Conroy Sno-Jet 1975 Thunder Jet User manual

TABLE OF CONTENTS
Warr anty 2
Safety Precautions 2
Clutching 3
Spark Plugs 5
Engine 5
Fuel &
Oils 14
Carburetion 15
Suspension 19
Track,
Studding & Skis 20
Sprockets &Chaincase 23
Modification Kits 23
Specifications 24
Parts L
ist 25
Index
Engine
Mount 26
Exhaust System 28
Electrical System 30
Drive System 32
Suspension 40
Ski 44
Steering 46
Body 48
Hood 52
Fuel System 54
Tool
Kit 56
Foreword
This manual is intended to o ffe r a parts list and racing suggestions for the 1975 ThunderJet. The
manual
assumes that the operator has a basic knowledg e of snowmobile mainten ance. The informa-
tion included in thi s manual was correct at the time of printing and should be of gr eat assistance in
sett ing up your new ThunderJet to race competit ively. As new information becomes available it will be
sent to your dealer.

THIS WARRANTY IS EXPRESSLY IN LIEU OF ALL OTHER
WARRANTY OBLIGATIONS, EXPRESSED OR IMPLIED, LE-
GAL OR CONVENTIONAL, INCLUDING THE LEGALLY IM-
PLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR THE PURPOSE
INTENDED ON THE PART OF SNO·JET,
INC., WHICH NEITHER ASSUMES , NOR AUTHORIZES ANY
OTHER PERSON TO
ASSUME FOR IT, ANY OTHER WAR-
RANTY RESPONSIBILITY IN CONNECTION WITH SNO·JET
SNOWMOBILES.
B. SnoJet Warranty - Specific Limitations
Snot Jet, Inc. limit s or exclude s from the terms of the War-
ranty General Statement, the following parts or services:
Excluded:
Time spent in the diagnosis of a cause of malfunction.
C onsum able items, in cluding: Spark plug s, con densers,
light bulbs , coil suspension springs, fuel filter s, brake
linings, ski-runners , slide runners, gaskets, windshields,
carburetor diaphragms or needle valves, oil seals, track
cleats or rivets.
Pistons damaged by improper conditions of use.
Assemblies, unless they cannot be placed in good condition
by replacement of component parts.
Normal operating services, including: Engine tune-ups , ad-
justments and alignments.
Limit ed to 30 Days From Commencement of Warranty :
Leaf
springs, throttle and brake control cables , electrical
wiring, upholstery.
Drive belts. (Only manufacturing defects acceptable.)
Skis . (Only welding failures ac ceptable .)
Engines:
Sno" Jet engines are guaranteed for the same period as the
snowmobile to which they are fitted, but for late season sales
there is no
extension beyond April 30.
Figure 1
TJ1
Storage:
Off-season storage and pre-season preparation are at the
discretion and expense of the owner. However, any failure
which occurs as a resu lt of inadequate seasonal preparation
shall not be covered unde rwarranty.
High Performance Models:
The Thunder Jet is considered a high performance model and
as
such is manufactured with high performance components .
These
components tend to be less durable and when coupled
with the more severe riding applications that a high per for-
mance model is subjected to, premature failures can result
which are not considered to be a defect in material or work-
manship . Any such failures would not be covered by warranty
if such abuse was present.
Warranty Repairs:
Your machine must be delivered to an authorized Sno·Jet
dealer for warranty repairs. While we expect that you will
return to your selling dealer for warranty repairs, if you are
vacat
ioning or have moved more than 50 miles away from
your selling dealer , any Sno" Jet dealer can perform warranty
repairs. If you travel out of your local area to purchase your
machine, you are expected to go back to that selling dealer
for your warranty service.
Safety Precautions
1. Before starting the engine:
a. Be sure throttle lever operates freely and returns to
idle posit ion .
b.
Check brake control.
c. Be sure no one is in front or back of snowmobile .
2. Check operation of kill switch and tether switch.
3. During warm up, use a stand with a protect ive guard that
w ill prevent any objects being thrown from the mach ine
and
caus ing injury. F igure 1
4. Be sure braking system is working properly .
5.
Gasoline is highly flammable , use caut ion when fil ling
the gas
tank . Always keep a fire extinguisher nearby.
6.
Wear protecting clothing as designated by your racing
association. Never ride without a helmet and eye pro-
tection .
7.
Test your machine only in designated areas.
8.
Courtesy and good sportsmansh ip is very important on
the race track. Do
not block or cut off fellow racers.
9.
Snowmobile racing is normally controlled by flags. Al-
ways obey the flag and stay alert. F igure 2
10. If you should experience problems during a race, raise
your left hand to warn other racers and get off the track.
tI'O ......
...... t-
WarrantrPOlrt:
A. General Statement
Snor Jet, Inc . guarantees each Snor Jet snowmobile against
defects in materials and workmanship ,provided the snow -
mobile is proper ly maintained and used in normal cond itions,
for the following period:
Ninety days from the date of retail purchase, or from
December 1, whichever date is the latest in the same snow-
mobile season, subject to the specific limitat ions listed
below .
(The
snowmob ile season is considered as December 1
through Apri l 30.)
In the case of a
purchase made after March 1, the warranty
will apply from the date of purchase until December 31 of the
same year, subject to the specific limitat ions listed below .
The
warranty is limited to the original retail purchaser.
The
obligation of sno: Jet , Inc ., under this warranty is limited
to the repair or replacement of defective parts by an autho-
rized Sno·Jet dealer , when parts are proven to be defective in
either material or workmanship and does not cover any
consequential damages.
This warranty is not valid if the Sno t Jet snowmobile has
been: misu sed, improperly maintained, damaged by acci-
dent, immersed in water, altered, RACED , repaired by un-
authorized persons, repaired with other than genuine sno ' Jet
parts or used on surfaces other than ice or snow .
All warranties are invalid on any Snor Jet snowmobile de-
livered to
the retail purchaser in the shipping crate and not
properly prepared for delivery .
2

