Yamaha MX250 Service Manual
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Yamaha MX250 Service Manual

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SERVICE MANUAL
MX250
MX300
LIT-19616-02-45
7VE-F8197-10

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Summary of Contents for Yamaha MX250

  • Page 1 SERVICE MANUAL MX250 MX300 LIT-19616-02-45 7VE-F8197-10...
  • Page 3 MX250, MX300 SERVICE MANUAL ©2017 by Yamaha Motor Corporation, U.S.A. 1st Edition, March 2017 All rights reserved. Any reprinting or unauthorized use without the written permission of Yamaha Motor Corporation, U.S.A. is expressly prohibited. LIT-19616-02-45...
  • Page 4 This manual was produced by the Yamaha Motor Powered Products Co., Ltd. primarily for use by Yamaha dealers and their qualified mechanics. It is not possible to include all the knowledge of a mechanic in one manual. Therefore, anyone who uses this book to perform maintenance and repairs on Yamaha machines should have a basic understanding of mechanics and the techniques to repair these types of machines.
  • Page 5 HOW TO USE THIS MANUAL This manual is intended as a handy, easy-to-read reference book for the mechanic. Comprehensive explanations of all installation, removal, disassembly, assembly, repair and check procedures are laid out with the individual steps in sequential order. •...
  • Page 6 SYMBOLS The following symbols are used in this manual for easier understanding. The following symbols are not relevant to every machine. SYMBOL DEFINITION SYMBOL DEFINITION Filling fluid Lubricant Special tool Tightening torque Wear limit, clearance Engine speed Electrical data Replace the part with a new one. Lithium-soap base grease Molybdenum disulfide grease Engine oil...
  • Page 7 INDEX GENERAL INFORMATION PERIODIC CHECKS AND ADJUSTMENTS ENGINE CARBURETOR ELECTRICAL TROUBLESHOOTING SPECIFICATIONS...

  • Page 8: Table Of Contents

    TABLE OF CONTENTS CYLINDER HEAD GENERAL INFORMATION DECARBONIZATION ......2-11 ADJUSTING THE VALVE MACHINE IDENTIFICATION ......1-1 CLEARANCE ........2-11 SERIAL NUMBER .........1-1 ENGINE SPEED ......... 2-13 STARTING SERIAL NUMBER....1-1 ADJUSTING THE ENGINE SPEED..2-14 RECOIL STARTER ......2-15 DIMENSIONS ..........1-2 FITTINGS AND FASTENERS.....
  • Page 9 INSTALLING THE FLYWEIGHT CARBURETOR SHAFT ASSEMBLY AND GOVERNOR FORK......3-19 REMOVING THE CARBURETOR....4-1 ADJUSTING THE GOVERNOR ..3-20 DISASSEMBLING THE CARBURETOR ... 4-2 ENGINE SWITCH AND OIL WARNING CHECKING THE CARBURETOR ..4-3 UNIT............3-21 CYLINDER HEAD COVER, CYLINDER HEAD ............3-22 ELECTRICAL CHECKING THE ROCKER ARM ..3-24 CHECKING THE PUSH ROD....3-24 CIRCUIT DIAGRAM ........
  • Page 10 ELECTRICAL ........7-7 TIGHTENING TORQUES ......7-8 GENERAL TORQUE SPECIFICATIONS ........7-10 LUBRICATION POINTS AND TYPE OF LUBRICANTS...........7-11 WIRE ROUTING DIAGRAM .....7-12 UPPER SIDE VIEW......7-12 ENGINE SWITCH AND OIL WARNING UNIT ........7-14 TCI UNIT ..........7-16 OIL LEVEL SWITCH (CRANKCASE COVER)..........7-18...

  • Page 11: General Information

    GENERAL INFORMATION MACHINE IDENTIFICATION SERIAL NUMBER The serial number is printed on the label “1” affixed to the position of the multi-purpose engine as shown in the illus- tration. • The first four digits identify the model, and the remaining MX300 digits indicates the production number.

  • Page 12: Dimensions

    DIMENSIONS DIMENSIONS DIMENSION CHART in (mm) 7/16 -20UNF-2B 88.5 6.31 72.2    5/16-24UNF-2B DEPTH0.71 4.05(103) 5/16-24UNF-2B    DEPTH0.71 3/8-16UNC2B    DEPTH0.71 *1: UNF: Unified fine thread (Unit: in) *2: UNC: Unified coarse thread (Unit: in)

  • Page 13: Top

    DIMENSIONS in (mm) 17.49 (444.5) MOUNTING BASE in (mm) 0.63 (16) 1.46 2.32 (59) (37)

  • Page 14: Left Side

    DIMENSIONS LEFT SIDE in (mm) 4.06 (103)

  • Page 15: Front

    DIMENSIONS FRONT in (mm) 15.36 (390.3) 7.2 (183) 8.16 (207.3) 3.48 (88.5)

  • Page 16: Important Information

    Be sure to use the correct special tool for the job to guard against damage. Oil, grease and seals Be sure to use genuine Yamaha oils, grease and sealers, or the equivalents. Expendable parts Always replace the gaskets, O-rings, cotter pins and cir-...

  • Page 17: All Replacement Parts

    • Make sure that the parts, move smoothly after each sec- tion of the machine is assembled. ALL REPLACEMENT PARTS We recommend the use of genuine Yamaha parts for all replacements. Use oil and/or grease, recommended by Yamaha, for assembly and adjustment.

