Instruction/ maintenance manual of the product 78 Troy-Bilt
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Pr of essional Shop Manual 78/83/90 Series Horizontal Shaft Engines (277/357/420 cc Engines) NOTE: These materials are fo r use by tr ain ed techni cians who ar e ex perien ce d in the service an d re.
For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
I Chapter 1: Introduction Professional Service Manual Intent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Fasteners.
II Chapter 4: The Fuel System and Governor Inspecting the fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Test fuel for alcohol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
III Chapter 7: Ignition System Troubleshooting the ignition system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Stop switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Remote (ignition) stop switch (snow engines) .
IV For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
Introduction 1 Professional Service Ma nual Intent This manual is intended to provide service de alers with an introduction to proven diagnostic and rep air proce - dures for 78/83/90 series MTD horizo ntal shaft en gines. Disclaimer : The information con tained in th is manu al is c orre ct at the time of writin g.
78/83/90 Series Horiz ontal Shaft Engines 2 • Be prepared in ca se of emergency: 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. • Replace any missing o r damaged safety labels on equipment being serviced.
Introduction 3 Fasteners • Most of the fastener s used on the MTD engine are metric. Some are fractional inches. For this reas on, wrench sizes are frequently identified in the text, and measurements are given in U.
78/83/90 Series Horiz ontal Shaft Engines 4 Model and serial number The model and serial number can be found on a white sticker with a bar code. The st icker is located on the right side of the engine at the bottom of the block. See Figure 1.1. NOTE: The serial number will always start with the model number .
Introduction 5 Maintenance The information in th is manual applie s to the MTD engine. Some basic principles may ap ply to engines pro- duced by other manufacturers. As the saying goes “an ounce of prevention is worth a pound of cure”. The same can be said a bout preventive maintenance on out door power equipment.
78/83/90 Series Horiz ontal Shaft Engines 6 Air filter (s ummer e ngines) Generally air filters come in two dif ferent types, a pleated-p aper element or foam. A combination of the two are used on the MT D en g i ne . See Figure 1.3. 1. The main functio n of the air filter is to trap air borne part icles before they enter th e engine.
Introduction 7 Oil type and capacity MTD engines use oil with a SF/SG API rating or better is the r ecommended oil for this engine. The winter engines use SAE 5W-30 oil and the summ er engines use SAE 10W-30 oil. The o il capacity for all of the 78/83/90 series engines is 37 fl.
78/83/90 Series Horiz ontal Shaft Engines 8 T o check the oil with a threaded dip stick: 1. T wist and remove the dip stick from the engine. 2. Clean the oil off of the tip of the dipstick. 3. Re-insert the dipstick without threading it in to get the oil level reading.
Introduction 9 Fuel Gasolines cur rently on the market are n ot pure gasoli ne. T oday’ s fuels have alcohol and other additives in them to reduce emissions. The fuel make up can vary seasonally and geographically . Fuel with alcohol added to it is sometimes referred to as “oxygenated fuel ”.
78/83/90 Series Horiz ontal Shaft Engines 10 T o replace the fuel filter: 1. Siphon the fuel out of the fuel t ank. 2. Remove the front fuel t ank shroud using a 10 mm wrench.
Introduction 11 V alve lash V alve lash is the clearance between the top of the valve stem and the rocker arm. The valve lash should be checked after the firs t 25 hour s of use and every 100 ho ur s a f ter that. V alve lash can be checke d and a djusted using the following steps: 1.
78/83/90 Series Horiz ontal Shaft Engines 12 6. Slowly pull the starter rope until air can be heard coming out of the spa rk plug hole. 7. Confirm that the piston is at T op- D ead- C enter on the compression stroke.
Introduction 13 13. Double-check the clearance af ter tightening the ja m nut, to confirm that it did not shift. Re-ad just if necessary . 14. Rota te the engine through several compression cycles: • Observe th e mo ve m ent of th e valv e gear .
78/83/90 Series Horiz ontal Shaft Engines 14 General torque specificat ion s Maintenance Chart 0 0 0 H ] L V 0 0 0 0 H ] L V .
