Yamaha JT1 Series User manual

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-~-tr::c';:}';.;·· .. :''= · • YAMAHA
JT1---JT2
SERIES · .· ·
. SERVICE -
MANUAL·

NOTICE
This manual has been written by Yamaha Motor Company for use by Authorized Yamaha
Dealers and their qualified mechanics. In light of this purpose it has been assumed that
certain basic mechanical precepts and procedures inherent to our product are a'lready
known and understood by the reader.
Without such basic knowlege, repairs or service to this model may render the machine
unsafe. and for this reason we must advise that all repairs and/or service be performed
by an Authorized Yamaha dealer who is in possession of the requisite basic product
knowledge. Other information is produced by the U. S. distributor. Yamaha International
Corporation. and is necessary to provide total technical coverage regarding the product.
The Research. Engineering. and Service Departments of Yamaha are continually striving to
further improve all models manufactured by the company. Modifications are therefore in-
evitable and changes in specifications or procedures will be forwarded to all Authorized
Yamaha Dealers and will. where applicable. appear in future editions of this manual.
YAMAHA JT SERIES SERVICE MANUAL
1st EDITION, DECEMBER 1973
2nd EDITION, MAY 1976
©DECEMBER 1973 YAMAHA INTERNATIONAL CORPORATION
BUENA PARK, CALIFORNIA 90620
LIT-11612-88-99
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FOREWORD
The new YAMAHA MINI ENDURO. though small in size. is a fully functional motorcycle in the
Yamaha Endure vein. The 60 cc engine. four. speed gearbox. tubular frame and fully functional
suspension combine to provide an ideal vehicle for the younger rider.
Its rugged design and excellent performance also make it an intriguing d1vers1on for older. more
exRerienced riders as well.
This service manual has been prepared 1n order to provide all Yamaha dealers and their service
techn1c1ans the repair 1nstruct1ons and technical information required to keep the MINI-ENDURO
in top cond1t1on We hope that you will find this manual most helpful and valuable in carrying out
this goal.
YAMAHA MOTOR CO., l TD.
ENGINEERING & SERVICE DEPARTMENT

MOOEL IDENTIFICATION
This manual was originally printed for the JT2 Mini-Enduro. It has been edited to
include information for the JT1, JT1 Land JT2M models.
Generally speaking, there were no major changes to any of the Mini-Enduro models
since the JT1 was released for the 1971 model year. The JT1 L (1971½ release) was
a JT1 with lighting kit. The JT2 had various internal engine changes to improve
reliability and carburetor changes to improve performance. JT2 lights were
upgraded in intensity. The JT2M was styled after the JT1 for those people who
wanted a Mini-motocrosser without lights. Additional identification information is
given in the table below.
IBM ID
STARTING IDENTIFYING
MOOEL YEAR NUMBER
ENGINE NUMBER
COLOR
JT1 1971
288
JT1-000101 Desert Orange
JT1 L 1971(½) 338
JT1-200101 Desert Orange
JT2
1972 288
JT1-050101 Mandarin Orange
JT2M 1972
288
JT1-300101 Competition Yellow