  • Page 18: Bearings And Oil Seals

    BASIC SERVICE INFORMATION BEARINGS AND OIL SEALS Install the bearing(s) “1” and oil seal(s) “2” with their manu- facture’s marks or numbers facing outward. (In other words, the stamped letters must be on the side exposed to view.) When installing oil seal(s), apply a light coating of lightweight lithium base grease to the seal lip(s).
  • Page 19 BASIC SERVICE INFORMATION Checking the connections Check the leads, couplers, and connectors for stains, rust, moisture, etc. Disconnect: • Lead • Coupler • Connector NOTICE • When disconnecting a coupler, release the coupler lock, hold both sections of the coupler securely, and then disconnect the coupler.
  • Page 20 BASIC SERVICE INFORMATION Check: • All connections Loose connection  Connect properly. • If the pin “1” on the terminal is flattened, bend it up. • After disassembling and assembling a coupler, pull on the leads to make sure that they are installed securely. Connect: •...
  • Page 21 BASIC SERVICE INFORMATION Check: • Continuity (with the digital circuit tester) Model 88 Multimeter with tachometer: YU-A1927 Digital circuit tester (CD732): 90890-03243 • If there is no continuity, clean the terminals. • When checking the wire harness, perform steps (1) to (4).

  • Page 22: Special Tools And Testers

    SPECIAL TOOLS AND TESTERS SPECIAL TOOLS AND TESTERS The proper special tools are necessary for complete and accurate tune-up and assembly. Using the correct special tool will help prevent damage caused by the use of improper tools or improvised techniques. •...

  • Page 23: Periodic Checks And Adjustments

    PERIODIC CHECKS AND ADJUSTMENTS INTRODUCTION This chapter includes all information necessary to perform recommended checks and adjustments. These preventive maintenance procedures, if followed, will ensure more reliable machine operation and a longer service life. The need for costly overhaul work will be greatly reduced. This information applies to machines already in service as well as new machines that are being prepared for sale.
  • Page 24 *1 ..The air filter and foam element need to be cleaned more frequently when using in unusually wet or dusty areas..Since these items require special tools, data and technical skills, have a Yamaha dealer per- form the service.

  • Page 25: Periodic Maintenance

    PERIODIC MAINTENANCE PERIODIC MAINTENANCE SPARK PLUG WARNING Check and adjust the areas around the cylinder head after the engine has cooled down completely. Remove: • Spark plug cap • Spark plug NOTICE Before removing the spark plug, use compressed air to clean the cylinder head cover to prevent dirt from falling into the engine.

  • Page 26: Fuel Leakage

    PERIODIC MAINTENANCE Install: • Spark plug Spark plug: 20 N·m (2.0 kgf·m, 15 lb·ft) To prevent threads from being damaged, temporally tighten “a” the spark plug before tightening it to the speci- fied torque “b”. FUEL LEAKAGE Check: • Leakage Check at fuel tank, fuel cock, fuel hose, and carbu- retor NOTICE...

  • Page 27: Replacing The Engine Oil

    PERIODIC MAINTENANCE Check: • Check that engine oil is at the specified level “a”. ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼ Oil level checking steps: a. Place the multi-purpose engine on a level surface. b. Warm up the engine for several minutes. c. Stop the engine. d.

  • Page 28: Air Filter Element

    PERIODIC MAINTENANCE Fill: 0 10 30 50 70 90 110130 °F • Engine oil Make sure to fill with the recommended engine oil from the oil filler hole in the crankcase. SAE 10W-30 Recommended oil brand: SAE 10W-40 YAMALUBE Available oil grade: 20 -10 0 10 20 30 40 50 °C API SE type or higher JASO MA or MB Available viscosity index:...

  • Page 29: Muffler

    PERIODIC MAINTENANCE Check: • Air filter element Damage  Replace. Clogged  Wash the element with white kerosene, and then dry it thoroughly. Oil the element and squeeze out the excess oil. WARNING Do not wash the element with gasoline or with acidic, alkalic, or organic solvents.
  • Page 30 PERIODIC MAINTENANCE Remove: • Spark arrester “1” Use a flathead screw driver to pry the spark arrester out from the muffler. Remove: • Muffler (Refer to “MUFFLER” on page 3-6) Decarbonize: • Muffler Tap on the muffler in the area shown in the illustra- tion to loosen carbon buildup, and then shake it out from the end of the muffler.

  • Page 31: Fuel Tank Filter

    PERIODIC MAINTENANCE Install: • Muffler (Refer to “INSTALLING THE MUFFLER” on page 3-7) • Spark arrester • Spark arrester screw Spark arrester screw: 1.8 N·m (0.18 kgf·m, 1.3 lb·ft) Align the spark arrester lump “a” with the hole “b” in the muffler pipe.

  • Page 32: Fuel Cock Strainer

    PERIODIC MAINTENANCE Check: • Fuel tank filter Damage  Replace. Dirt/clog  Clean. Clean the fuel tank filter with clean gasoline, and then dry it thoroughly. Install: • Fuel tank filter • Fuel tank cap WARNING Make sure that the tank cap is tightened securely. FUEL COCK STRAINER WARNING Do not smoke, and keep away from open flames,...