3
Yellow
spring is standard, but to increase engagement A.P.M. you
c
ould install a green spring which is stronger.
Part
Number Color S pr ing Compress ion Rate
050760 Y
ellow 50 Ibs. @in.
050786 Green 64 Ibs. @in.
NOTE : New
springs will have a higher ten sion than a used
spr ing. Norm ally , a spring will "se at" afte r approx imate ly 50
mil es. Be sure to install a used spring when adju sting your
drive
system s ince spring change s could later alter y our ad-
ju
stments .
TRACK CLEAR SLOW-MAINTA IN RACE OVER
POSITION
TJ2
Figure 2
DRIVE SYSTEM
Weights: The roller arm weights primari ly control the hold
A.P.M. The weight will also have a slight effect on the en-
gagements and
shift pattern. 8000 A.P.M. must be main -
t
aine d for best efficiency. Below is a weight chart li sting
heavier and lighter weights. Lighter wei ghts would incr ease
the
hold A .P.M . and a heavier weight would decrease the
A.P.M .
WEIGHT
CHART
CAUTI ON: All six weights must be the same. Never mix
lighter or heavier weights .
HIGHER HOLD R.P.M.
LOWER HOLD R.P.M.
NOTE: Some ra cing as sociations may not allow changing
clutch wei
ght s from standard in stock classes.
Ramps: Engagement, shift p att ern, and Hold A.P.M. are all
affected by the r
amp angl e. We do not re com mend mod ifi-
cation of the ramp
since this must be accurately matched to
the torque curve of the
engi ne. Below is a chart which
indi cates e ffe ct of ramp angle on the shift pattern. ( Fig ure 4)
Part Number 1.0.
Num ber
Out side Dia. Wei ght in Grams
050780 175 .437 3.725 li
ght er
050779 135 .471 4.479
ligh ter
050777 107 .491 4.958
light er
050776 106 .530 5.958 standard
050778 123 .568 6.992 heavier
'.'."
:
,
--
,\
,,\
,-,
,,,
,,,
:
,/ ,
,,
,\
,\
."
\
.'
--r\.
,-\
-,,
-
The 1975 Thunder Jet Drive System is ad justable for different
tracks or track cond itions . There are adjustments for both the
drive c
lutch as well as the driven clutch (convertor.)
Before attempt ing to change clutch ing , we should d iscuss
the terms involved.
Hold R.P.M.- ·Hold A.P .M. is the maximum A.P.M . the
eng ine
will achieve with the drive clutch fully shifted . En-
gagement R.P.M . is the po int at which the drive clutch
sheaves squeeze the drive b elt and move the sled . The
recommended maximum A.P.M. for the 1975 Thunder Jet 440
is 8000 A.P
.M. In the past some snowmobile racers have felt
that by increa sing A.P.M ., more hors epow er could be ob-
tained. This is not tru e! As
indic ated by the graph below ,
hors epower a ctually d ecr eased over 8000 A.P.M . Figure 3.
345678910
HoRSE POWER .... . ..
RPMxl000
TORQ UE - -- ---
TJ3
Figure 3
Since 8000 A.P.M. must be maintained for maximum output ,
the
clutch must be adjusted to th is specification . The driven
clutch can be adjusted to achieve this setting or the drive ·
clutch can be corrected.
Drive
Clutch: The drive clutch has three possible variables
which are the spring , roller arm weights, and ramps.
Sprin g: The c lutch spr ing primarily controls the engage-
ment A
.P.M. However, a stronger spring will also slightly
increase the hold A.P.M. and increase shift back . A yellow
LOWER ENGAGEMENT R.p,M. HIGHER ENGAGEMENT R.P.M.
TJ4
Figure 4

4Driven
Clutch (Convertor): Spring tension can be adjusted
for different conditions or desired hold R.P.M. The driven
clutch spring is always set w ith 1200of preload. There are
four settings possible for the spring. If the spring is set
tighter, the H old R.P .M. of the clutch will increase. If the
spring is set looser, the Hold R.P.M. will be lower. (Fi gure 5.).
(Figure 7). Then rotate the movable sheavel cover assembly
clockw ise until all clearance is taken up in that direction .
With the square, scribe a second line on the movable sheave
in
line w ith mark on stationary sheave (see f igure 7.)
The
distance between the two lines on the movable sheave is
to be measured. The
allowable tolerance is up to .156"
(5/32") . Anything below .156" is within to lerance and will
function properly . Any clutch that has .156" or more playas
described must be repaired.
TJ5
Figure 5
Clut ch Alignment: Clutch alignment is important to both
performance and belt life . There are three considerations
when considering clutch alignment. Namely , par alleli sm,
offset, and center to center distance . Offset is important to
keep the
drive belt centered. Offset can be checked by
placing a straight edge from the drive clutch to the driven
clutch as shown in Figure 6. Distance "A" should be set at
.550. TJ7
DRIVE CLUT CH TROUBLE SHOOTING
DRIVEN PULLEY TROUBLE SHOOTING
Figur e 7
Cause and
Remedy
Too much tension on spring .
Move back one
notch at a time
and test.
Too
little tension on spring.
Move one notch at a time and
test.
Check o
ffs et. No grease or oil
is to be evi dent on belt or
driven
pull ey.
RPM Too
Low
Problem
RPM
Too High
Black or Rubber From Belt
on Driven Pu
lley
Prob lem Cause and Remedy
C
lutch Engagement Too Low Wrong ramp. Check for proper
spring ,ramp, and weights for
engine cc size.
Clutch Engagement Too Hig~
Wrong ramp or spring. Check
ramp and weight application
or dirty moveable parts . Lubri-
cation
on drive clutch-very
little graphite or dry lubricant.
Hold RPM Too High
Weight is too light or wrong
ramp. Install heavier weight or
correct ramps.
Hold RPM Too Low
Weight too
heavy. Install
lighter w eight and check en-
gine for possible horsepower
loss .
Black or Rubb er From Belt
Check offset. No grease, gas
on
Clutch
or oil to be evident on belt and
clutch.
Figure 6
Para
lle lis m: Parallelism can be checked by plac ing a
stra ight
edge at the top and bottom of the clutch di ameters
and comparing the measurement. The measurem ent should
be
identical. If Dimension " A" was different than B, the en-
gine or jack shaft may be forward or backward .
If the
same check is made at top and bottom location s on the
clut ches, these dimensions should also be the same . If these
dimen s ions were different, this would indicate that the
engine or jack shaft was too high or too low .
Center to Center D
ime nsi on: The clutches must be the
p
roper distance apart for proper operation of the drive
sy
stem. See Figure 6 for proper adjustment. Be sure the drive
belt is 44 ±3/16". Since variances in drive belt circ umfer ence
would make c enter to center distance incorrect. Aft er ad jus t-
ment the drive belt must have 1/8 " cle arance around the drive
clutch hexa gon shaft .
D. C
hecking Clutch Wear
The
following is a positive method to check for allowable
wear tol erance. Remove the clutch from engine and r emov e
cover hou
sing and spring from clutch. With the spring
removed, replace the cover
housing and secure with the three
cap
screw s.
While hol din g the stationary sheave securely, rotate the
mov a ble sheavel cover asse mbly cou ntercl ockwi se un til all
clearance is taken up in that direction.
With a combin ati on
squa re, scribe a line on the sta tionary and movable sheaves
~--~------