CONTENTS
CHAPTER 1 GENERAL .....................................................................................................................
1- 1 Profile ··············································································································--···--·--··
1- 2 Features ··--··--···----·--········--···--·--·················--·--·--·--·--·--················································ 2
1- 3 Specifications and Performance ·----·--·············································--·--------·------·--··--·--· 3
1- 4 Performance Curves ·······················--····--··--······--···--····----·--·--····································· 5
1- 5 Tools and Instruments for shop service ··································--··--·--···--··------·--·---------- 6
CHAPTER 2 YAM AHA AUTOLU BE ·--·----·--------·----·--·----······ ·················--·--··--·--·------····--··------·--·--······ 7
2- What is Yamaha Autolube? --------------------------···························---------------------------------- 7
2- 2 Features of Yamaha Autolube --··----------··--···--···················--·----------------·--·--··--··----·--··· 7
2- 3 Handling the Oil Pump --------------··------------------··----·························----··--------------------·-- 7
CHAPTER 3 ENGINE ·························································-------------------- ----···································· 1I
3 - Engine Remova1 ----------------------------------···--······································----··------------·--------·· I 1
3 - 2 Cylinder Head ···········································--··------------------------···----····----······················ 15
3- 3 Cylinder ·······················--···········································--·--·----·--·--··--······················--······ 16
3- 4 Piston Pin --··········--······--··················--···--············--·--·------------·------··--··························· 17
3 - 5 Piston Ring ·········--·········--·--·--································--····----------------··--··························· 18
3 - 6 Piston ----·--·--·······--·--·--·--·······--······--············································----·--·----··········.......... 19
3 - 7 Flywheel Magneto ···································--·--·----··--------··----·--·--·--··········--········----·------· 20
3- 8 Crankcase Cover (RH)·····························--·--·--------------·--·--···························--··--·---- 21
3- 9 Clutch ············································----·--·--·--··--·······--······················--····--···--··--····--·--··· 22
3-10 Primary Drive Gear··················--····--·····················--·--------------·························--·--·----· 27
3 - 11 Kick Starter Mechanism ·····································----------··--·--···················--·----------··---- 27
3-12 Shift Mechanism ----·············--···································--------------························--·------··-- 29
3-13 Rotary Valve····--·····--········································--······------····························------········ 31
3 - 14 Drive Sprocket ················································--······----------·····························--··----···· 33
3 - 15 Crankcase --·--························--·········----------------·----------··························--------------·--····· 35
3 -16 Transmission Assembly ----·····--·--······································--------------------··················---- 36
3 - 17 Crankshaft ·--··--··--·········--········································--··----·······························--·--·----···· 38
3-18 Bearings and Oil Seals ..............................................--·······················----········----······· 41
-3-19 Carburetor ····--·····--·························--··········--·············--·--·················------·--·····--·--········ 42

3 - 20 Air Cleaner ................................................................................................................ 44
CHAPTER 4 CHASSIS .................................................................................................................... 46
4- 1 Front Wheel .............................................................................................................. 46
4- 2 Rear Wheel ............................................................................................................... 49
4- 3 Rear Wheel Sprocket ............................................................................................... 53
4 - 4 Tires and Tubes ........................................................................................................ 54
4- 5 Front Forks ............................................................................................................... 54
4- 6 Rear Shocks ............................................................................................................. 57
4- 7 Gas Tank .................................................................................................................. 58
4 - 8 Rear Swing Arm ...................................................................................................... 58
4 - 9 Steering Head ........................................................................................................... 60
4- 10 Oil Tank and Tool Box............................................................................................ 61
4 - 11 Frame ........................................................................................................................ 61
'
4 - 12 Handlebars ................................................................................................................ 61
4-13Miscellaneous ............................................................................................................ 61
CHAPTER 5 ELECTRICAL SYSTEM ............................................................................................... 62
5- 1 Electrical Equipment..................................................................................................... 62
5- 2 List of Electrical Components ..................................................................................... 62
5- 3 Ignition System-Function and Service .......................................................................... 63
5- 4 Ignition Timing .. ......................................................................................................... 63
5- 5 Ignition Coil ..... ........................................................................................................... 64
5- 6 Condenser .. .. ... . ...... .. ... ............... ...... ... .... .................. .. . ... .. ... . ... . .. .. ......... ... .. .. .. .... ... .. ... .. 64
5- 7 Charging System........................................................................................................... 65
5· 8 Battery ............................................................................ •···· • •·· •• •· • •··· ••• •·· •··· •· •····· ··· ·· ······· 67
5- 9 Checking the main Switch ............................................................................................. 69
5- 10 Spark Plug ....................................................................................................................... 69
APPENDICES .................................................................................................................. 72
I Parts and Service Bulletins.................................................................................. 72
II Parts Illustrations.............................................................................................. 82
Ill Maintenance and Lubrication Intervals.................................................................... 86
IV Pre-operation Check List.................................................................................... 87
Torque Values for Common Components................................................................... 87
V Cleaning and Storage Hints ............................................................................... .. 88
VI Metric Inch Conversion Tables.............................................................................. 89