  • Page 33: Breather Hose

    PERIODIC MAINTENANCE BREATHER HOSE Check: • Breather hose “1” Crack/damage  Replace. Poor connection  Connect. CYLINDER HEAD DECARBONIZATION Remove: • Cylinder head assembly (Refer to “CYLINDER HEAD COVER, CYLINDER HEAD” on page 3-22) Eliminate: • Carbon deposits (Refer to “CHECKING THE CYLINDER HEAD” on page 3-24) Install: •...
  • Page 34 PERIODIC MAINTENANCE Pull the recoil starter slowly, and then set the piston at TDC (top-dead-center) on the compression stroke. Check the piston position by inserting a screw driver into the spark plug hole. Measure: • Valve clearance (Between the rocker arm and valve stem end) Out of specification ...

  • Page 35: Engine Speed

    PERIODIC MAINTENANCE Adjuster Valve clearance Turn clockwise Decrease Turn counterclockwise Increase c. Tighten the locknut “1”. Valve adjuster locknut: 7 N·m (0.7 kgf·m, 5.2 lb·ft) ▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲ Install: • Cylinder head cover gasket • Cylinder head cover Cylinder head cover bolt: 11 N·m (1.1 kgf·m, 8.1 lb·ft) •...

  • Page 36: Adjusting The Engine Speed

    PERIODIC MAINTENANCE ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼ Measurement steps: a. Move the throttle lever “1” to the high engine speed position “a”. b. Check the high engine speed. ▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲ Measure: • Low engine speed (with no load) Out of specification  Adjust. Low engine speed: 1900–2100 r/min ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼...

  • Page 37: Recoil Starter

    PERIODIC MAINTENANCE Adjust: • Engine speed ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼ Adjustment steps: a. Loosen the throttle stop screw “1”. b. Move the throttle lever “2” to direction “a” or “b” until the high engine speed is obtained. High engine speed: 3750–3850 r/min Direction “a” High engine speed is decreased.

  • Page 38: Fittings And Fasteners

    PERIODIC MAINTENANCE FITTINGS AND FASTENERS Check: • All fittings and fasteners Looseness  Tighten. Rough movement  Replace the defective part(s). Damage/pitting  Replace. 2-16...

  • Page 39: Engine

    ENGINE ENGINE INSPECTION MEASURING THE COMPRESSION PRESSURE Measure the compression pressure after checking and adjusting the valve clearance. Warm up the engine for several minutes. Remove: • Spark plug cap • Spark plug NOTICE Before removing the spark plug, use compressed air to clean the cylinder head cover to prevent dirt from falling into the engine.
  • Page 40 ENGINE INSPECTION Reading Diagnosis Higher than • Worn cylinder, piston, and piston ring(s) without oil • Defective piston, piston ring(s), valve(s), and Same as cylinder head gasket without oil • Improper valve timing and valve clearance ▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲ ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼ Testing step (above the maximum specification): a.

  • Page 41: Air Filter

    AIR FILTER AIR FILTER 1.6 N · m (0.16 kgf · m, 1.2 lb · ft) 0.9 N · m (0.09 kgf · m, 0.66 lb · ft) 7 N · m (0.7 kgf · m, 5.2 lb · ft) 9 N ·...

  • Page 42: Fuel Tank

    FUEL TANK FUEL TANK 10 N · m (1.0 kgf · m, 7.4 lb · ft) 10 N · m (1.0 kgf · m, 7.4 lb · ft) 29 N · m (2.9 kgf · m, 21 lb · ft) 6 N ·...

  • Page 43: Checking The Fuel Tank Cap (Canister)

    FUEL TANK CHECKING THE FUEL TANK CAP (CANISTER) Check: • Fuel tank cap (canister) “1” Cracks/damage  Replace.

  • Page 44: Muffler

    MUFFLER MUFFLER 10 N · m (1.0 kgf · m, 7.4 lb · ft) 10 N · m (1.0 kgf · m, 7.4 lb · ft) 1.8 N · m (0.18 kgf · m, 1.3 lb · ft) 20 N · m (2.0 kgf · m, 15 lb · ft) 20 N ·...

  • Page 45: Installing The Muffler

    MUFFLER INSTALLING THE MUFFLER Install: • Gasket • Muffler assembly • Muffler nut “1” • Muffler nut “2” • Muffler bolt “3” Temporarily tighten the nut “1”, nut “2”, and bolt “3” until just comes in contact with the surface. Tighten: •...

  • Page 46: Recoil Starter

    RECOIL STARTER RECOIL STARTER 5.4 N · m (0.54 kgf · m, 3.9 lb · ft) 7 N · m (0.7 kgf · m, 5.2 lb · ft) Order Job/Parts to remove Q’ty Remarks Removing the recoil starter Remove the parts in the order listed. Recoil starter assembly Starter handle Drive plate...

  • Page 47: Disassembling The Recoil Starter

    RECOIL STARTER DISASSEMBLING THE RECOIL STARTER Remove: • Starter handle “1” Before untying the knot “a” above the starter handle, make a knot “b” on the rope so that the rope is not pulled into the case. Remove: • Drive plate screw “1” •...