Varying terr ain and oper ating conditions may require spark
plugs of different heat ranges. Sustained cross- country
operation , full throttle operation or pulling may require a c old
plug . Trail riding or extensive slow speed oper ation may re-
quire a
somewhat hotter plug.
After operatin g the engine as it will be run in normal snow
conditions , remove the spark plugs and observe the color of
the
center electrode insulator. ( Figure 8)
Acracked,
fouled or d irty plug is to be replaced. Do Not clean
and use
old spark plug s, grit may be released into the
cyl
inder , causing severe damage.
1. Tan or light
brown in sulator tip - indicated correct type
and proper heat range.
2.
Light gray or white insulator tip - indicates over-
heat ing caused by a lean
carburetor setting , improper
timing , overloading or incorrect (hot) spark plug . Con-
tinued operation can damage plug as shown . If not cor -
rected
engine damage will occur. If the carburetor is
properly adjusted , replace the plug with one of a colder
heat range.
3.
Black insu lator Tip - indicates fouling caused by ex-
cessive oil, a
rich carburetor setting or incorrect (cold)
spar k plug. If gasloil mixture and carburetor adjust-
ment is correct ,replace spark plug with one of a hotter
heat range.
5
Assemb ly
1. Install the
starter spring in the starter case , and hook
the
outer end of the starter spring ont o the starter case
hook.
Note: Apply Aero
shell no. 7A to the entir e surface of the
starter spring.
2.
Unti e the knot of the start er handle side, and insert the
starter rope end into the center of the sheave
drum .
Wind the starter rope around the sheave drum, and
bring out the starter rope end through the starter case
hole . Then, thread the starter rope through the st arter
handle and connector , and make a knot on the rope end.
When the
starter rope is drawn out of the start er case, it
is necessary to turn the sheave
drum about 1/3 turn to
the
left from the free state of the starter spring and tie
the starter rope to the starter handle . This procedure is
needed to
allow the starter handle to return smoothly to
position .
3.
Install the thrust wa sher and drive plat e spring on the
starter case
center bolt.
Position the drive pawls so that they fit respectively in
the three recessions in the
drum sheave. (The pawls
must be directed to the left.) Place the r
eturn spring on
the
pawls . The return spring helps the drive pawls re-
turn to their original positions . Take care not to mis lo-
cate it.
4.
Hook one end of the shorter return spring onto the drum
sheave, and insert the bent end into the drive plate hole.
Then ,
turn the drive plate to the left from the free state
of the return
spring so that each pawl w ill move into the
next recession.
5.
Insta ll the fla t screw and spr ing washer, and lock with
the nut .
B. Drive
Clutch
Removal
1. The
dr ive c lutch assembly is attached to the crankshaft
on the left side of the engine.
2. To remove the drive
clutch aspecial tool (shaped lik e a
bolt)
must be inserted in and tightened . Figure 10
021
Lean
Rich
020
019
Norma l
Figure 8
Engine
A. Rec oil Starter
Disass
embly
1. The starter pulley assem bly is held to the flywheel mag-
neto. (Figure 9)
2. The parts
from the starter assembly can be removed
after the nut, spring washer and flat washer are re-
moved.
Care
should be taken not to mi slocate the three return
springs .
Figure 9
Fi
gur e 10
Figu re 11
3. Before
installing the drive clutch on the cranks haft , clean
the crankshaft and
fixe d sheave with a dry cloth.

6c.
REMOVAL OF FLYWHEEL MAGNETO
1. Loosen the four bo lts and remove the crankcase cover.
Figure 12
2.
With the starter assembly removed you will not be able to
remove
the magneto assembly. The first step in this pro-
cedure is to remove
the flywheel securing nut with asoc-
ket
wrench 1.02 ins. (26 mm .).
(In
this case, to loosen or tighten the flywheel , a "bar"
should be inserted into the hole in the pulley. The flywheel
can be locked.)
Figure 13
3. Remove the bolts that hold the starter pulley .Once these
bolts are removed, the pulley can be set aside.
Figure 14
4. The
flywheel magneto can now be removed. This requires
the use of
special puller. Be sure to use the three short
screws with the puller when removing the flywheel.
(Note: Immediately after the flywheel is removed, take the
woodruff key off the shaft and attach it to the flywheel
so that it does not become lost.)
Figure 15
5.
All parts of the magneto assembly, except for the mag-
neto backing plate, have been removed. The only t ime that
the magneto backing plate needs to be removed is in the
case of
complete eng ine teardown. The backing plate has
to be removed in
order to split the cases and this is ac-
complished by removing the hold-down screws , w ith an
impact screw dr iver.
Figure 16
D. REMOVAL OF TOP END
Note:
During d isassembly keep matching parts together ; right
piston with right cylinder, etc.
Remove the cyl
inder heads. Each head is held down by
eight nuts .
Break each
nut loose , in pattern,before removing com -
pletely.
Figure 17

