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"The ABC's of Carburetion"

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MIKUNllM SUPER lUNING

~ . . . -- ,

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"

PARTS LIST

,. .. ",. ... ,. "''''' "."" ,,..,., " . ..... " " , .. ,,", .... M _., .. ". ... ...... " , ... ":,,, .... j" ""1" " . -.- " ,. N',,'" .... 'u " ... ,,' " . _ .. ,'" " ,. "" ... '" .... ," ... ,," " -".,. ,. ....... ,., .... ia' .. ,"". " - "," " ,. ,,_.m , ... ,,, , ...... ,,- .. _" '" • -,,, .... i .. ....,"', ,,- -'" " . """" -,. -: .. ... " .. .... .. " , .. ,,,,, " -"" -"'-"" -... " n'.,'" .... ,,'" " , - '" " .".:oo ..... ,,,' _:,,, .... i." ".,,, ... ., '.,,"''' " """" _11>1 ...... ,,' .... 1 .. -," ", .. ",",. " -, .. -" ... ,;" ... . i .. -,. ". --• ...... " 1><,""- "".;" ,,- "''I'" " .... ,1<. """ ... ,"",'W ," , ""' • ~'" - '""' " -""' •• '<1','''' -, . _. •• .. ......

, . •• , •• ,. •• ,. •• •• " u. ". ". " . ". ,.. n. ,. . n . 00. u . u. "-... " ". ". " ,.. ~

" .12. n. ". ". •• " "

"'bbor C. p, Thm"" "'~,. c.~ .. .... J~ ... ~ Lo"~u'. c.h1. AdJu.t.r '!:'P. IIlxln< 0._ .. reb., IJ.c Te .. Slr!»l, t1,.,,, VaL •• Pl.,., No"" ... " au "0" .......

"" ~=n~ .,., .. V . ...

~ ... l. Jot hbtor C.~, ' <onlo, Iy""" Cop, St.« .. Fl~",., Lnor ''' y, '<or""1 '1'con loo_1 '~d"" ....... ,g,h'''1''5 O,<1UI. 'tu'e< rL ...... PIu ..... . l,or."'J Sy,,,,, • oty, Kid"" C_-'OT Gu ku , tlo., ChPIo<t B,tfle PLot_, ll ... Cu.o.e, r ll" Jet "n, PIa," Am " '".. ~I~ ...... a ~,~ .. :!'100' ""_ .. lI,allor. 11 .. , 0. ..... Plo" Plul!. n .... ' 0....., .. Air J*( 0.;, 'ec ... . ,.'n~ . .u, JrdJ"" ""I .. ,~. .... oa , :.Il. Adl""""' .«." ..,.., ... . U1>. .... j .,. •• ". lI_hor , I~.dl. " ..... ... ,.. n_l •• S . at A .. ! ""P, 1 .. 1 loo<oI"lr.& >!Iln Jot Ph,., 'r.,,, :rob, .. , .. ....", 'or"", "',.t ....... ".

.. ,,'" .... m ,vu", ... .... .... '" -" ,," i',. "".",. .... "" .. ,""" "" .. " _m ~,~ ""f" ..... ," .... m. .. ~ .... 11' ..

-'" .... ,," ...,,,,» V'<1I11>' .. ,.." .. >om --., .,-.-". ..... " .. -" .. -,,, . "',.. .. .. .. I'. .. .. ,<. .0,," .",,, -'" "" .. ", ... ,,'" .. ,,"', .,,'" .. .,,, """ "'I" --- ..... , .. "" ..... "" .. " . -" .... , ..

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"'''''"

foreword

This manual is intended as a gu ide tor users 01 Mikuni ~a rburetor.iwho went to Itllrn the adjusting

method to the best performance from 011 ' products. In motorcycles, special luning of the engine is now

comideretJ ill rootine prllctice. The il rfOW~ that app-.'f in the drawi ng~ in th i ~ text show the direction

in which air, fuel and an air-fuel mixture flows, r;rsp~c \i~ely_

~FuCI . Mixture

Mounting angle fo re and aft inclination of the carb should not exceed apprOK 20 degrees

from hor;~ontal _

r --- - - - •• - -- - - - --- - -- - -- • • - --- --- --- --- - --- - - ---., - ' ~ -- - - - - . - _ .- - - - - - - - - -- - -- - - - - - -- -- --. - . - .- -- - - . ~ I , , , , , ,

T i,,"01U1l ." ,. , ,

:t: , ' , . , , , , , , needle

Air jet r Oil . , outl. t

,- ~. aCrew

--- -- ----- --- -"

Needle b.I •• ~ jet

N.edle ,,, /

Starter ,_" J , Main J" '---

~ __ .:: ~:: : : : ::_-~~:: :-.-::: 3£:: ::::::::: ::::::-___ u u u1 "'Midur.!I

, . 2.

3.

4.

TABLE OF CONTENTS

What b a earburetor 1 . ....•...... ...... · .. . ... ... -. . . . . .. - .... . Air-full! miJtture rClt;Q ....... _ ... ..... . · ... ... ... ...... ... .... . FuneriQn and eonstruCtlon _. _ ..... .... . · ......... - . .... . -.. -. - ... . 3-1 . Low·speed tuel sys1lfTl (p ilot 'VItam) • _ . · .. . ....... ........ . -. - .