  • Page 48: Assembling The Recoil Starter

    RECOIL STARTER Check: • Drive pawls “1” • Drive plate “2” Wear/damage  Replace. • Springs “3” • Compression spring “4” Cracks/damage  Replace. ASSEMBLING THE RECOIL STARTER Install: • Starter rope “1” • Starter spring “2” • Sheave drum “3” Engage the spring hook “a”...
  • Page 49 RECOIL STARTER Install: • Starter handle • Drive plate screw “1” To set the spring preload, hook the starter rope into the sheave drum slot “a” and turn the sheave drum assembly counterclockwise four times, and then unhook the starter rope from the slot.

  • Page 50: Flywheel

    FLYWHEEL FLYWHEEL 10 N · m (1.0 kgf · m, 7.4 lb · ft) 7 N · m (0.7 kgf · m, 5.2 lb · ft) 100 N · m (10 kgf · m, 74 lb · ft) 7 N · m (0.7 kgf · m, 5.2 lb · ft) Order Job/Parts to remove Q’ty...

  • Page 51: Removing The Flywheel

    FLYWHEEL REMOVING THE FLYWHEEL Remove: • Flywheel cover • TCI unit Remove: • Flywheel nut “1” • Washer • Starter pulley Attach the sheave holder “2” to hold the flywheel. Primary clutch holder: YS-01880-A Sheave holder: 90890-01701 Remove: • Flywheel “1” •...

  • Page 52: Installing The Flywheel

    FLYWHEEL INSTALLING THE FLYWHEEL Install: • Woodruff key • Flywheel Install: • Starter pulley • Washer • Flywheel nut “1” Flywheel nut: 100 N·m (10 kgf·m, 74 lb·ft) Tighten the flywheel nut “1” using the sheave holder “2” to hold the flywheel. Primary clutch holder: YS-01880-A Sheave holder:...
  • Page 53 FLYWHEEL Measure: • TCI unit air gap Out of specification  Adjust. Feeler gauge set: YU-26900-9 Thickness gauge: 90890-03180 TCI unit air gap: 0.4–0.6 mm (0.0157–0.0236 in) Adjust: • TCI unit air gap ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼ Adjustment steps: a. Loosen the TCI unit bolts “1”. b.

  • Page 54: Governor

    GOVERNOR GOVERNOR THROTTLE LEVER ASSEMBLY 8 N · m (0.8 kgf · m, 5.9 lb · ft) 7 N · m (0.7 kgf · m, 5.2 lb · ft) 1.1 N · m (0.11 kgf · m, 0.81 lb · ft) 7 N ·...

  • Page 55: Flyweight Shaft Assembly

    GOVERNOR FLYWEIGHT SHAFT ASSEMBLY Order Job/Parts to remove Q’ty Remarks Removing the flyweight shaft assembly Remove the parts in the order listed. Fuel tank Refer to “FUEL TANK” on page 3-4. Fuel tank stay Refer to “FUEL TANK” on page 3-4. Refer to “ENGINE SWITCH AND OIL Engine switch and oil warning unit WARNING UNIT”...

  • Page 56: Removing The Flyweight Shaft Assembly And Governor Fork

    GOVERNOR REMOVING THE FLYWEIGHT SHAFT ASSEMBLY AND GOVERNOR FORK Remove: • Crankcase cover • Governor assembly Remove: • Clip “1” • Governor fork “2” • Washers “3” Remove: • Flyweight shaft assembly “1” • Washer “2” Remove the flyweight shaft assembly by tapping the weight shaft from outside of the crankcase cover.

  • Page 57: Assembling The Flyweight Shaft Assembly

    GOVERNOR Check: • Weight • Flyweight shaft • Collar • Washer • Circlip • Weight shaft Wear/damage  Replace. ASSEMBLING THE FLYWEIGHT SHAFT ASSEMBLY Install: • Weight shaft “1” • Circlip “2” • Washer “3” • Collar “4” • Weights “5” •...

  • Page 58: Adjusting The Governor

    GOVERNOR Adjust: • Governor (Refer to “ADJUSTING THE GOVERNOR” on page 3-20) ADJUSTING THE GOVERNOR Adjust: • Governor ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼ Adjustment steps: a. Loosen the governor arm bolt “1”. b. Turn the governor arm “2” clockwise until it stops. c. Turn the governor fork “3” clockwise until it stops. d.

  • Page 59: Engine Switch And Oil Warning Unit

    ENGINE SWITCH AND OIL WARNING UNIT ENGINE SWITCH AND OIL WARNING UNIT 7 N · m (0.7 kgf · m, 5.2 lb · ft) 4.5 N · m (0.45 kgf · m, 3.3 lb · ft) Order Job/Parts to remove Q’ty Remarks Removing the engine switch and oil...

  • Page 60: Cylinder Head Cover, Cylinder Head

    CYLINDER HEAD COVER, CYLINDER HEAD CYLINDER HEAD COVER, CYLINDER HEAD 11 N · m (1.1 kgf · m, 8.1 lb · ft) 20 N · m (2.0 kgf · m, 15 lb · ft) 7 N · m (0.7 kgf · m, 5.2 lb · ft) 2.3 N ·...
  • Page 61 CYLINDER HEAD COVER, CYLINDER HEAD 11 N · m (1.1 kgf · m, 8.1 lb · ft) 20 N · m (2.0 kgf · m, 15 lb · ft) 7 N · m (0.7 kgf · m, 5.2 lb · ft) 2.3 N ·...

