Cycle News - Archive Issues - 1970's

Cycle News 1974 04 23

Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles

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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. • 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 ! • • 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 '

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