Introduction 15 Frequently used specific ations Minimum Maximum in. mm in. mm Intake valve lash 78 0.004 0.10 0.006 0.15 83 0.004 0.10 0.006 0.15 90 0.004 0.10 0.006 0.15 Exhaust valve lash 78 0.006 0.15 0.008 0.20 83 0.006 0.15 0.008 0.20 90 0.006 0.
78/83/90 Series Horiz ontal Shaft Engines 16 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
B ASIC T ROUBLESHOOTING 17 Definitions T roubles hooting - The act of gathering information by pref orming tests and direct observations. Diagnosis - Developing and testing theories of what the prob lem is, based on the information gathered in trouble- shooting.
78/83/90 Series Horiz ontal Shaft Engines 18 IV . Unusual exhaust tone There are tools that the te chnician can use in order to define the problem, such as: 1. Interview the customer . 1a. Get a good descript ion of their complaint. 1b. If it is an int ermittent problem, verify what conditions aggravate the problem as best as possible.
B ASIC T ROUBLESHOOTING 19 D. Rope jerks back . This usually indicates that the piston is stopping before top dead center on the com - pression stro ke and is being dr iven back down by compression or com bustion. The lik ely suspects are: I. Compression that is unusually hig h.
78/83/90 Series Horiz ontal Shaft Engines 20 VI. Check valve timing/actuation. VII. Check exhaust. 3. S tart s, runs poorly 3a. S tarts, then di es I. Run the engine with a sp ark tester in-line between the spark plug wir e and the spark plug or use an oscilloscope and see if the spark goes aw ay at the same time the engine dies.
B ASIC T ROUBLESHOOTING 21 b. Intake blocka ge • An intake blockage up-stream of the carburetor will cause a rich fuel/air mixtur e and constrict the amount of air that the en gine can draw in, limiting performance. • The in t ake va lve not fully o pening.
78/83/90 Series Horiz ontal Shaft Engines 22 • Low octane fuel • Over-heating engine (check fo r 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.
B ASIC T ROUBLESHOOTING 23 Rep airing the problem The third step in the trou bleshooting process is to repair the problem. This step consists of: A. Form a diagnosis b y using all of the informa tion g athered fro m the tro ubl eshooting th at was p erforme d.
78/83/90 Series Horiz ontal Shaft Engines 24 Prime test T o perform a prime test: 1. Prime the engine th rough the carburetor throa t us ing a squirt bottle, filled with clean fresh gasolin e. 2. Make sure the throttle is in the run posit ion and the safety key if fully inserted.
B ASIC T ROUBLESHOOTING 25 7. Compare the re sults to the following chart. Leak-down T esting Result s Symptom Possible cause Air escaping from the breather Worn cylinder or piston rings.
78/83/90 Series Horiz ontal Shaft Engines 26 Compression test T o perform a compression test: NOTE: Compression should be in the range of 40-75 PSI (2.8-5.2 Bar ). • Disconnect the high-te nsion lead from the spark plug and ground it well away from the spark plug hole.
B ASIC T ROUBLESHOOTING 27 PCV testing The PCV (Positive Crankcase V entilation) valve is loca ted in the valve cove r and allows the crankcase pressure to escape.
78/83/90 Series Horiz ontal Shaft Engines 28 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
A IR I NTAKE S YSTEMS 29 MTD builds horizont al crank engines for snow bl ower s and chore performers. The dif ferences between snow engines and chore engines are the muffle r and the air intake system. Therefore the air intake system for the snow and chore engines will be discussed sepa rately , as will the muf flers in a later chapter .
78/83/90 Series Horiz ontal Shaft Engines 30 Air filter (s ummer e ngines) Generally air filters come in two dif ferent types, a pleated-p aper element or foam.
A IR I NTAKE S YSTEMS 31 5. Install the air filter by following the previous step s in reverse orde r . NOTE: When installing the air filte r , the hole in the bottom of the paper element must fit over the ri ser in the air filter base. See Figure 3.6.
78/83/90 Series Horiz ontal Shaft Engines 32 Air filter base and control p anel (summer engines) T o remove/replace the air filter base and control panel: 1. Remove the air filter followin g the steps described in the previous sec tion. 2. Remove the three screws that ho ld the air filter base to the intake elbow .