  • Page 62: Checking The Rocker Arm

    CYLINDER HEAD COVER, CYLINDER HEAD CHECKING THE ROCKER ARM Check: • Rocker arm • Wear/damage/cracks  Replace. CHECKING THE PUSH ROD Check: • Push rod runout Runout limit: 0.15 mm (0.0059 in) Out of specifications  Replace. CHECKING THE CYLINDER HEAD Check: •...

  • Page 63: Installing The Cylinder Head Assembly

    CYLINDER HEAD COVER, CYLINDER HEAD INSTALLING THE CYLINDER HEAD ASSEMBLY Install: • Cylinder head • Cylinder head bolts “1” to “5”. Tighten the bolts to the specified torque in two steps and in order from “1” to “5”. Cylinder head bolt: 1st: 12 N·m (1.2 kgf·m, 8.9 lb·ft) 2nd: 50 N·m (5.0 kgf·m, 37 lb·ft) 3-25...

  • Page 64: Valve

    VALVE VALVE Order Job/Parts to remove Q’ty Remarks Removing the valve Remove the parts in the order listed. Refer to “CYLINDER HEAD COVER, CYLIN- Cylinder head assembly DER HEAD” on page 3-22. Valve cotter Valve spring retainer Valve spring Exhaust valve Intake valve Valve stem seal Intake side only.

  • Page 65: Removing The Valves And Valve Springs

    VALVE REMOVING THE VALVES AND VALVE SPRINGS The following procedure applies to all of the valves, valve springs and related components. Remove: • Valve cotter “1” • Valve spring retainer “2” • Valve spring “3” • Valve “4” • Valve stem seal (Intake side only) Remove the parts using the valve spring compres- sor “5”.
  • Page 66 VALVE Measure: • Valve stem diameter “a” Valve stem diameter (Intake): 5.948–5.963 mm (0.2342–0.2348 in) Valve stem diameter (Exhaust): 5.940–5.955 mm (0.2339–0.2344 in) Limit (Intake): 5.918 mm (0.2330 in) Limit (Exhaust): 5.910 mm (0.2327 in) Out of specifications  Replace. Measure: •...

  • Page 67: Checking The Valve Seats

    VALVE Installed compression spring force (Intake): 91.20–104.93 N (9.12–10.49 kgf, 20.50–23.59 lbf) Installed compression spring force (Exhaust): 91.20–104.93 N (9.12–10.49 kgf, 20.50–23.59 lbf) Installed length (Intake): 29.0 mm (1.1417 in) Installed length (Exhaust): 29.0 mm (1.1417 in) Measure: • Valve spring tilt “a” Tilt limit: 1.7 mm (0.0669 in) Out of specifications ...

  • Page 68: Valve Lapping

    VALVE Insert the valve into the valve guide and use a valve lapper to contact the valve face with the valve seat. Do not rotate the valve while the valve face is contacting the valve seat. Measure: • Valve face contact width “a” Make sure that the contact width along the entire valve face is within specifications.

  • Page 69: Installing The Valves And Valve Springs

    VALVE Clean off all of the lapping compound from the valve face and valve seat. Apply fine lapping compound on the valve face and lap the valve as described in step Once the contacting surface of the valve face is pol- ished and becomes shiny, apply mechanic’s blue lay- out fluid to make sure that there are traces of even contact in the center of the valve face.

  • Page 70: Piston, Camshaft, Crankcase, And Crankshaft

    PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT 27 N · m (2.7 kgf · m, 20 lb · ft) 27 N · m (2.7 kgf · m, 20 lb · ft) 10 N · m (1.0 kgf · m, 7.4 lb · ft) 10 N ·...
  • Page 71 PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT 20 N · m (2.0 kgf · m, 15 lb · ft) Order Job/Parts to remove Q’ty Remarks Camshaft Valve lifter Connecting rod cap Crankshaft Connecting rod Piston pin circlip Piston pin Piston Piston ring set 3-33...

  • Page 72: Removing The Camshaft And Valve Lifter

    PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT REMOVING THE CAMSHAFT AND VALVE LIFTER Remove: • Camshaft “1” Remove the camshaft “1” when the camshaft gear mark “b” and the crankshaft gear mark “a” are aligned. Remove: • Intake valve lifter “1” • Exhaust valve lifter “2” CHECKING THE CAMSHAFT Check: •...

  • Page 73: Checking The Valve Lifter

    PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT Camshaft lobe dimensions: Lobe height “a” (Intake): 32.1–32.2 mm (1.2638–1.2677 in) Limit: 31.9 mm (1.2559 in) Lobe height “a” (Exhaust): 32.5–32.6 mm (1.2795–1.2835 in) Limit: 32.3 mm (1.2717 in) Base circle diameter “b” (Intake): 25.98–26.08 mm (1.0228–1.0268 in) Limit: 25.78 mm (1.015 in) Base circle diameter “b”...

  • Page 74: Installing The Valve Lifter And Camshaft

    PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT INSTALLING THE VALVE LIFTER AND CAMSHAFT Install: • Intake valve lifter “1” • Exhaust valve lifter “2” Install: • Camshaft “1” NOTICE Be sure to align the camshaft “1” gear mark “b” with the crankshaft gear mark “a”. CHECKING THE CRANKCASE COVER Check: •...