t
•
Note:
On reassembly , the head bolts are torqued, gradually and
in
pattern to:
16.6 - 19.5 ft-Ibs. (2.3 - 2.7 m -kgs.)
Figure 18
2. The next step is to remove the
cylinders.
when the cylinders are lifted off it is advisable to sl ip a
clean rag beneath the
piston . This will stop any carbon,
broken
parts , or other contamination from falling into the
engine .
Figure 19
3. The
piston assembly is removed at this time. LJse needle
nose pliers to remove the piston pin clip.
Use a soft drift pin to push the piston pin out to allow a
pai r of pliers to
grip and completely remove the piston
pin . Once the p iston pin has been removed the piston can
be lifted
off the rod and set aside. Be sure to remove the
needle
bearings in the upper rod end so as not to loose
them when the engine is moved.
E. REMOVAL OF CRA NK SHA FT ASSEMBL Y
1. Remove the four bolts that hold the engine m ountin g
br
acket. The engine is now free of all extr a external part s
and
attachment s. The cases are now ready to be split, if
so des ired.
Figure 21
2.
This illustration shows the proper sequence for removing
the case bolts. This sequence must be followed in order
to avoid distortion of the two crank cases.
Fi
gure 22
3. Use
soft hammer to lightly tap around the entire edge of
the cases.
This will free the cases in case any adhesion is
present. The cases can
now be split by hand. As is shown
in the illustration , the cases have knock pins (locating
pins ).
Be sure
that the locat ing pins are immediately gathered
up and kept in a safe place so that none are lost before
the eng ine is assembled again.
7
CL
TJ23
Figure 20
Figur e 23

8
A. CRANKSHAFT ASSEMBLY
1.
After removal of the crankshaft , wear of the connecting
rod big end, crank pin and bearing should be checked.
Excessive
wear of the connect ing rod big end is checked
by m,easuring the axial
play of the connecting rod small
end when it is moved from side to side with your fingers.
The axial
play should not exceed 0.08 in. (2 mm.).
After the Crank pin and bearing are replaced, maximum
axial play should not exceed 0.032 - 0.04 in. (0.8 - 1.0
mm.).
+0
2.36 - 0.02 ins.
+0
(60 - 0.05 rnrn.]
TJ27
<De flection >
7.01 ±0
.004 ins.
178 ± 0. 1 mm .
Figure 27
W idth of assembled
cran kshaft
measured value
0.08 in. (2 mm .)
TJ24
Figure 24
2. Use a feeler gauge to measure
for the correct free play of
0.01 - 0.03 in. (0.25 - 0.75 mm .).
Free play
should not be confused with axial play . Free
play is the clearance between the rod and crank
shims .
I
Keep the free play within
0.01 - 0.03 in.
(0 .25 - 0 .75 mm .]
~l
~
-
~~
1
,1
rr
jT
I
~
,
I
J-
~
-
TJ25
Figure 25
3. Set the crank
shaft on V-blocks so that the bearing sec-
tions are posit ioned on the V-blocks, and check for align-
ment with a dial gauge.
5.
Correction of the crankshaft deflect ion can be accom-
plished by tapping the flywheels with a brass hammer
and/or using wedges placed between the flywhee ls.
TJ28
Figure 28
6.
Installation of the crankshaft in the crankcase should be
carefully done. Each of the crankshaft bear ings has a lock
pin
hole in the outside race and two aligning marks offset
90° from the lock pin hole. The aligning marks should be
matched with the sealing surface of the upper crankcase
half so as to insure correct matching of the knock pin in
the
knock pin hole .
Should the crankshaft be assembled in the cases with the
bearings misaligned, the knock pin will be pushed into
the body of the crankcase and the bearing w ill not be
properly secured after the crankcases are sealed. Damage
to
both the crankshaft and crankcases can therefore oc-
cur .
,
Figure 26 TJ29 •
4. Crank shaft run -out can be checked by placing the crank-
shaft ends in V-blocks .
By u
sing a dial indicator the defle ction of the crankshaft
should be measured at the places shown.
1. Align the marking with 3. Upper case
sealing surface
2.
Knock pin
Figure 29

7. The bottom half of the crankcase can now be f itt ed over
the crankshaft and o nto the upper crankcas e half. The
crankcase bolts can now be placed in the case. Torque
the crankca se bolt s to 7.2 ft-l bs. (1.0 m-kg s.) for initial
t ightening and 14.5 ft -lbs. (2.0 m-kg s.) for final torque .
Figure 30
B. PISTON
AND PISTON RING MAINTENANCE
1. The p iston -to-cy l inder wall clearance should be checked
before r eassembly .
This is
done by measuring the largest outside diameter of
the
pis ton and the smallest inside d iameter of the cy linder
bore. The largest diameter of the piston is checked 0.4 in.
(10 mm .)
above the bottom of the piston skirt.
New piston to cylinder wal l clearance is 0.018 - 0.019
in. (0.045 - 0.050 mm .).
Fi
gur e 31
2. The pi
ston pin fit can be checked by inserting the piston
pin into the p iston and checking the amount of resistance
encountered .
The
pis ton pin should fit into the p iston with a snug
thumb-press fit.
A
lso check the needle bearings and needle bearing cage
for excessive wear, or
overheating .
Figure 32
3. Remove the
piston ring from the pist on, and place it in the
cyl inder so that it is in tight contact with the cylinder w all.
By using the piston head , position the ring in par allel to
the cylind er top edge . Measure the ring end gap with a
feeler
gage . If the gap mea sured is in the range of 0.014 - .
0.020 in. (0.35 - 0.55
mm.), the ring is in satis fac to ry
condition . If more than 0.020 in. (0.55 mm .), the ring face
is ex
cessively worn.
Remove the ring from the cylinder , and keep it in a free
position .Then, measure the ring end gap. If the gap
measured is less
than 0.014 in. (0.35 mm .), the ring is
considered to be fat igued with heat.Replace it with a new
one; otherwise , power loss or ring breakage may result .
When installing a ring on the p isto n, avoid opening the
end gap apart more than 0.63 in. (16 mm .), because the
ring may break .
Note :
If
piston dome or piston skirt is bad ly scored or damaged ,
it
would be advisable to replace the piston. Once seizure
has occurred and the piston has been damaged seizure
can more read ily reoccu r.
TJ33
Figure 33
4.
Before replacing the piston ring,remove carbon accumu-
lations from the gap between the ring and the groove . The
piston ring must be installed so that the locating pin
in
the groove is between both ends of the ring as illus -
tr ated . This is the most v ital requirement to be observed
when the p iston is ins erted into the cyltnder.
No ck pin
Piston ring end
TJ34
Figure 34
5. The up and
down movement of the p iston ring in the pis-
ton groove can be calculated from the side cle arance as
illustrated . This side clearance is created by wear on the
ring
botto m. If the side clea rance is measured more than
0.019 in. (0.05
mm. ), the ring should be replaced. The il-
lustrati on shows the sect i ona l view of a new p iston ring
ins
talled in the ring gro ove.
9