A IR I NTAKE S YSTEMS 33 NOTE: All summer engines built on or after Ja nuary 1, 201 1 will have a hose from the evaporative emis - sions (EV AP) system attached to the control panel above the breathe r hose. See Figure 3.10. 9. Pull the control pane l straight off the front of th e engine.
78/83/90 Series Horiz ontal Shaft Engines 34 Carburetor and Insulat or 1. Siphon the fuel out of the fuel t ank. NOTE: Dispose of drained fuel in a safe and responsible manner . 2. Remove the fuel t ank front bezel using a 10 mm wrench. See Figure 3.1 1.
A IR I NTAKE S YSTEMS 35 4. Remove the Heat box (sno w engines) or the control panel (summer engines) by following the steps described in appro priate section of this chapter . 5. Disconnect the throttle linkage and spr ing. See Figure 3.13 . 6. Slide the carburetor o f f of the mounting studs.
78/83/90 Series Horiz ontal Shaft Engines 36 NOTE: An insu lator block separates the carburetor from the cylinde r head. There is a gask et on each side o f the insulator . See Figure 3.16. NOTE: The gaskets are dif ferent, and ther e is an orientation to the insulato r .
F UEL S YST EM AND G OVERNOR 37 The function of the fuel system is to st ore fuel, mix the fuel with air in the correct ratio and deliver it to th e intake port. The fuel system consists of the following components: • Fuel tank • Fuel lines • Fuel filter • Carburetor NOTE: When working on the fuel systems, look at the whole system.
78/83/90 Series Horiz ontal Shaft Engines 38 T est fuel for a lcohol Fuels curren tly on the market contain a wide array of ad di tives. Some of these additives oxygenate the fuel. Oxy - genated fuel re duces emissions, and is required in some pa rt s of the United S tates.
F UEL S YST EM AND G OVERNOR 39 Fuel t ank vent The fuel tank vent perfor ms the important t ask of allowing air into the fuel tank. As fuel is being used by the engine, the fuel level in the t ank drops. The dropping fuel level then create s a vacuum in the tank.
78/83/90 Series Horiz ontal Shaft Engines 40 The fuel filter The fuel filter is located in th e fuel t ank nipple . It can be remo ved and cl eaned 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 t he carb cl eaner through th e barb end an d out of the screen.
F UEL S YST EM AND G OVERNOR 41 The fuel t ank T o remove the fuel tank: 1. Drain the tank. 2. Remove the four screws that secure the fuel t ank using a 12 mm wrench. See Figure 4.5. 3. Lift up on the fuel t ank and disconnect the fuel line. See Figure 4.
78/83/90 Series Horiz ontal Shaft Engines 42 Evaporative (EV A P) emissions system Gasoline is made from the grad uated distillation of cr ude oil. It consists of a mult itude of individual hydrocarbons and has a boiling range of 86 - 410 ° F (30- 210 ° C) 1 .
F UEL S YST EM AND G OVERNOR 43 This system operates as follows: 1. The gasoline evaporate and lets of f vapors. 2. The vapors e xit the fuel ta nk through the roll over valve vent. NOTE: The fuel cap used with the char coal canister sys - tem is not vented.
78/83/90 Series Horiz ontal Shaft Engines 44 T roubleshooting the EV AP System There are four failure modes for the EV AP system: • A leak in the system - will allow dirt ingestion. • A blockage in the system (between the canist er and the fuel t a nk) - acts a plugged vent.
F UEL S YST EM AND G OVERNOR 45 T esting the roll over va lve The roll over valve ven t has two functions. The first fu nction is to vent the tank and th e second function is to close off the vent if the tank is inverted. T est the roll over valve by: 1.
78/83/90 Series Horiz ontal Shaft Engines 46 Charcoal canister T o remove/replace the char coal canister: 1. Disconnect the hoses from the canister . See Figure 4.15. 2. Remove the screw from the canister mounting strap using a 10 mm wrench. See Figur e 4.