  • Page 75: Checking The Cylinder And Piston

    PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT CHECKING THE CYLINDER AND PISTON Check: • Piston wall • Cylinder wall Vertical scratches  Replace the cylinder, and replace the piston and piston rings as a set. Measure: • Cylinder warpage Measure the warpage on the contact surface of the cylin- der head at six points using a straight edge and feeler gauge set.

  • Page 76: Checking The Crankcase

    PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT “C” = maximum of D b. If out of specification, replace the crankcase assembly, and replace the piston and piston rings as a set. c. Measure piston skirt diameter “P” with the microm- eter. a = 10 mm (0.4 in) from the piston bottom edge Piston diameter: 79.96–79.98 mm (3.1480–3.1488 in) Limit:...

  • Page 77: Checking The Piston Pin

    PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT CHECKING THE PISTON PIN Check: • Piston pin Blue discoloration/grooves  Replace the piston pin and then check the lubrication system. Measure: • Piston pin hole inside diameter “a” Out of specification  Replace. Piston pin hole inside diameter: 19.004–19.015 mm (0.7481–0.7486 in) Limit: 19.045 mm (0.7498 in)
  • Page 78 PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT Piston ring Top ring Side clearance: 0.03–0.05 mm (0.0011–0.0019 in) Limit: 0.10 mm (0.0039 in) 2nd ring Side clearance: 0.02–0.04 mm (0.0007–0.0015 in) Limit: 0.10 mm (0.0039 in) Install: • Piston ring (into the cylinder) Use the piston crown to level the piston ring near bottom of cylinder “a”, where cylinder wear is lowest.

  • Page 79: Checking The Crankshaft

    PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT CHECKING THE CRANKSHAFT Check: • Crankshaft sprocket “1” Damage/wear  Replace the crankshaft. Measure: • Crankshaft runout limit Out of specification  Replace. Use a dial gauge. Dial indicator gauge: YU-A8428 Dial gauge: 90890-03097 Runout limit: 0.8 mm (0.0315 in) Measure: •...

  • Page 80: Installing The Piston And Piston Ring

    PISTON, CAMSHAFT, CRANKCASE, AND CRANKSHAFT Install: • Connecting rod “1” • Connecting rod cap “2” Connecting rod cap bolt: 20 N·m (2.0 kgf·m, 15 lb·ft) Tighten the cap bolts so that the crankshaft does not move while the oil clearance is being measured. Remove: •...

  • Page 81: Installing The Crankshaft

    • Piston pin “2” • Piston pin circlips “3” • Make sure that the “YAMAHA” mark “a” on the con- necting rod faces toward the crankcase cover. • Make sure that the “ ” mark “b” on the piston head faces toward the push rod.
  • Page 82 • Piston with the connecting rod “1” position • Make sure that the “ ” mark “a” on the piston head faces toward the push rod. • Make sure that the “YAMAHA” mark “b” on the connect- ing rod faces toward the crankcase cover. Install: •...

  • Page 83: Removing The Carburetor

    CARBURETOR REMOVING THE CARBURETOR Order Job/Parts to remove Q’ty Remarks Removing the carburetor Remove the parts in the order listed. Fuel tank Refer to “FUEL TANK” on page 3-4. Air filter Refer to “AIR FILTER” on page 3-3. Carburetor assembly Link rod Spring Gasket...

  • Page 84: Disassembling The Carburetor

    DISASSEMBLING THE CARBURETOR DISASSEMBLING THE CARBURETOR 0.7 N · m (0.07 kgf · m, 0.52 lb · ft) 1.1 N · m (0.11 kgf · m, 0.81 lb · ft) 7 N · m (0.7 kgf · m, 5.2 lb · ft) 1.3 N ·...

  • Page 85: Checking The Carburetor

    DISASSEMBLING THE CARBURETOR CHECKING THE CARBURETOR Measure: • Float height “a” Out of specification  Replace. • Lift up the float so that the tip of the float valve lightly contacts the float arm, and then measure the float height “a”.

  • Page 86: Electrical

    ELECTRICAL CIRCUIT DIAGRAM...
  • Page 87 CIRCUIT DIAGRAM 1. Spark plug 2. TCI unit 3. Oil warning unit 4. Oil level switch 5. Engine switch Color code Black Black/White Yellow...

  • Page 88: Electrical Components

    ELECTRICAL COMPONENTS ELECTRICAL COMPONENTS 1. Spark plug 2. TCI unit 3. Oil warning unit 4. Engine switch 5. Oil level switch...

  • Page 89: Checking The Switch Continuity

    ELECTRICAL COMPONENTS CHECKING THE SWITCH CONTINUITY Check each switch for continuity with the tester. If the con- tinuity reading is incorrect, check the wiring connections O.L” and if necessary, replace the switch. NOTICE Never insert the tester probes into the coupler termi- Ω”...

  • Page 90: Checking The Spark Plug Cap Resistance

    ELECTRICAL COMPONENTS Check: • Spark plug condition Wear/damage  Replace. (Refer to “SPARK PLUG” on page 2-3) Check: • Ignition spark gap Out of specification  Perform the ignition system troubleshooting, starting with step 8. (Refer to “ENGINE DOES NOT START” on page 6-1) Minimum ignition spark gap: 6.0 mm (0.24 in)

  • Page 91: Checking The Air Gap

    ELECTRICAL COMPONENTS ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼ a. Remove the spark plug cap from the high tension cord. b. Connect the digital circuit tester () to the spark plug cap as shown. Model 88 Multimeter with tachometer: YU-A1927 Digital circuit tester (CD732): 90890-03243 • Positive tester probe  Spark plug side “1”...