10
0.01 - 0.019 in.
(0.03 - 0.05 mm )
TJ35
Figure 35
Honing or rebori ng should be done if this tolerance is
exceeded. "Taper" of the cy
linder bore is measured by
moving the
micrometer from the top to the b ott om of
the cy
linder being careful to miss the port openings. (Fig-
ure 39) The
differen ce in these measurements (between
top and bottom) sho uld not exceed 0.002 in. (0.05 mm .).
If
upon inspection of the cyl inder bore, scoring of the
cyl
inder wall is obse rved, hone or rebore until these
scores disappear.
Befo re assemb ly, a lways clean the carbon from the ex-
haust port. A lso wash out the cy linde r with solvent or
soap and
water to insure that all hone mater ial has been
removed.
Failure to do
thi s may result in premature we aring or
scoring of the cyl inder and p iston.
•
TJ38
TJ39
--
i---- ..,
,
..
1.. ...J
Figure 38
Figure 37
Figure 39
2. Since the formation of carbon is a prod uct of two - stroke
operation, decarbonization at regular int ervals is essen-
tial. Removing the cylinder head and scraping the carbon
from piston
cro wn and cylinder head should be done at
every
tun e-up or every 40 hours of oper ati on. (Figure 37)
During every
other de carboni z ation, the cylinder should be
removed and the exha
ust port decarbonized.
Figure 36
Installing the piston
The p iston shou ld be installed so that the arrow mark on the
piston crown po ints to the exhaust port (toward the front of
the engine). (Figure 36)
TJ36
Remedies:
1) If any piston ring is found stuck , poor sea ling will result.
2) If the cyl inder wall is
scuffed , poor sealing will resu lt as
in the case of the piston ring being s
tuck . In the above two
cases, the piston ring shou ld be replaced with a new one,
and at the same time, the cyl
inde r wall should be checked
for scrat ches.
3) Whether the co
nta ct between the piston ring and the
cyli nder wall is good or not will have a great e ffect on
sea
ling . The enti re area of the piston ring face must be
in tight
cont act with the cyl inde r wall. If excess wear is
found on the upper or the lower half of the piston ring
face, the piston ring
groo ve is cons idered to be worn
unevenly, causing the ring to
twi st.
If imitation pisto n rings (
oth er than g enui ne Yamaha
part s) are used, defects may easily occ ur, and in the
worst case, serious engine
trou bles may result.
C. CYLINDER MAINTENANCE
1. When the cylin der and piston are removed, there are
several ch
ecks w hich sho uld be pe rforme d. First of all
the cyli
nder bore should be checked for wear or scori ng
each
tim e it is removed. The "out of round" and "taper"
can be checked
with an inside m icrometer . The "ou t of
round" of the
cylinder bore is checked by moving the
micrometer in a circular direction 90' at a time . The dif-
ference in measurements should not exceed 0.04 in.
(0.10
mm.) . (F igure 38)

In stalling the cyli nder head
Cyl
inder head t ighten ing torque :
16.6 - 19.5 ft -Ibs.
(2.3 - 2.7 m-kqs.] M10P1.25 x 4
3.
Turn the flywhee l magnet in the direc tion counterc loc k-
wise, and set the
ignition tim ing at (1.8 mm. ) B.T.D.C..
This adjustment can be
done by turn ing the pulser.
Loosen the
two pan head screws tig htening the pulsa r,
adjust the pul ser so that al
ignmen t mark on the flywhee l
magn et is a
ligned with tim ing mark on the pulse r.
11
All
nut s must be ti ght ened evenly.
Figu re 40
Figure 41
D. I
GNITI ON TIMING ADJUSTMEN T
1. Install
the dial gauge and dial ga uge stand in the p lug hol e
of the cylinder head.
2.
Turn the flywh eel ma gne t to bring the piston to top dead
center, and set the dial guage to zero.
T J4 ~
1. Pan head screw
2. Tim ing mark
3. A lig nment mark
Figure 43
No te:
The difference in the ignition tim ing betw een the two
cylinder s should be zero.
Figure 42
TJ42

12
E. ELECTRICAL
1. Wiring Notes
a. Connections must be done accurately.
Special care is requ ired
for connection of the ground cir-
cuit and ignition coil.
b. The C.D.1. unit and ignition coil are installed in the
specified position . If the position is to be changed, a dry
and
airy place should be selected.
c. To remove the rotor, be sure to use the
rotor puller (an
accessory tool).
Avoid using ahammer, or the rotor may
be damaged.
d.
Handle the C.D.I. unit with special care.
If you
should drop carelessly, the incorporated electronic
components will be damaged.
2. Assortment of C.D.I. Unit, Coil &Magneto
Ignition Magneto
C .D.I. unit
811528 811693 811692
811279
3. Electric Circuit
Spark Plug
Right Cylinder
Gray
~~","<--r-r-~- -----~
Black/White II
~\ Cm=-"':
1';'OP80"OO
C.D.I. Unit
Orange -Jl~~~~
· .,_ - 4 :>- - -~ ,---<.(E--- -«<f
TJ44
Figure 44