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OVERALL WIDTH 27.6(700)---I
GROUND LEVELIW/0 RIDER
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>----------------<JVERALLENGTH 62.0(1.575)>------+-----4-,
DIMENSIONS/PROFILES TYPICAL FOR ALL MODELS
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Front fork
Chain guard
Muffler
Side stand
magneto
Sprocket
Rear wheel
Tail light (JT!L Light
Rectangular)
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GENERAL-Specifications and Performance
1-3 Specifications & Performance JT2
Model YAMAHA
60 JT2
<JTI>
Dimensions .
Overall length 62.4
in ( 1580 mm)
<62.0>
Overall width 27 6 ,n (
700 mm )
Overall height
366 ,n ( 930 mm)
Wheelbase
41.5 ,n (1055 mm)
Mm
ground clearance
63 ,n (160 mm)
Weight,
Grr:iss
60 kg
1321bs
Performance ·
Fuel consumption 176m1le/9al (19 mph)
(on paved level roads)
Climbing ability 22 degrees
Min. turning radius
59.1 in. (1,500 mm)
Braking distance 24 6 ft/22 mph (7 5 m/35 km/h)
Engine
Model JT1
Type 2 stroke, gasoline
Lubricating system Separate lubricahon
(YAMAHA
Autolube)
Cylinder Single. forward inclined.
Displacement 3,54 cu
,n (58 cc)
Bore x Stroke 1 654 in x 1 654 in_ (42 mm x 42 mm)
Compression ratio 6.4 1
Max
power 4.5 hp/7.500 rp.m
Max torque 3
62ft-lb/5.500 r p.m (0 5 k9-m/5.500 rpm)
Starting system Kick starter
Ignition system Magneto 1gmt1on
Ignition timing
18 mm BTD.C
Carburetor
Type
M J
(See 3-19-C)
Corb. Specs.
J N
Air cleaner Wet .
foam rubber
Transmission
Clutch Wet .multiple-disk
Primary reduction system I Gear
Primary reduction ratio 3.895 (74/19)
-3-

GENERAL-Specifiaations and Performance
Model
YAMAHA 60 JT2
Gear box
Type Constant mesh, 4-speed forward
Reduction ratio
1st 3077(40/13)
2nd
1.889 (34/18)
3rd I
304 ( 30/23)
4th 1.038
(27/26)
Secondary reduction system Chain
Secondary reduction ratio 3153(41/13)
011 capacity
0.85 US qts (800-850 cc)
Chassis
Frame Tubular-Double loop
Suspension system. front
T elescop1c fork
Suspension system. rear Swmging arm
Cushion system front Coil sprmg. oil damper
Cushion system. rear Coil sprmg,
011 damper
Steering system
Steering angle 47° both right
and left
Caster
63 5°
Tra1!
2 7 in (68mm)
Brakmg system
Type of brake Internal expansion
Operation system.
front Right
hand operation
Operation system. rear Right foot operation
Tire size
Front 2
50-t5-4PR
Rear
2 50-t5-4PR
Dynamo
Model
F11-L46
<Fl 1-L42>
Manufacturer HITACHI
ltd
Tanks
Gasoline tank capacity
11 us
gals(43/)
011 tank capacity
11 us qts (1.0 I)
-4-

GENERAL· Performance Curves
1-4 PERFORMANCE CURVES
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60 JT ENGINE PERFORMANCE CURVES
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Maximum Power 4.5 B 1-P /7,500 rpm
Ma;ii;imum Torque 3.62 ftlbs/5,S00rpm
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Horse power
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ENGINE SPEED (rpm)
3-port Cylinder Exploded View
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Exhaust port transfer par
Inlet port
- 5 -

GENERAL-Tools and Instruments for Shop Service
1-6 Tools and Instruments for Shop Service
The following tools and instruments are required to service the JT2
1 General Tools
1) Plug wrench 23x29 mm.
2) A set of wrenches
3) A set of socket wrenches
4) Plastic tip hammer
5) Steel hammer
6) Circlip pliers (ST type)
7) Circlip pliers (RT type)
8) Needle nose pliers
9) Pliers
10) Phiilips-head screwdriver
It) Phillips-head screwdriver(l)
12) Phillips-head screwdriver (M)
13) Phillips-head screwdriver (S)
14) Slot-head screwdriver(M)
15) Slot-head screwdriver (S)
16) T-handle socket wrench
2 Special Tools and Instruments
I Clutch holding tool
2 Crankcase disassembling tool
3 Crankshaft assembling tool
4 Flywheel magneto holding tool
5 Flywheel magneto puller
Fig 1-6-1
F,g t-6-2
In addition an electro-testor, tachometer.(engine rpm meter) hy..drometer ,etc. should and available.
3 Other Material
1) Yamaha Bond (No.5)
2) Gear 011
3) Grease
4) Wiping material
5) Yamaha Bond No.4
6) Wood0J1 box
7) Oiler
8) Oil jug
The use of a wooden box as shown in the above
photo will facilitate engine service and overhaul.
Consumable parts (such as gaskets) and replace-
ment parts must also be on hand.
Fig. 1-6-3
-6-

