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Bultaco Mikuni conversion
By Marty Gregory
With the recent success of J im
Pomeroy on the World Championship circuit, the Bultaco Pursang
has become more popular t han
ever with asp iring race rs. Alw ays a
good handler, th e 'Sang has a
reputa tion fo r being o ne o f' th e
mo st forgivin g bik es avail able,
m a ki ng it a n exce lle nt ch o ice for th e
Novice rid er as well as th e Exp ert .
Un fo r tuna tely , the Huls also have a
repu ta ti o n for p oor m ann e rs re ga rd in g
st ar ting , idl in g, clean run ni n g and
carb ure tion in ge ne ral.
Th e 36 mm Am a l conce ntric
ca rb ure tor , w h ich comes as stan d ard
equ ipm en t o n - the new m odel 2 50 and
360 l' ursangs, works well when new and
pr op erl y je lled . Ho wever , th e - s lid e
se e ms
to w ea r o u t prematu rel y a nd
me rely inst alling an acce ssory air filt er
will alter th e je tti ng whi ch is a ge nui ne
bear to change . T he 34m m Mikuni fro m
a ' 7 1 Yamah a !\IX gives perfo rmance
eq ual to, if not surp assing, the Arn al ,
allows th e b ike to st art o n the fir st kick
co ns tan tl y, and is simpl e to t une due to
the acc essabi lity of the main je t whi ch
lives in side a bolt o n th e side of th e
carb o
Th e installa tion of the Japan ese
fue l-air mixer is su pe r-sim ple and
requi res o nly a wr en ch or two and a file
or grinder. You need a carb , so me
silicone seal, a pi ece o f rad iat o r hos e
and (if you wa nt to d o it righ t ) a K&N
air filte r. With som e "carefu l co nsumer"
tac tics , yo u sh o uld b e ab le ' to p ick up
th e whole nu mb er fo r arou nd forty
b uc ks. Pu t ti ng it on only takes abou t an
ho ur if y o u ' re sort a slow lik e us.
Step 3
> 0..
~ <
o
.
Once you get the mounting re lati o nshi p
right, give t he Mikun i's " 0 " ring a sho t
of si licone seal to p reven t leak age and
bolt it o n using the 8mm flat washers to
kee p it from slipp ing to the sid e in
eithe r d irec ti o n. Ta ke care not to
overtighte n th e nuts as t hi s will
com press th e rubber mou nt ing b lock
uneve n ly and cause a ir leaks. '
UJ
er
o
>
f-
er
''"
"
o
f-
o
Step 4
Mod ify t he ca b le to fit t h e Mikuni. Be
su re t hat t he slid e is h itting both t he top
and t he bottom of the carb o We had an
accessory th rottle on our, bi ke and had
to short en th e cab le by abo ut an inch .
This will vary w it h each mac h ine .
Step 5
In o rd er t o get th e a ir int ake hos e to f it
t he Mikuni, w e cut it down to ab ou t
o ne inch .on each sid e and insert ed a
length o f radiator ho se (1 '% " I.D.) int o
it . So me me asur ing and t r im m ing is
needed her e t o get th e d istance betw ee n
t he carb a nd t he airbox right. Be sure to
get t he t yp e of rad iato r hose w ith a
green stri pe o n it and no sp ring insid e .
It's fa irly easy t o cut and is gas resista nt.
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r
a.
Th is is how the K&N fil t er mounts inside the a ir bo x .
Step 8
T ake you r new lv- carbu ret ed scoot er out
to your favorit e test ing ground and read
so me p lugs . T he Mikuni comes st ock
w ith a numbe r 310 ma in jet and our
b ike ran pretty lea n o n it. (No te : The
super -breathinq K&N had a lot t o do
w it h t his.] The nu mb er 60 pilo t jet w ill
allow t he bi ke to sta rt after a couple of .
kic ks but a richer one w oul d be a littl e
better. What we're try ing t o get across is
t hat the st an d ard Mlkunl-for -Yamaha
jett ing wi ll get yo u int o the prove rb ial
ball park but changes wil l be need ed for
every mac h ine. Exp erim ent . A good ru le
of thu mb is to go ric her on t he ma in jet
unt il it b lubbers on top end and t hen go
down on e size . Re me mber : the ma in jet
co ntro ls top end , t he needle posit ion
contro ls rnid-ranqe and the pil ot jet
hand les st art ing and bottom en d
respo nse.