-- - ------
ENGINE TROUBLE SHOOTING 13
Prob lem
Symptom
Check
Engin e turns freely but will not start No Spark I
gniti on switch 'off'
Emerge
ncy switc h 'o ff'
Spark plug
Igni tion co ils
COl Unit
I
gniti on wi ring cut , p inched , or frayed
No Fuel (spark plug dry) Fuel tank em pty
Vapor lock
(Hot eng ine on ly)
Fuel
filt er p lugged
Fuel line disc onn ected
Fuel pump impulse line d
isconnected
For eign ma teria l or ice in carbureto r
Choke disconnected
Carbu re
tor loos e on eng ine
Hol e in pisto n
Broken ring(s)
No
Comp ress ion Spark plug loose
Head gasket blown
Hole in piston
Broken ring(s)
Excessive wear
Flooded (spark
plug wet) Check
for no spark (above)
Choke engaged
Too
much oi l in fuel mixture
Wrong
spar k plug
Exhaust plugged
Check
for low compr ession (above)
Carburetor inlet valve le
akin g
Carburetor float level too
high
Back Firin gSpark plu g
wire s reversed
Impuls e line di sco nnected
Bad spark
plug
High tension lead resistor defecti ve (if
equippe d)
COl
unit def ective
Loo se or frayed i gniti on wiring
Ign
ition sw itch faulty
Improper ignition timing
Eng ine wi ll not turn over Seized Before D
isassemb ly of en gine
check: Carb uretor adju stment s
Spark
plug heat range and reach
Fuel
for oil, additives, etc.
Igniti on timing
During Disassemb ly of engine
ch eck: Loo se components
Gaskets
-to rn, mis sing, damaged
Seals-leaking, missi ng, damaged
Unauthorized mo
difi c ati ons
Foreign
materia l in engine
Engine star ts but has no power ,
Spar k plug(s) wet Bad spark
plug
poor accele ration
Choke partially engaged
Carbureto r
low speed mixtu re
Spark
plug res istor bad (if equipped )
Low
comp re ssion
Spark plug(s) dry Fuel
system (same as no fuel check)

14
GAS/ OIL REQUIREMENTS
Your Sno*Jet is equipped with a 2-cycle en-
gine.
THE ONLY LUBRICATION IT GETS IS FROM
THE OIL THAT YOU MIX WITH THE GASO-
LINE.
Follow the recommendations and procedures
b
elow to ensure maximum engine perfor-
mance and reli
ability .
GASOLINE
Never use Naptha,
Methanol , Fuel oil or sim i-
lar produ
cts .
Use PREMIUM GAS
for all Sno*Jet/Y amaha
engines.
OIL
- Use
only high quality AIR-COOLED 2-
CYCLE OIL.
- If the container is
not print ed to show that
oil is suitable for air cooled engine, DO
NOT USE IT.
- Never use outboard
motor oil, or the pre-
mixed g
asl oil fuel that is available in some
locations.
- There are some specially formulated "syn-
thetic " o ils available that require a dif -
ferent
mixing r atio to normal. If these are
used, the
mixing ratio shown on the con-
tainer shou ld be employ ed.
MIXING RATIO
For all Sno*Jet
F/A en gine s is 15:1.
MIXING PROCEDURE
Use a separate
contain er to mix the oil and
gasoline.
IT
CANNO T BE MIXED PROPERLY IN THE
SNOWMOBILE TANK.
1.
Half -fill container with gas.
2. Add the
full quant ity of oil (oil at room
temp erature mixes easier).
3. Agitat e.
4. Add balance of gas. A git ate well until gas
and oil are completely blended.
NOTE: A paddle is more
effect ive for m ixing
the fuel than shaking the conta iner.
CAUTION: If you store a
cont ainer of pre-
mix ed fuel , always agita te it will
before
using .
Figure 45

CARBURETION 15
.->---- -- 31
-- - ---12
- - - - - -- 13
~
'-0
1- - - -30
)- -------29
28
~------ -27
---- ---26
fA 25
~- ---24
_~ " _ """ I 23
~ ll".:'Io--..... ' '- ,,& 22
~/~ 21
#J ~ ~ ~20
19
--- ---18
---- ----- 17
-- - ----16
---- --- -15
-- ----- -14
7--- --
5-- ---- ---+-~
6--- ~
4- --- -
1- ---
2-- -
3
-----
8 - - - - - --11

16
TJ47
Pilot Jet
Large Clearance
Small Clearance Medium Clearance
r-igure 47
C.
Intermediate System (Figure 48)
When the throttle valve is opened further, the pilot outlet
of the idle system cannot supply the required fuel. This
shortage has to be made up with fuel entering from the
bypass and main system needle jet .Fuel passing
through the main jet in the intermediate range is metered
by the clearance between the needle
jet and jet needle.
Air entering from .the main air intake mixes with the fuel
being discharged from the needle jet. The jet needle
moves up and down with the throttle valve to control the
fuel
mixture in the intermediate range. The throttle cut-
away also affects the mixture in the intermediate range,
but has .no effect above one half throttle opening.
TJ46
SP~
Mixture
To Venturi
Float Chamber
The purpose of the starter system is to enable easy
starting when the engine is cold. In the starter system,
fuel and air for starting the engine are metered with
entirely independent jets. The air metered in the starter
jet is mixed with fuel from the float bowl and is broken
into tiny particles in the emulsion tube. The mixture then
flows into the plunger area, mixed again with air coming
from the air intake port for starting and is delivered to the
engine in the proper air-fuel ratio through the fuel dis-
charge outlet. The starter is opened and closed by
means .of the starter plunger. Since the starter is con-
structed so as to utilize the negative pressure of the inlet
pipe, it is important that the throttle valve is closed when
starting the engine .
The 1975
Thunder Jet is equipped with dual 36 mm MikUlo.
VM
type slide carburetors. Experience has shown the Mikuni
VM carburetor provides the best performance and tuning
capacity available at this time. There are several distinct
functions of the M ikuni, and it is important to understand
their operation.
A. Starter System (Choke) (Figure 46)
MIKUNICARBURETOR
Operation
Figure 46
''''---Needle Jet
B. Idle System (Figure 47)
The
air which enters through the pilot air intake passes
around the outside of the air screw where it is metered
and
enters the pilot jet bleed holes. The air entering the
pilot jet bleed holes mixes with the fuel that is being
metered by
the pilot jet . The fuel and air mixture being
drawn further up the fuel channel is again mixed with
air that enters through the bypass. This mixture is drawn
into the pilot outlet to mix with air flowing through the
main bore into the engine.
TJ48
Figure 48

