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! Three International Six Days Trial Championships
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by Dr. Jan Schulmann
With 1972 marking the third J awa
World Trophy and Silver Vase win in a
row it js worthwhile to examine in some
detail the machines ridden by
Czechoslovak Trophy and Silver Vase
teams to victory in the recentI.S.D.T.
held
at Spindleruv Mlyn in
Czechoslovakia.
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guides inside the drum. The kickstarter
pedal is si tuated on the righ t in the
front part of the side cover. Th.e pedal
operates the starter qU!'£lrant matching
the counterpart gear with ratchet
carried by the primary drive l(ear.
Final drive is by special 5is - 1/4 in.
roller chain, to tally enclosed in a special
chaincasc. The
two-funnel type
chaincase contributes considerably to
the long service life of the chain.
reducing at the same time maii1.tenance
to a minimum.
The six motorcycles for the Trophy
and the eigh t for the two Silver Vase
Teams contest havc been prepared by
the Jawa Research and Development
Cen tre in Prague. Holders for two years
running of both the I.S.D.T. premier
awards, Jawa was careful. not to
experiment and to direct their effort
rather to improve reliability - a decisive
factor in what is called "The
Motorcycling Olympics."
Note 'rear wheel hub torque anchor, brake pedal with operating rod, kickstarter pedal, upswept
exhaust pipe with robust silencer and gearbox side cover. I.S.D.T. Jawas of all displacements look.
alike externally.
The two six volt ignition circuits are
separately connected. A master switch
serves for switching on either or both at
one and the same time. The circuits are
supplied with power from the J awa
ferrite alternator generating enough 12
V 0 I t curren t for the motorcycle
headlamp and tail lamp.
Accordingly, no- major changes were
made to the machines that had provcd
their worth in 1970 in Spain and in
1971 on the Isle of Man. The
watchword was improvement
improvemen t in reliability, in power and
torque, as far as permitted by the
existing motor design, and in weight,
meaning weight reduction wherever
possible.
The upswept exhaust pipe withrobust silencer provide the only clue for
telling the machines of the individual
classes apart. Ou t of the silencer end
piece of the 175 protrude two small
pipes; out of that of the 250 three pipes
and the two big machines possess just a
narrowing tail piece of different bore.
The air for the carburetor comes via
a large size micronic paper element
situated in the large fiberglass induction
silencer chamber situated between the
frame tubes under the saddle.
All the Jawa J.S.D.T. motorcycle
competing in the 11'5,250, 350 and
500cc classes are powered with single
cylinder. two-stroke motors. Iden tical. in
appearan·ce, the machines of the
individual capacity I classes differ in
minor details. So, for instance, the
cycles entered in the 500cc class are
modified three fifties, the cylinder bore
being increased from 7.S to SOmm giving
a displacement of 362cc.
Motor and Transmission
Observe the rubber piping running through cylinder head fins to keep down resonance noise, the
three-position ignition switch under the fuel tank, the short gear pedal and the speedometer drive
neatly clamped to the frame down tube. The side cover provides access to the crankshaft
assembly.
OOPS!! SHOULD
HAVE READ
"ESCAPE BY TRAIL BIKE"
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STRan
83.00. Cyde _
Pr....uets. p.o. Ik>x
498 Dept. CNP. I.ong _ . CI."90801.
COIif. RlISldentlldd 5"" Tlx
LL NEWI ALl WEATHEII
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2 Stroke-
factory replacement piston rings
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RACING
super-rings
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Blends perfectly
,
not only unbreakable, but chrome
p1atad for elltra long life too l!
now for-Hodaka 100cc d,kes.
Bultaco 125, Bultaco 250. DKW I
Sachs 125, Penton 125
FOR ALL FACTORY PISTON SIZES
ASK YOUR LOCAL DEALER
poppy
820 N. Hollywood Way
Burbank. Ca. 91505
(213) 845·6766
CASTOR
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with reqular gas,'
aviation qas or
alcohol and stays
blended in extreme eold or ex·
treme h.lt. AIlords maximum lubrication with blend
ol 32·40 to I ratio. No port clogging
or pIuq problems. Reduces wear to
blrest minimum. Keeps rinq. free. Car·
bon residue % by weight 0.002. Ash
residue % by weight 0.004. Pour Point
·45°.