YAMAHA AUTOLUBE·What is Yamaha Autolube? Features of Yamaha Autolube, Handling of the Oil Pump
CHAPTER 2 YAMAHA AUTOLUBE
( Automatic, Separate lubrication Sys tern)
2 -1 What is Yamaha Autolube?
Conventional 2-stroke engines are lubricated by oil pre-
mixed in gasoline. but YAMAHA ·s Autolube furnishes an
automatic, separate lubrication system, That 1s. the oil
in a separate oil tank is automatically regulated by the
oil pump and fed to the engine according to engine
speed and load
2-2 Features of Yamaha Autolube
YAMAWI AlJTOLUBE OIL
•. STRAIGHT ~IN£
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Fig 2-1-1
The oil pump 1s dnven by the engine through a reduction gear ,and is connected to the carburetor throttle cable,
which in turn is controlled by the occellerotor grip The oil pump automatically regulates the volume of lubricating oil
according to engine speed and throttle valve opening, thus pumping the optimum amount of oil for engi_ne !ubncation
under any operating condition.
This "automatic. separate lubrication" does not merely eliminate disadvantages in the conventional pre-mix system. but
11 further improves the perform,,nce and efficiency of 2-stroke designs by eliminating certain oil-starvation conditions
which formerly existed
The Autolube feeds an optimum amount of lubricating oil to the engine under any operating condition, thus
featuring .
* Less oil consumption
* Less carbon accumulation
* Less exhaust smoke
* Improved lubricating efficiency.
2 The Auto[ube simplifies fuel supply. thus featuring:
* Using straight gasoline directly in the gas tank
* Less fuel contamination.
3 The Auto!ube improves the reliability of lubrication. thus eliminating.
* Special care concerning oil/fuel mixing ratios.
2 -3. Handling the Oil Pump
The oil pump is a precision-machined assembly. Make no attempt to disassemble it. When you remove the oil pump
from the engine. protect 1t from dust dirt. ·etc,. and after reinstalling lt. bleed and adjust the pump correctly. Proper
handling will keep the pump free from trouble.
-7-

YAMAHA AUTOLUBE·Handling of the Oil Pump
The oil pump is similar in both construction and operation to other Autolube systems. The only difference is the employ-
ment of a 4,0 ¢ plunger.
2-3-1 Checking Minimum Pump Stroke
1 Checking
a Fully close the accelerator grip.
b Turn the oil pump starter plate in the direction
of the arrow marked on the plate. Then measure
the gap between the adjustment pulley and
the adjustment plate. Keep the gap as wide
as possible by observing lt with the eye prior
to measuring,
c. Insert a feeler gauge (015 mm) into the gap
If the qap allows it to enter
......Stroke Is correct
If the gap does not allow
......Stroke is insufficient
2 Adjustment
a Remove the adJustment plate lock nut. and
then remove the adjustment plate
Fig 2-3-1
Fig 2-3-2
Fig 2-3-3
-8-

b Install a O1 mm adiustment shim where the
adJustment plate was
c Reinstall the adjustment plate lock nut. and
measure minimum stroke When the gap allows
a O20 mm, feeler gauge to enter but does
not allow a O25 mm. the stroke 1s correctly
adiusted
Stroke adJustment tolerance....Q.20 to 0.25 mm.
YAMAHA AUTOLUBE·Handling of the Oil Pump
Fig 2-3-4
2-3-2 Carburetor and Autolube CiJble Adjustment
Follow the preceedmg in section 2-3-1 steps to check minimum stroke. and adJust 11 1f incorrect Then ad1ust the
pump and carburetor as described m the steps below
I I Throttle Cable Adjustment
a AdJust the carburetor with the engine at
td!e. and remove all but 1 mm of slack
from cable B
9 To bring the play of the throttle cable into
correct adJustment. loosen or tighten the
throttle cable adiustment screw
• To check this adiustment lightly pull throttle
cable B. and engine speed should slightly
increase from 1dlmg r.p.m. after 1 mm
of travel.
b Next. adJust throttle cable(A) so that the
gap as shown in Fig 2-3-6 be!ow w1!! bE
between 0,5 and 10 mm (0 02 ,..__,0 04 in~
e Check the p!ay of the throttle cable (A)
by pu!hng on the outer part of the throttle
cable If the play 1s excessive or msuff1cent.
ad1ust the play using the odiustor.
-9-
Throttle cable 8
Fig 2-3-5
Throttle cable(A)
Fig 2-3-6
Adjusting play of the throttle
cable B to 1 mm,(0,04 in.)
Look nut
Adjustor Lock nut
Cable guide(A)
+-landle grip

