to verify the right amount of tension on the starter rope and che

Starter and Charging System73Starter pulley and recoil spring If damage is suspected, the recoil may be disassembled by:NOTE: The recoil spring is nes

78/83/90 Series Horizontal Shaft Engines2• Be prepared in case of emergency: Keep a fire extinguisher nearby Keep a first aid

78/83/90 Series Horizontal Shaft Engines745. Carefully lift the spring and pulley out of the recoil housing. See Figure 6.9.NOTE: If the spring is und

Starter and Charging System75Electric starter The electric starter is only available on the snow engine. It is powered by an extension cord that i

78/83/90 Series Horizontal Shaft Engines764. Pull back on the starter approximately 1/2”. Then angle it away from the engine while sliding it out of t

Starter and Charging System77Charging system Some engines are equipped with a charging system. The charging system used on MTD engines consists of

78/83/90 Series Horizontal Shaft Engines78Charging system testingTo test the charging system:1. Locate the charger harness. It will be by the right ha

Starter and Charging System79StatorTo remove/replace the stator:1. Remove and ground the spark plug wire.2. Remove the flywheel by following the s

78/83/90 Series Horizontal Shaft Engines80For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

Ignition System81Troubleshooting the ignition system The purpose of the ignition system is to provide a spark in the combustion chamber at the pro

78/83/90 Series Horizontal Shaft Engines82Stop switch All MTD horizontal engines that are in use in North America have a stop switch built into th

Ignition System83Remote (ignition) stop switch (snow engines)To test the remote stop switch:1. Remove the engine shroud by following the proce-dur

Introduction3Fasteners• Most of the fasteners used on the MTD engine are metric. Some are fractional inches. For this reason, wrench sizes are frequen

78/83/90 Series Horizontal Shaft Engines84Test for ignition that won’t turn offMTD engines are turned off by removing the spark from the engine. This

Ignition System85The module The coil in this ignition system is an inductive discharge magneto, contained in a single module.• The inductive disch

78/83/90 Series Horizontal Shaft Engines86Module removal1. Unplug the spark plug.2. Remove the engine shroud by following the steps procedures in Chap

Ignition System87Installing the module and setting the air gapNOTE: If just setting the air gap, loosen the module mounting screws first then follow t

78/83/90 Series Horizontal Shaft Engines88Flywheel The flywheel holds the magnets. These magnets induce a field in the module which in turn produc

Ignition System89About 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 tha

78/83/90 Series Horizontal Shaft Engines90For Parts Call 606-678-9623 or 606-561-4983www.mymowerparts.com

Exhaust91The exhaust system is a frequently overlooked component of an engine. It is important to make sure the muffler is in good condition and free

78/83/90 Series Horizontal Shaft Engines92Muffler removal/replacement1. Remove the two nuts that hold the manifold pipe to the cylinder head using a 1

Exhaust936. Remove the four screws that secure the outer heat shield to the muffler. See Figure 8.6.7. Slide the outer heat shield off of the muffler.