CLEAN ACCELERATION
WITHOUT SMOKE, CONTAINS
NO SYNTHETICS (Imitations)
LOOK FOR THE
CHECKERED CAN
"BlendzaU" Corp.
1433-37 First St.
~n,Calil.95320
The crankcase-gearbox unit of the
we H-p roven single cylinders is a
one-piece electron * alloy casting with
electron alloy side covers. The design.
permitting the crankshaft assembly and
gearbox mechanism to be inserted and
withdrawn from the side, is used by
J awa for many years and has proved of
the greatest advan tage offering the
possibility of easy and speedy checking
of the mechanisms and replacement of
faulty parts )Vhen necessary.
The cylinder barrel is a special light
alloy cas.ting with pressed-in steel liner.
Generous Imning provides a large heat
transfer surface area. The cylinders are
four port, i.e. wit" two main and two
auxiliary scavenging ports. The cylinder
head. too, is a special light alloy casting
with fan-shaped finning. Two sparking
plugs and a decompressor valve for easy
starting are provided in the head.
Each of the two crank webs with
their journals are one-piece forgings. As
in the past. lightening holes in the crank
pin ·had sometimes been the cause of
cracks, full crank pins were used this
year. The smaJJ end i. supported on free
needles guided by side shims, whereas a
needle roller bearing with cage is
employed for the big end. The light
alloy pistons are fitted with two steel
compression. rings with chromium
plated working face.
The 175 and 256cc motor crankshaft
assembly ~ carried on the left in a
needle roller bearing with outer steel
race and on' the righ t in a roller bearing
with side race for axial location. That of
the two bigger motors employs roller
bearings on both sides without any
othe~ axial location.
The primary drive from the
crankshaft to the dry clutch is by 'spur
gears. The clutch plates are steel with
Ferodo lining. The clutch drum is
prOvided with internal involute gearing
for clutch plate guiding. Clutch control
is by cable operating via a lever the
pinion engaging a rack carried in a
special thrust bearing on the clutch
pre~sure plate.
The gearbox of the two smaller
capacity machines differs from that
used in the two bigger ones. The 175
and 250cc employ a six-speed, the 350
and 500cc a five-speed box. The design
of the gearboxes and of the shifting
mechanism is the same. The gearboxes
are of the two-shaft type with widely
supported low toothed gears for extra
reliability.
Motor lubrication is by gasoline and
oil mixture at the rate of 25 to 1.
Czechoslovak M2T two stroke oil or
Castrol 30/40 is the lubricant employed.
Frame and Running Gear
The close hame is welded of circular
section chrome-molybdenum steel
tubes. It is. very rigid owing to the direct
fixing of the top and down tubes in the
steering head and to the additional
brace in that portion. It will stand
considerable load. as for instance the
machine leaping cross country obstacles.
The pivoted rear fork is extra rigid
with wide support. Girling or Koni
suspension units with progressive
uncovered coil springs a;re used.
The front fork is a modified CZ
mo tocross unit. The modification
provides bigger suspension travel - from
the original 170mm to 200mm - giving
a much better ride. The simple sheet
steel front fender is bolted to the front
fork bottom lug. supported on rubber
qushes. The rear fender, fuel tank and
saddle pad are fiber glass.
The brakes. front and rear, are
mechanical in temal expanding single
cam. Front brake control is by
handlebar lever "1'd cable. the rear 6rake
is operated by pedal and rod. Lining
material is Rubocol, riveted or glued in
accordance with the rider's wishes. The
brake drum liners are special cast steel
for better wcar resistance.
The front wheel come. from the CZ
motocross machine with -the exception
of the liner mentioned. The rear wheel
is provided wi th a high strength
duralumin forged hub, linked with the
duralumin chainwheel by means of the
cush-drive rubbers in a similar fashion as
on the roadster J awas. The new Barum
rear tires passed their test in the course
of the season and during the Six Days.
The front part of the motor cycle
features the most markcd change
compared to last year's models. The
heavy separate. headlamp and fron t
competition number plate have been
replaced with an ABS thermo-plastic
moulding fonning both the competition
number plate and headlamp shell
housing the original electro-optical
parts.
Equipment comprises the electric
hom, speedometer with trip recorder
and special toolkit stowed in the leather
bag which is fastened on top of the fuel
tank with the tank holding strap.
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magnesium
Shifting is by revolving drum, the
shifting mechanism proper operating
,three selector forks carried in two
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