  • Page 92: Checking The Ignition Coil (Tci Unit, Secondary) Resistance

    ELECTRICAL COMPONENTS ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼ a. Remove the ignition coil (TCI unit). b. Connect the digital circuit tester () to the primary terminal. Model 88 Multimeter with tachometer: YU-A1927 Digital circuit tester (CD732): 90890-03243 • Positive tester probe  Black/White “1” • Negative tester probe  Body ground “2”...

  • Page 93: Checking The Engine Switch

    ELECTRICAL COMPONENTS CHECKING THE ENGINE SWITCH Check: • Engine switch continuity Out of specification  Replace. ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼ a. Disconnect the engine switch connectors. b. Connect the digital circuit tester () to the engine switch connectors as shown. Model 88 Multimeter with tachometer: YU-A1927 Digital circuit tester (CD732): 90890-03243...

  • Page 94: Checking The Wire Harness

    ELECTRICAL COMPONENTS b. Check the oil level switch continuity. ▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲ CHECKING THE WIRE HARNESS Check: • Terminal of the connector Contamination, rust or disconnection  Clean or replace the connector.

  • Page 95: Troubleshooting

    TROUBLESHOOTING ENGINE DOES NOT START Checking steps Possible remedy Check the engine oil level. Add the oil if it is insufficient. (Refer to “ENGINE OIL LEVEL” on page 2-4) Check the fuel level. Add the fuel if it is insufficient. Check if the fuel has deteriorated.

  • Page 96: Engine Starts But Stalls

    ENGINE STARTS BUT STALLS ENGINE STARTS BUT STALLS Checking steps Possible remedy Check the fuel level. Add the fuel if it is insufficient. Check if the fuel tank cap (canister) is clogged. Clean or replace. Check if the fuel cock is clogged. Clean.

  • Page 97: Engine Speed Does Not Increase

    ENGINE SPEED DOES NOT INCREASE ENGINE SPEED DOES NOT INCREASE Checking steps Possible remedy Check the spark plug for dirt and check the spark plug gap. Clean, adjust, or replace the spark plug. Check the air filter element for dirt. Clean.

  • Page 98: Engine Speed Is Uneven

    ENGINE SPEED IS UNEVEN ENGINE SPEED IS UNEVEN Checking steps Possible remedy Check the fuel level. Add the fuel if it is insufficient. Check if fuel has deteriorated. Replace the fuel. Check if the fuel tank cap (canister) is clogged. Clean or replace.

  • Page 99: Governor Operation

    GOVERNOR OPERATION GOVERNOR OPERATION Checking steps Possible remedy Check that the governor link operate smoothly. Adjust or replace. Check that the governor spring is stretched. Replace. Check the governor adjustment. Adjust. (Refer to “ADJUSTING THE GOVERNOR” on page 3-20) Check the governor weight and governor bushing function. (Refer to “CHECKING THE FLYWEIGHT SHAFT ASSEM- Adjust or replace.
  • Page 100 GOVERNOR OPERATION MEMO...

  • Page 101: Specifications

    Compression ratio 8.4 : 1 Standard compression pressure 450–550 kPa (4.5–5.5 kg/cm², 65.25–79.75 psi) Net output* 6.4 kW (8.7 PS) (MX250) 7.0 kW (9.5 PS) (MX300) Fuel Fuel Unleaded gasoline (Gasohol [E10] acceptable)* Fuel tank capacity 5.5 L (1.45 US gal, 1.21 Imp gal) Engine oil quantity 1.0 L (1.06 US qt, 0.88 Imp.qt)
  • Page 102 GENERAL SPECIFICATIONS * The power rating of the engine indicated in the SPECIFICATIONS is the net power output tested on an engine model and measured at 3600 r/min. Actual power output for an installed engine will vary depending on numerous factors, including the operating speed of the engine, environmental conditions, and other variables.

  • Page 103: Maintenance Specifications

    MAINTENANCE SPECIFICATIONS MAINTENANCE SPECIFICATIONS ENGINE Piston: Piston-to-cylinder clearance 0.033–0.047 mm (0.0012–0.0018 in) Limit 0.06 mm (0.0024 in) Piston diameter 79.96–79.98 mm (3.1480–3.1488 in) Limit 79.91 mm (3.1461 in) Measuring point (from piston 10.0 mm (0.3937 in) skirt bottom) Oversize 1st 80.25 mm (3.1594 in) 80.50 mm (3.1692 in) Piston pin hole inside diameter...
  • Page 104 MAINTENANCE SPECIFICATIONS Warpage limit 80.04 mm (3.1512 in) Crankshaft: Big end side clearance 0.20–0.65 mm (0.0079–0.0256 in) Runout limit 0.8 mm (0.0315 in) Crank pin outside diameter 33.969–33.984 mm (1.3374–1.338 in) Limit 33.9 mm (1.3346 in) Connecting rod: Small end diameter 19.006–19.020 mm (0.7482–0.7488 in) Oil clearance 0.006–0.015 mm (0.0002–0.0005 in)
  • Page 105 1.7 mm (0.0669 in) Carburetor: Type/manufacture RUXING Bore size ø22 (MX250) ø24 (MX300) Main jet 1.03 mm (0.040 in) (MX250) 1.14 mm (0.044 in) (MX300) Main air jet 1.55 mm (0.061 in) 1.8 mm (0.070 in) Pilot air jet 1.0 mm (0.039 in)
  • Page 106 MAINTENANCE SPECIFICATIONS Pilot outlet 0.9 mm (0.035 in) (MX250) 0.8 mm (0.031 in) (MX300) Valve seat size 1.6 mm (0.062 in) Main nozzle 6B (MX250) 6A (MX300) Pilot jet 0.41 mm (0.016 in) (MX250) 0.44 mm (0.017 in) (MX300) Pilot screw...