F UEL S YST EM AND G OVERNOR 47 Roll over valve T o remove the roll over valve: 1. Gently pry the gro mmet from in b etween the r oll over valve and the fuel t an k. See Fi gure 4.18. NOTE: Leaving the hose connected to the roll over valve will help prevent it from falling inside the t ank once the grommet is removed.
78/83/90 Series Horiz ontal Shaft Engines 48 Choke 78/83/90 series MTD engine s a re equipped with a chok e. The snow engines are also eq uipped with a primer , both of which must be used to st art the engine. NOTE: The choke should be opened af ter the engine st arts.
F UEL S YST EM AND G OVERNOR 49 The choke rod is part of the heat box assembly mounted on the front of the carburetor . See Figure 4.22. NOTE: The choke rod can be bent s lightly to facilitate adjustment. T o adjust the choke rod: 1. Remove the choke knob and the engine shroud by following the step s described in Chapter 3: Air Int ake Systems.
78/83/90 Series Horiz ontal Shaft Engines 50 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 depre ssed.
F UEL S YST EM AND G OVERNOR 51 Carburetors If diagnosis indicates a fuel problem, inspect the ca rb uret or . This is important even if problems are identified elsewhere in the fuel system . IMPORT ANT : the fuel must be tested for alcohol content before diagnosing anything else on the engin e.
78/83/90 Series Horiz ontal Shaft Engines 52 Disassembly and re building of the carburetor 1. Clamp-off the fuel line to prev ent fuel spillage, then disconnect th e fuel line from the carburetor . 2. Disconnect the prime r hose. 3. Remove the carburetor by following the steps de scribed in Chapter 3: Air Intake an d Filter .
F UEL S YST EM AND G OVERNOR 53 8. Remove the pin that the float hin ges on to remove the float. NOTE: The float is not adjustable. S pring tension against the float v alve begins to bu ild from the horizontal position, p utting progressively more pressure between t he tip of the valv e and the seat .
78/83/90 Series Horiz ontal Shaft Engines 54 NOTE: The main jet secur es the emulsion t ube in the central column of the carburetor . See Figure 4.30. 10. The throt tle stop screw has a large plia ble lip around the head of the screw . That lip secures a mete ring plug for the p ilot and trans ition ports.
F UEL S YST EM AND G OVERNOR 55 12. Examine the metering plug: See Figure 4.33. • Fuel, drawn from the central column via the long fuel feed leg, is me ter e d by th e br ass orifice in the tip of the mete rin g plug . • Air , drawn from the emulsion air po rt, is metered by the size of the brass orifice at the entran ce to the port.
78/83/90 Series Horiz ontal Shaft Engines 56 Fuel shut-off valve T o prevent the needle valve from un-se ating and flooding the engine while being towed, engine s designed to go onto log splitters have a fuel shut-o ff valve built into the carburet or .
F UEL S YST EM AND G OVERNOR 57 6.Remove the fuel shut-of f lever . See Figure 4.38. NOTE: There is a passage cast into the shut -of f lever . When this passage lines up with the two posts in the carburetor , fuel will fl ow to the needle valve. 7. Remove the rubbe r gasket.
78/83/90 Series Horiz ontal Shaft Engines 58 Governor The engine speed is controlled by a balance between the force applied by a spring (pulling the throttle open) and a flyweight mechanism within the engin e applying force to the govern or arm (pushing the throttle closed).
F UEL S YST EM AND G OVERNOR 59 Governor shaf t T o remove or replace the governor shaf t: 1. Remove the engine from the equipment that it pow - ers. 2. Remove the governo r arm by following the previously described step s. 3. Remove the flywheel by following the step s described in Chap ter 7: Ignition Systems.
78/83/90 Series Horiz ontal Shaft Engines 60 Governor cup and the govern or gea r The Governor gear and cup are locate d inside the crankcase cover . The flyweights and the gover no r cup ar e inter locked on this family of engines. The gover nor gear and cup ar e serviced as a c omplete assembly .
F UEL S YST EM AND G OVERNOR 61 Governor adjustme nt T o adjust the governor: 1. Remove the fuel tank by following the pr ocedures described in the fuel t ank section of this chapter . 2. Loosen the governor arm nut but do not remove the nut completely .