D. High Speed System
Fuel from the float
chamber passes throug h the main jet
and enters the needle jet. Air entering the
main air intake
passes through the air bleed in the needle
jet and mixes
wit h the fuel being discharged from the needle jet. Air
entering from the main bore also mixes with the fuel and
then enters the
engine at the proper air fuel mix ture . The
clearance between the needle jet and the jet needle is
greater than the metered orifice of the main jet. There-
fore, the jet needle has no affect on the high speed sys-
tem. Note : The main jet only a
ffects the high speed sys-
tem. (From one half to full thro
ttl e.)
TJ49
Figure 49
E. Float
Syst em
The carbure
tor mus t provide a proper mixt ure of fuel at
differ ent thrott le open ings and engine speeds. To ac-
comp lish
thi s, the fue l level in the carbure tor must be
maintained. The
floa t chamber functions to serve thi s
purpose. The fuel from the tank enters the float cha mber
through the fuel inlet passage between the inlet nee
dle
valve and seat. This fills the chamber to the level where
the
float arm rises to s hut off the flow of the fuel by
seating the inlet needle valve against the valve seat. As
the fuel is
consume d, the float nee dle wi ll ope n and then
c
lose to main tain a constant fuel level. A spring is
inco rporated in the needle valve wh ich is in contact with
the float arm lip. The purpose of
thi s spring is to prevent
the needle valve from vibrating so that a constant fuel
level can be maintained even when ridi ng over roug h
ter rain.
FUNC TION ,
SelECTION , AND IDENTIFICATION OF
COMPONENTS
A. Pilot Air Screw and Pilot Jet
The pil ot air screw regulates air that mixes w
ith fuel from
the pilot jet. Turnin g pil ot air scr ew IN, richens m ixtur e,
OUT, leans
mixture . The pilot air screw should be nor -
mally po sitioned V2 to 2 turns out from full in, or seated
posit ion. Less than V2 turn indicates pilot jet to be too
small and the next size larger
pilot jet should be used .
More than 2 turns
out , shows the p ilo t to be too large
and a
smaller jet is needed. An engine may idle smoothly
out of these settings ,however problems may occur in
the
transit ion to mid range throttle operation. Pi lot jets
are numbered in increme nts , of 5, such as 20-25-30. The
lowest
numbe rbeing the smaller orfi ce and leanest,
while higher
numbers are the richest mixtures.
B. Slide Figure 50
The
slide controls fuel mixture between 0& V2 throttle .
The use of the
throttle valve w ith d ifferent size cut -
aways wi ll
contro l the discharge rate of fuel whi ch af-
fects the fue l air ratio . The largest thrott le valve cut away
numb er provides a leaner mixture. Thi s is only effective
with in the lower range of throttle opening and has no
effec t above one half thrott le open ing . (Figure 50)
\TJ50
Figure 50
Thrott le slides are stamped with a numbe r (1.5, 2.0, 2.5,
3.0 , 3.5, etc .); these
numbers reflect the degree of cut-
away. The larger the number , the greater portion of
throttle slid e cut-away, which results in a leaner in itial
fuel-air mixtu re.
C. Jet Needle
The needle can be set in one of five d
ifferent pos it ions
by moving the needle c lip to a different groove. For fine
tuning purposes, number the grooves from 1-5, starting
from the top groove dow n. The number 1 groove gives
the leanest se
tting , whereas the number 5groove gives
the richest sett ing . (
Fig ure ~1 ) .
2 3 4 5
JW =:I===:== =====:::>
TJ51
Figure 51
Jet needles can be id
entifi ed by the numb er stamped on
them . Example 60P1-3. The firs t numb er is the l ength of
the needl e, #6
being 60 millimeters in length, #2 is the
shortest and #9 is the longest. The letter
indic ates a
scale of richness resulti ng
from needle taper measured
from the middle notch , 2 letters show double taper. A,
being the leanest and Z, the richest. The second
num -
ber is a
product ion mode l number that may in dicate a
change in beg
inning point of taper. The last num ber after
the dash is the clip po sition in notches from the top of
needle.
17