ENGINE· Handling of the Oil Pump
2) Autolube Cable Adjustment
With the throttle cable properly adjusted, check the pump
guide pin location. It should be no closer than 1.5mm
(.060") to the raised boss of the pump pulley. If it is too
close, loosen the lock nut (see Figure 2-3-7) and turn the
adjusting bolt until adjustment is correct.
lock nut. Tighten
Next, turn throttle to full open position, the guide pin
should not strike the raised boss of the pump pulley in this
position, if it does, loosen the adjusting bolt slightly to
provide clearance.
2-3-3 Bleeding
Lock nut
Guide pin
Fig 2-3-7
When the pump has been removed or the Autolube oil has run out. air will enter the pump. The air will cause
an irregular flow of oil after the pump 1s mounted again or the 01! tank 1s refilled In order to prevent such an irreg-
ular flow of oil. b' leed the pump in the following manner
I) Remove the bleeder bolt.
2) Next. rotate the starter plate in the direction of
the arrow marked on the plate. Continue turn-
mg the plate until no air comes out with the
011 and tighten the bleeder bolt To facilitate
this bleeding, fully open the accelerator grip
and rotate! the starter plate. As the plunger
stroke becomes greater, the air can be quickly
bled.
Fig 2-3-8
Fig 2-3-9
-10 -

ENGINE-Engine Removal
CHAPTER 3 ENGINE
This chapter c;lescribes the disassembly and reassembly of the engine, its removal from the chassls, and the necessary
service data. However, except when overhauling the crankshaft assembly, transmission, shifter mechanism or bearings
and oil seals in the crankcase, it is suggested that engine be serviced without removing it from the chassis. This will
save a lot of time and labor.
Preparation for disassembly of the engine :
1) All dirt. mud, dust. and foreign material should be thoroughly removed from the exterior of the engine assembly
before removal and disassembly. This will prevent any harmful foreign material from entering the interior of the
engine assembly.
2) Before engine removal and disassembly, be sure you have proper tools and cleaning equipment so you can per-
form a clean and ·efficient job.
3) During disassembly of the engine .. clean all parts and place them in trays in order of disassembly. This will make
assembly time faster and easier, and insure correct installation of all engine parts.
3 -1 Engine Removal
Start the engine and warm it up for a few .minutes,
then turn off the engine and drain the transmission
oil
(Volume of o;I 800-850 cc 10.85 US qt)
YAMALUBE 4-CYCLE
or
SAE lOW-30 "SE"
2 Pull out the seat fitting pin and remove the seat
by pulling it backward.
-11-
Fig 3-1-1
Fig 3-1-2

ENGINE· Engine Removal
3 Disconeect the fuel line at the bottom of the fuel
tank. Remove the tank mounting bolt (13) and
lift the fuel tank off.
4 Remove the connector from the magneto lead wire
and remove the plug cap,
5 Remove the muffler
a) Remove the exhaust pipe nut.
Use the special wrench. Hammering on the nut will
only damage it and/or the cylinder.
b)Remove the muffler holding bolts
(13 mm)
-12-
F,g 3-1-3
Fig 3-1-4
Fig 3-1-5
Fig 3-1-6

ENGINE-Engine Removal
6 Remove the change pedal
(10 mm)
Fig 3-1-7
7 Remove the !efthand crankcase cover
($ screwdriver)
Fig 3-1-8
8 Disconnect the master link and remove the chain.
Fig 3-1-9
When reconnecting the chain be sure the master link is facing in the correct direction.
driving direction
After reconnecting the chain adjust the free play to 25 mm. ( 1 in.) up and down at the center of the lower
section with the rear wheel on the ground.
-13-

ENGINE-Engine Removal
9 Remove the pump cover and pump cable
((£) screY_tdriver)
10 Remove the oil pipe and plug the hole with a
bolt to stop the flow of oil. from the tank
Remove the fuel line and remove the carburetor
(8 screwdriver)
12. Remove the four engine mounting bolts
(13 mm)
Fig 3-1-13
Fig 3-1-10
Fig 3-1-11
Fig 3-1-12
Fig 3-1-14
-14-