  • Page 107: Electrical

    MAINTENANCE SPECIFICATIONS ELECTRICAL Ignition system: Ignition timing at 3600 r/min BTDC 20 ± 3 0.8  ± 20 % Primary coil resistance Secondary coil resistance 11.7 k ± 20 % Spark plug cap resistance 3.75–6.25 k at 20 (68F) TCI unit air gap 0.4–0.6 mm (0.0157–0.0236 in) Minimum ignition spark gap 6.0 mm (0.24 in)
  • Page 108 TIGHTENING TORQUES TIGHTENING TORQUES Item Thread size Tightening torque M6  1.0 Air filter cover nut 1.6 N·m (0.16 kgf·m, 1.2 lb·ft) M6  1.0 Air filter element nut 0.9 N·m (0.09 kgf·m, 0.66 lb·ft) M6  1.0 Duct bolt 7 N·m (0.7 kgf·m, 5.2 lb·ft) M8 ...
  • Page 109 TIGHTENING TORQUES Item Thread size Tightening torque Pilot jet — 1.1 N·m (0.11 kgf·m, 0.81 lb·ft) Throttle valve screw — 0.7 N·m (0.07 kgf·m, 0.52 lb·ft) Choke valve screw — 0.7 N·m (0.07 kgf·m, 0.52 lb·ft) Main nozzle — 1.6 N·m (0.16 kgf·m, 1.2 lb·ft)

  • Page 110: Tightening Torques

    GENERAL TORQUE SPECIFICATIONS GENERAL TORQUE SPECIFICATIONS This chart specifies torque for standard fasteners with standard I.S.O. pitch threads. Torque specifi- cations for special components or assemblies are included in the applicable sections of this book. To avoid warpage, tighten multi fastener assemblies in a crisscross fashion, in progressive stages, until full torque is reached.

  • Page 111: Lubrication Points And Type Of Lubricants

    LUBRICATION POINTS AND TYPE OF LUBRICANTS LUBRICATION POINTS AND TYPE OF LUBRICANTS Part name Type of lubricant Oil seal lip Governor fork oil seal lip Bearing Connecting rod big end Crank pin Crankshaft journal Connecting rod cap bolt Piston pin Piston Crankshaft gear teeth Valve stem...

  • Page 112: Wire Routing Diagram

    WIRE ROUTING DIAGRAM WIRE ROUTING DIAGRAM UPPER SIDE VIEW 7-12...
  • Page 113 WIRE ROUTING DIAGRAM 1. Spark plug cap 5. Engine switch lead (ground) 2. TCI unit lead 6. Wire harness protector 3. Oil level switch lead 7. Oil warning unit 4. Engine switch A. The spark plug cap should face the direction indicated in the illustration. B.

  • Page 114: Engine Switch And Oil Warning Unit

    WIRE ROUTING DIAGRAM ENGINE SWITCH AND OIL WARNING UNIT 7-14...
  • Page 115 WIRE ROUTING DIAGRAM 1. Plastic locking tie 5. Oil warning unit lead 2. Wire harness protector 6. Engine switch 3. TCI unit lead 7. Oil warning unit 4. Clamp A. As an indication of the tightening position of the plastic locking tie, the terminals are collectively set to the center of the wire harness protector so as to be bundled.

  • Page 116: Tci Unit

    WIRE ROUTING DIAGRAM TCI UNIT 7-16...
  • Page 117 WIRE ROUTING DIAGRAM 1. High tension cord 4. Oil warning unit lead 2. Flywheel 5. TCI unit 3. Oil warning unit A. Make sure that the high tension cord is not pinched when installing the flywheel cover. B. Adjust the TCI unit air gap between 0.4–0.6 mm (0.0157–0.0236 in) on both sides. C.

  • Page 118: Oil Level Switch (Crankcase Cover)

    WIRE ROUTING DIAGRAM OIL LEVEL SWITCH (CRANKCASE COVER) 7-18...
  • Page 119 WIRE ROUTING DIAGRAM 1. Clamp 2. Oil level switch A. Touch the tube (gray) with the grommet. B. Route the oil level switch lead as shown. Color code Black Gray 7-19...
  • Page 120 APPENDIX CIRCUIT DIAGRAM...
  • Page 121 1. Spark plug 4. Oil level switch 2. TCI unit 5. Engine switch 3. Oil warning unit Color code Black Black/White Yellow...
  • Page 122 MEMO...
  • Page 124 2017.03×1 !

This manual is also suitable for:

Mx300

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