78/83/90 Series Horiz ontal Shaft Engines 62 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
Lubrication 63 Oil type and quantity MTD engines use oil with a SF/SG API rating or better is the r ecommended oil for this engine. • The winter engines use SAE 5W- 30 oil. • The summer engines use SAE 10W-30 oil. • The oil capacity for all of the 78/83 /90 series engines is 37 fl.
78/83/90 Series Horiz ontal Shaft Engines 64 Oil dip stick NOTE: There are two typ es of dip sticks that can be found on MTD engines; a thr eaded dip stick that was used on older engines and a quarter turn dip stick that is used on engines currently bein g pr od u ced .
Lubrication 65 Dip stick tube removal T o remove/replace the dip stick tube: NOTE: The procedure to remove the dip stick tub e is the same for both styles of dip stick. NOTE: A threaded dip stick and tube assembly is inter - changeable with a quarter tu rn dip stick and tube assembly .
78/83/90 Series Horiz ontal Shaft Engines 66 Lubrication system MTD uses a splash lube syst em for it s horizontal shaft engines. The connecting r od ha s a dipper on it that “splashes” oil around the insid e of the engine. See Figure 5.5. NOTE: The ca m an d the bala nce s haft were removed for a bett er view of the lubrication system.
Lubrication 67 Positive crankcas e ventilation valve T o remove the valve cover and PCV v alve: NOTE: The PCV valve is located inside the valve cover . The function and test procedures for the PCV valve is cove red in Chapter 2: Basic T roubleshoot - ing.
78/83/90 Series Horiz ontal Shaft Engines 68 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
Starter and Charging System 69 There are two st arter systems available for the MTD engine. Th e fir st one is a recoil starter and th e second is a 1 10 volt electric starter . All of the 78/83/90 series MTD engines eq uipped with an e lectric starter , ar e also equipped with a recoil starter .
78/83/90 Series Horiz ontal Shaft Engines 70 Start er Cu p The st arter cup is a steel cup that is bolted to the flywheel. 1. Inspect the inside of the starter c up. See Figure 6.2. NOTE: If the starter was fa iling to engage the fly - wheel, and the edge s of the slots inside the cup are bu rred or damag ed, replace t he starter cup.
Starter and Charging System 71 Start er Ro pe 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 th e rem - nants of the old rope and install a new rope.
78/83/90 Series Horiz ontal Shaft Engines 72 9. Give the starter a couple of te st pu lls to ve rify the right amount of te nsion on the starter ro pe and check the action of the pawls. NOTE: If starter rope te nsion needs to be adjusted, hook the rope into the notch in the pulley and wind the pulley a couple of turn s to add tension-.
Starter and Charging System 73 St arter pulley and recoil spring If damage is suspected, the recoil may be disa ssembled by: NOTE: The recoil spring is nested within th e starter pulley and both part s are sold as a single part num ber . 1. Remove the starter by following the steps described earlier in th is chapter .
78/83/90 Series Horiz ontal Shaft Engines 74 5. Carefully lift the spring and pulley out of the recoil housing. See Figure 6.9. NOTE: If the spring is undamaged, but h as been removed from the pulley , the spring may be re-wound.
Starter and Charging System 75 Electric st arter The electric starter is only available on the snow engine. It is powered by an extension cord that is plugged into househ old 120 volt AC current. The starter a nd switch assembly are one pi ece and are not serviceable.
78/83/90 Series Horiz ontal Shaft Engines 76 4. Pull back o n the starter approx imately 1/2”. Then angle it away from the engine while sliding it out o f the engine. See Fig ure 6.13. NOTE: Before condemning a star ter make sure to bench test it. T o bench test a st arter: A.
Starter and Charging System 77 Charging system Some engines are equipped with a charging system. The charging system us ed on MTD engines c onsist s of three component s, th e rotor , stator and the rectifie r .
78/83/90 Series Horiz ontal Shaft Engines 78 Charging syst em testing T o test the charging sys tem: 1. Locate the charger harnes s. It will be by the right handle bar of the sn ow thrower when the engine is installed. See Figure 6.16. 2. S tart the engine and run it at full throttle.