O nce yo u get t he jetti ng right, vou
may w ish to indul ge yo urself a b it . Pu ll
o ver to you r b uddy's truck and shu t off
th e Pursa ng. When a large enough crowd
has gath er ed , give it o ne kiCK to start it
and wal k away wh ile it id les per fec t ly.
That will b low at least several m ind s fo r
su re !
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Mikuni conversion for Bultaco
Step 1
Unbolt the Am al. Remove sundry gas
line s, a ir cle aner hose and throttl e cable .
You need not rem ove the cabl e from
t he t h rott le hou sin g end .
Seal hose to intake rubber.
Step 6
Rat·tail file is applied to mount.
Step 2
With a file or grinder , open the end s of
the Mikun i's bolt holes so that the stock
man ifold bolts w ill fit pr operly. It is
wise to ta ke th is st ep slowl y and ma ke
constant fittings to insure a correc t
al ignment. O ur b ike (a Mk . V 350 ) had
t he man ifo ld m ounted at a gro ss ly
of f -camb er a ngle and it was necessa ry t o
an gle the ho les a b it to get th e carb to
sit st ra ight and tall. Cave at empto r!
Secure the radiator hose in t he st oc k
hose w it h a little silico ne sea l. Yo u can
use t he st ock hose clam p to mount t he
mod if ied ho se to the carbo Be e xtra
ca refu l that t h e ho se is clam ped eve nly
and ti ght ly to t he ca rbureto r as any
leakage at t h is po int w ill suck in grunge
and barf yo ur engine. A variation on · a
th eme here is to take th e inta ke be ll
(velocit y stack ) off the Amal, gr ind the
thread s off it and epoxy it to the
Miku n i. Th is procedure elim inates t he
need for the rad iator hose as the stoc k
ho se w ill fit perfectly on t he bell.
earb installed, throttle cable modified. check the slide return.
Parts list and approximate prices
1 -
Y am ah a Mi k un i 34 m m Flange M ount Carburetor No. 28 4 · 14 10 1-60. St oc k Carb on
'7 1 360 MX . . .Available new & used at most Yamaha shops. . .$25 to $40.
- K&N filter for Bultaco Pursang (Whatever Mk. you've got) or a K&N for a Suzuki
TM ·12 5 w ill fit w e l l. A r oun d $8 .00 most every w here .
- 6 " Le ngth o f Gr een St r ipe rad iat o r hos e , 1'% 1.0 . a vailab le at m o st a ut o pa rts houses
for ar o und $ 1 .50 . (Note: if yo u p lan t o u se the A mal i ntake be ll on the Mi ku n i . you
will no t need the ho se .l
2 - 8mm Flat Washers. . .5 cents .
MISCE LLANEOUS ITEMS : A' bit of silicone seal or some related n on -hard en ing
adhe siv e, some f oa m st rip s and assort ed m ain and pil ot j et s f or the M ikun i.
Jetting Specifications
Step 7
Test bike:
Inst all the K&N a ir filt er. It 's a neat id ea
to glu e some foa m strip aro und t h e
mo ut h o f th e K&N to seal it well w here
it sits o n the a irbo x. If you ca n' t get t he
off icia l Bultaco re placement K&N , you
can use on e d esigned fo r a Suzuk i
TM 125. It f its. Be sure to ta pe t he ho les
in t he Suzuk i fi lter sh ut befo re yo u put
it in.
Altitud e
350cc Bultaco Pursang Mk. V (With K&N
f i lt er and b uilt-i n silencer) .
:
2800 feet
T emperat ure
"
Mid 60' s
Mai n jet • . . . . . . . . .• . • .. • . . • No . 3 10 (stand ard wit h carb) - currentl y using N o . 320
Pilot jet
Needle posit ion
51 cutaway
ide
No. 60
Middle Notch (No. 6DH2 needlel
2.5
NOTE : These j etti ng specs are merely wh at wo r ked o n o ur te st b i ke. Each machi n e will
require changes and e xp er ime nta t ion. Th ese size s will get you close eno ugh t o get a go od
idea o f what you need !
37 '