18 D. Needle
Jet
The needle jet comes in various sizes and works in direct
r elation with the need le. The needle jet or ifice d iameter
remains constant through the entire length of the jet ,
and
therefore , cha nging the needle jet wi ll have a greater
affect on mid -range opera tion than a groove change on
the
jet needle. Needle jets can be ident ified by a numbe r
stamped on them . Example 159P4, f irst three digits are
model number . F irst alphabet is scale of richness . A is
lean, Z is rich.
Last number is mino rchange. 0 is
leanest, 9 is
richest . Lette r in dic ates orf ice size, earliest
alphabet lett er is leanest. Number ind icates orf ice size
a
lso ,however it is more exact in calib ration , being in
incr ements of 10 d igits, the larger numbe rbeing the
richest .
MI
D-RANGE TUNING - Fine tun ing the mid -range is
s
imilar to tuning the main jet. For e xamp le, use a rich
need le
position and a rich needle jet unt il four-cyc ling
is evident . When the four -cyc ling affect exists , install
the
next leaner need le or need le jet to insure the proper
fuel-air mixture is obta ined .
NOTE: All the adjustmen ts mentioned previously will over-
lap into the
next graduated range, which is necessary to
provide
smooth accelerat ion . A main jet change will affect
the mid -range operation by appro ximately 10% . If a one
groove needle change is made, it will affe ct the main jet by
approx imately 10% .
ADJU STMENT OF F uEL LEVEL
MODEL 32 3'
3.
me" .5
9-. 66 . 8
2- .90 .66
-. 71
H'
mm IS _ 17 2\
-23
17 -19
TJ52
Figure 52
~ A I R
IDLE
TJ53
Figure 53
Adju stm ent and Synchronization
Idle Adju stm ent (Figure 53)
1.
Carefully rotate the idl e air screw(s) c lockw ise unti l it
contacts the carb u retor seat .
2. Rotate
both idle mi xture scr ews one (1) revol uti on
counterclockwise . This is the norma l idle air loca tion .
3. If
the idle mixture is too rich, rotate the idle m ixture screw
out (counter clockwise) to lean out mi xture .Reverse this
procedure if mi xture is too lean.
4.
Idle R.P.M. should be set just below clu tch e ngagement.
5. To adjust idle speed,rotate idle adjust ing screw (s)
clockwise until carburetor slide just beg ins to rise.
6. When the
throttle slide(s) just begins to rise . R otate idle
screw % turn cloc kwise or until required idle R.P .M. is
reached. NOTE :
Mode ls with dual carbs, ad jus t both car-
buretor idle speed screws equa lly for proper idle.
Synchronizing Carburetors
The slides must be synchronized so that they to p-out to-
gether . This should not be done at idle position because
the
adjustment here may vary from one to the other , but
should rather be done at an interme diate point. (Figure 54)
•
026
F
igure 54
'j
.r.
... , 'j
(l
tY ~
I
\'r-,J .r
1. Remove intake si lenc er.
2. Raise one s
lide unt il it is pos itioned near the top of the
bore.
3. Then ,
holding the thrott le steady,observe the other
carburetor slide and see if it is in the same position .
4. If
not, adjust the cable housing end to make the slides
the
same .
E.
Main Jet
Main jet - the funct ion of the main jet is to contro l the
fuel
discharge in the high speed range. However, the
main jet wi ll also have a vary ing deg ree of effect in the
v
icin ity of one half thrott le open ing . The main jets are
numbered so that the larger the number, the b igger the
open ing and consequent ly a richer fuel air m ixtu re. Main
jets are
numbe red from "0" to "200" in increments of 5,
and over "200" in increments of 10. A 10%
mixture ad-
justment is realized with each change of main jet. The
main jet controls the m ixture from 3 /.1 to full thrott le.
To
determ ine if the co rrect main jet is ins talled:
1. Run the mach ine on hard packed surface at full
th rottle , and
2.
After operation at full thrott le, do not idl e eng ine.
3.
Stop engine and remove the spark plug.
4. Spark,plug po
rcelain should be a brown co lor . If plug
is dark and sooty , the main jet may be too large. Con-
versely, if the porce lain is l ight gray or wh ite , the
main jet may be too sma ll.
NOTE: Operate engine a suffic ient time to achieve
spark plug coloration.
Float Adjustment
The float adjustment shou ld be chec ked for properloca-
tion. Measure the d
istance shown in F igure 52. The 36
mm
carburetor should be .70" . If adj ustment is neces-
sary , bend
the sma ll tab on the float arm wh ich regulates
the inlet needle.

Trouble Shooting
Malfunctions that occur when adjusting a carburetor can be
traced to
either too rich a mixture or too lean a mixture .
Sym
ptoms are as follow:
1. When air fuel mixture is too rich:
a. Engine noise is dull and intermittent.
b. The cond ition grows worse when the engine is hot.
c. The condition grows worse when the starter (choke)
is opened.
d. The
condition may improve slightly when the air
silencer is removed.
e.
Exhaust gases are heavy.
f. Spark plugs become fou led.
2. When
air fuel mixture is too lean:
a. The
engine becomes overheated.
b. The condition
improves when the starter is opened.
c.
Acceleration is poor.
d. Spark
plugs burn.
e. The
revolutions of the engine fluctuate and lack of
power is noticed .
Carburetor Specifications
Adjustment
The track should have one inch slack between the cleat and
the hi-fax . Thi s should be checked with the mac hine sup-
ported in the rear and
pulling down at the bottom center of
the
tr ack. If incorr ect, adju st the two idl er wheel adjust m ent
bolts located in the rear of the skid frame (Figur e 55).
Alignment
Track alignment is very important to both durability and per-
formance. The track
should be aligned so the slide rail hi-fax
is centered on the
cleat and not rubbing on tr ack b elt s or
guid es.
To Adjust :
1. Run track slowly, if track runs to one side.
2.
Turn adju ster bolt , on that side only ( Figure 55).
3. Recheck track alignment and if
correct , tight en locknut.
19
Suspension
The suspension is designed to permit maximum adjustments
for varying track and snow cond itions. There are ba sically
two (2) adjustments.
Carburetor Type
Main Jet
Jet
Needle
Needle Jet
Slid e
Pilot Jet
Air Screw
Bypass
VM 36
250
6DH3-3
1590 -8
3.0
35'
11/2
1.4
ADJUSTERS
015
Figure 55
Front Springs
The front springs control ski pressure and track angle . In-
creasing front swing arm spring ten sion will li ghten ski
pressure and
allow for better track contact. Decreasing front
spring tension wi ll increase ski pressure and permit skid
frame and track to move up into the tunnel for bett er flota-
tions in heavy snow conditions .
Rear
Springs
The rear swing arm sp rings should be adjusted for ri der
weight. The suspension should be adju sted to allow arm
movement for good ride but prevent bottoming on all but
severe bumps.
Track
The
track is a steel cleated type for good traction and ease of
studd ing .