Starter and Charging System 79 Stat or T o remove/replace the st ator: 1. Remove and ground the sp ark plug wire. 2. Remove the flywheel by following the step s described in Chap ter 7: Ignition System. 3. Remove the baf fle that covers the charger harn ess using a 10mm wrench .
78/83/90 Series Horiz ontal Shaft Engines 80 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
Ignition System 81 T roubleshooting the ignition syst em The purpose of the igniti on system is to provide a sp ark in the combustion chamber at the proper time to ef ficiently ignite the fuel/air mixtu re. The step s in tro ubleshooti ng the ignition system are: 1.
78/83/90 Series Horiz ontal Shaft Engines 82 Stop switch All MTD ho rizontal engines that are in use in Nor th America have a stop switch bu ilt into the throttle lever assembly . MTD engines used on snow blowers have an additional stop (ignition) switch in the engine sh roud.
Ignition System 83 Remote (ignition) stop switc h (snow engines) T o test the remote stop switch: 1. Remove the engine shroud by following the proce - dures describe d in Chapter 3: Air Int ake System. 2. Disconnect the two wires from the remote switch.
78/83/90 Series Horiz ontal Shaft Engines 84 T est for ignition that won’t turn off MTD engines are tur ned of f by removing the sp ark from the engin e. This is accomplished by shor ting the primary windings of the coil to ground . If the engine does not stop when the key is remove d and/or the throttle is moved to the stop p osition: 1.
Ignition System 85 The module The coil in this ignition s ystem is an inductive discharge mag neto, cont ained in a single module. • The induct ive discharge magn eto has a two leg design. • The magneto is energized by the p assin g of a magnet mounted in the flywheel.
78/83/90 Series Horiz ontal Shaft Engines 86 Module removal 1. Unplug the sp ark plug. 2. Remove the engine shrou d by fo llowing the steps procedures in Chapte r 3: Air Intake Systems. 3. Remove the front fuel t ank shroud using a 10 mm wrench. See Figure 7.
Ignition System 87 Inst alling the module and setting the air gap NOTE: If just setting the air gap, loosen the module mounting screws first then follow the s ame steps as described below . 1. Rotate the flywheel so that the magnet s are away from where the module is mounted.
78/83/90 Series Horiz ontal Shaft Engines 88 Flywheel The flywheel holds the magnet s. These magnets induce a fiel d in the module which in turn produces a sp ark. It also controls the timing of the ignition system by c ontrolling when the magnets are introduced to the module.
Ignition System 89 About the sp ark plug • The spark plug is a F6R TC, gapped to 0.026” - 0.030” (0.65 - 0.75 mm ). NOTE: The F6RTC plug is the only plug that is EP A certified for the MTD engine. • Wear rate will vary some what with severity of use.
78/83/90 Series Horiz ontal Shaft Engines 90 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
Exhaust 91 The exhaust system is a frequently over looked comp onent of an engine. It is important to make sure the muffler is in good condition and fr ee of debris and/or insects. NOTE: A blocked muf fler will result in poor performance. If a muf fler is comp letely blocked the engine may not start.
78/83/90 Series Horiz ontal Shaft Engines 92 Muffler removal/replacement 1. Remove the two nuts that h old the manifold pipe to the cylinder head using a 13mm wrench. See Figure 8.3. 2. Lif t the muffler of f of the engine. 3. Clean all of the gasket material of f of the cylinder head and the mu ffler .
Exhaust 93 6. Remove the four screws that secure the outer heat shield to the muffler . See Figur e 8.6. 7. Slide the outer heat shield of f of the muff ler . NOTE: The inner heat shield is se rviced with the muffler . 8. Install the muf fler by following the previous steps in reverse orde r .
78/83/90 Series Horiz ontal Shaft Engines 94 Muffler removal/replace ment (snow engines) T o remove/rep lace the muff ler: 1. Remove the control p anel by following the proce - dures described in Chapter 3: Air Intake System. 2. Remove the muffle r heat shield by: 2a.
Exhaust 95 6. Remove the two muf fler nut s using a 13mm wrench and lift the muf fler of f of the engine. See Figure 8 .1 0. 7. Clean all of the gasket material of f of the cylinder head and the mu ffler (if reusing the muf fler) NOTE: The MTD engine uses a graphi te exhaust gasket.
78/83/90 Series Horiz ontal Shaft Engines 96 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
Cylinder head 97 Cylinder head removal The Cylinder head of the MTD engine ca n be removed without removing the en gine from the piece of equipment. T o remove the cylinder head: 1. Disconnect and ground the spark p l ug high tension lead. 2. Remove the sp ark plug using a 13/1 6” or 21mm wrench.
78/83/90 Series Horiz ontal Shaft Engines 98 8. Remove the s crew that fa stens 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 eng ine o r moved to the side.
Cylinder head 99 NOTE: The 490 engine has an over sized bo lt with a Belleville washer in the number 4 position. The wrench size and torq ue is the same as the other three bolt s. See Figu re 9.6. 14. Lift the cylinder head of f of the engine. 15. Carefully clean all sealing surfaces of all gasket resi - due.
78/83/90 Series Horiz ontal Shaft Engines 100 Cylinder head inst allation 1. Place a new head gasket on the cylinder , allowing the alignment dowels to hold it in place. See Figure 9.8. NOTE: MTD uses graphite head gasket s that have a bead of silicon on them.
Cylinder head 101 Va l v e s The valves and valve seat s can be serviced by g rinding and lapping or the head can be replaced. Dependin g on local machine and labor costs, it is probably more economical to rep lace the cylinder head versus servicing the valves.
78/83/90 Series Horiz ontal Shaft Engines 102 4. Lif t the springs off of the valve stems. 5. Slide the valves out of the cylin de r hea d. NOTE: Only the intake valve h as a valve guide seal. See Figure 9.12. 6. Inspect the valve seat. See Figure 9.13.
Cylinder head 103 10. T est th e valv es 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 clea ning solvent into the combustion chamber (just enou gh to cover the valves).
78/83/90 Series Horiz ontal Shaft Engines 104 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
Crankshaft, piston and connecting rod 105 The exact procedure a technician uses to disassemble an engine dep ends on the type of repairs needed . This chapter is written as a set of procedures that should provide the user with sufficie nt information to complete any fea - sible repair to the eng ine short block assembly .
78/83/90 Series Horiz ontal Shaft Engines 106 12. Remove the camshaf t. 13. Remove the ba lance sha ft. See Figure 10.2. NOTE: The valve tappet should be kept ri ding against their original lobes. Once broken in, switching the tappets to run o n dif ferent cam lobes will cause rapid t appet and cam wear .
Crankshaft, piston and connecting rod 107 16. Push the piston out of the cylinder . NOTE: Sometimes a ridge of carbon builds up where the cylinder me ets the head. If this happ ens, the piston can be removed from inside of th e cylinder block. 17. Remove one of the piston pin retaining r ings.
78/83/90 Series Horiz ontal Shaft Engines 108 Crankshaft inspection 1. Inspect the cranksha ft journals a nd 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 afte r it is found to be OK visually .
Crankshaft, piston and connecting rod 109 Piston Inspection 1. Clean the piston and re move all carbon from the rings and ring groves. 2. Clean the cylinder bore an d remove all carbon. 3. Insert one ring into the cylinder . Push it down about one inch from the top.
78/83/90 Series Horiz ontal Shaft Engines 110 7. Measure the gap b etween the ring an d the ring land using a feeler gau g e an d co mpare th e me a sur e - ment to the chart at the end of this chapter . See Figure 10.1 1. 8. Measure the piston pin bo re on both sides of the piston using telescoping g auges or vernier caliper .
Crankshaft, piston and connecting rod 111 Connecting rod inspection 1. Inspect the connec ting rod for cracks or any signs of damage. 2. Install the ro d cap and tighten to a torque of 106 - 124 in-lbs (12 - 14Nm ).
78/83/90 Series Horiz ontal Shaft Engines 112 Balance Shaf t (483 & 490) There are two primary motions that gene rate most of the vibrations in single-cyl inder engines; the r otatio n of the crankshaft, and the reciprocating motion of the piston .
Crankshaft, piston and connecting rod 11 3 Reassembly 1. Install the governor shaft. NOTE: The governor shaft MUST be installed before the cran kshaft is installed. 2. Clean the cylinder 2a. Remove all gasket material from all mating surfaces. 2b. Clean the cylinder and cran k case cover .
78/83/90 Series Horiz ontal Shaft Engines 114 6. Install the balance s haft by: 6a. Pre-l ube the balance shaf t with clean 10W-30 motor oil 6b. Rotate the cranksh aft until the tim ing mar k on the larger gear point s to the 7:30 position. 6c. Insert the balance shaf t while aligning the tim - ing marks.
Crankshaft, piston and connecting rod 11 5 Engine specifications chart S pecification New Min New Max Service Limit in. mm in. mm in. mm Bore (digit 2&3 of the model number) 78 3.071 78.02 3.072 78.03 3.08 0 78.23 83 3.268 83.02 3.269 83 .03 3.277 83.
78/83/90 Series Horiz ontal Shaft Engines 116 in. mm in. mm in. mm Intake lobe height 78 1.248 31. 70 1.252 31.80 1.2 37 31.42 83 1.281 32.54 1.285 32 .64 1.269 32.24 90 1.281 32.54 1.285 32 .64 1.269 32.24 Exhaust lobe height 78 1.248 31.70 1.252 31.
Crankshaft, piston and connecting rod 11 7 in. mm in. mm in. mm Balance shaf t bearing max. ID (cylinder block) 78 N/A N/A N/A N/A N/A N/A 83 0.591 15.00 0.591 15 .00 0.592 15.03 90 0.591 15.00 0.591 15 .00 0.592 15.03 Compression ring to land max. clearance 78 0.
78/83/90 Series Horiz ontal Shaft Engines 118 Minimum Maximum in. mm in. mm Intake valve lash 78 0.004 0.10 0.006 0.15 83 0.004 0.10 0.006 0.15 90 0.004 0.10 0.006 0.15 Exhaust valve lash 78 0.006 0.15 0.008 0.20 83 0.006 0.15 0.008 0.20 90 0.006 0.15 0.
Crankshaft, piston and connecting rod 11 9 Engine torque valu es chart Fastener T orque 78 83 90 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.
78/83/90 Series Horiz ontal Shaft Engines 120 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
Failure Analysis 121 A properly maintained engine will provid e years of service. Occasionally an engine will fail. An important part of working on engine s is finding out why th ey failed.
78/83/90 Series Horiz ontal Shaft Engines 122 5. When particles enter the combustion chamber , the up and down motion of the piston grind s the p arti - cles into the s ide of the cylinder walls and damages the cylinder wall, piston and piston rings 6.
Failure Analysis 123 8. Because the oil suspends the abrasive p articles, the engine components that are immersed in oil will show definite signs of abrasive ingestion especially ar ound the connecting rod and main bearing jou rnals. See Figure 1 1.5.
78/83/90 Series Horiz ontal Shaft Engines 124 Insuffic ient lubricatio n The bearing surf ac es in an en gine ar e no t sm oo th . As a result of the machining processes to make the engine part s, there are little peaks and valleys that are on ly vi sible on a microscopic sca le.
Failure Analysis 125 Engine Overs peed The MTD engine is designed for a maximu m speed of 36 00 rpm. When the go vernor is unable to control the engine rpm , the engine can accelerate past the safe maximum spee d. When an engine runs beyond it s designed speed, a few things happen: 1.
78/83/90 Series Horiz ontal Shaft Engines 126 Overheated The MTD engines are air cool ed engines. Because of this, clean liness of the engine is very important to th e life of the engine. Dirt, grass and sludge all form an insulating layer on the engine.
Failure Analysis 127 Mechanical Breakage/ W ear Sometimes an engine fails because a part br eaks. There are gene rally three causes of a b roken p a rt, out side of the previously discussed engine fa ilur es. They are abuse, wear , and manufacturing defect s.
78/83/90 Series Horiz ontal Shaft Engines 128 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com.
MTD Products Inc - Product Trai ning and Education Department FORM NUMBER - 769-04951A 10/2010 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.
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