.. . • 1

.. .. 1

.. " I

. . 2 - 3

3-2. Mlin fuel system Imain system) ••.••.• •• • •• • • • •• •• .••••• • ••• . • • __ _ 3

3·2·1 . Primary typII •. ..••• ....•. . . .• . ... .. • . . •••• ••• . . • • . ..• . •.. . •

3·2·2. BlINK! tvJM ••• .••••••. ... •. .. . • ....•. .• .. . . . • _ .•.... . . ... .•

3-3. Float system ••. • ..• . .•• ,..... ... •.. _ •••..• . _ . . • • • • • • • • . . . . . . .. . 5

3-4. Slar tin~ dewiCII (starter l'(Item) ..•...... .. ....... . ....• .. . .•. . .... . o - e

Tunlni UJJ of engl"" Ind 5ell!elion o r tIpIIIrt ... ~ of I;IIrburetor ... .

4-1 . 5ellCtion of apeflur, of carburet or ••.••.•. . .•• • •• .

. . . . . , . , . . . . . . .

.. .. .... .. , .... 4-2. Sizes of Mikuni ClrbUreton •.•••••• ..... ....... ..... ... .... ....

. . 6 - 7

.. _ .. 7

5. calburetor setting ... . ... . .......... ..... • . •. ...•....•.•..•.... _ .. ... 7

&1. Sell!etinnolmlli n jet . • .. ..... . ... ... • •• . • .• ••• . ..•• ••. ..... .• .. •.. 8

&2. StI~tion of nudle jet . nd jet nHdll! ..... . .• . . . •.••. .. • • • • .• _ __ .. _ _ 9 - 1 0

6-3. Pilnt jetandthlalow lYstom . ...... . .. ..•... . • . • ....•.•......... 10 - 11

~. Th. ""'''Wly ij.jze 01 throttle Vllve .... . . ..••.... • . • ..•.•. • ••... ....... 1 1

fl. MalntlllNllell. . . . . . . . . . . . . . . . . . . . . . . . . .... .. ................ _ . .. 12

1. Tuning uJJ of ~rbutelor for nctl'llll . • . . • • • . . •••.•••••••.•.•••• •..•.... 13

7·1. The amount of incnmlng air In relation 10 meteorological condltlom ..... .... .... 13

1. Funotion of II carburetor

The function of a carburetor is to produce combustible air·fuel mixture, by breaking fUllllnto tiny

particles (in the form oj vapor) aoo by mix.ing tne fuel with air in it proper ratio , and to deliver the

mixtur. to the engine. A proper ratio (milnure ratio or air.fuel ratio) means an ideal air-fuel mixture that

can burn without I~a\ling an ooccess of fuel or air. Whether the proper mi:<ture ratio i~ maintai"ed or rlOt

hold5 the key 10 the efficient engine operation.

2. Air-fuel mixture required by the engine tF ig. 1,

The ratio of a mixture of fuel and air is ~lIed the mixture I"8tio or the eir·fuet retio and is generally

e~pres:red by the weisht Vrupurtion. TileurtltiC<llly, the .,mount of air required for complet~ combustion

of 1 gram of tuel unde r normal conditions Is:

Mixture ratio = t 5 Qrems of air 1 !:ram of fuel

..... .... theoretical mix.ture ratio

In realitv. varying mi~ture ratios are reQuired for the engine, depending on operating conditions.

Although the reqJired mixture ratio vari~ more or le~s with the type. the cooling efficiency. etc. of the

engine, the mixture ratio wown in Fig. 1 is

required for ordinary engines. In tile high speed

r~nge, the ratio of ~oout 12 to 13 grams of .ir for

1 gl<lm of fuel plOduces tile ma~imLJm output.

However, in the cose of an engine with low

cooling efficiency. a somewhat richer mixture 110

to 12 !lr~ms of "ir against 1 gram of fuel) may be

reqLJired to prevent seizure of the engin~.

3. Functlorl$ lind l;:(,)I'1struction

ft.-ot". ""I". <>pOning ("I Fig. 1

The engine of a vehicle is operated under a wide range of conditions, from idH rlg with the throttle

valve (1) "maining almoet cloelld to th~ full load (the maximum output) with t"'e t"'mule valve fully

op€ned. In order to meet tile requirements for the proper mixture ratio under these varying conditions, a

low·speed fuel system (the pilot wstem) and a main fuel system lthe main system) are provided in

Mi~uni VM.type carburetors.

-,-

3·1 . low·speed fuel system Ithe pilot system) ... Fig. 2 and fig. J

SineI' the engine is operated with th ~ thrott l ~ va lve almost closed ilt id ling o r in the low speed ran9'<,

the velocity of i ir flowing through the ,.,."edk> jet (2) i, Slo ..... Con"",qu~ntly, a negative pre!iSlJr~ i t ron'}

enough to draw fue l f rom the needle jet in the main fuel system is not created. Thl' fuel rupp ly during

this low >p~9d op<lriltion is controilltd by meal1$ of the pi lot outlet (3) and th ~ byp~$$ (4) that are

provided close to the engine. When the de-gree of the throttle villl/e opening is mull at idling, fU<l1

metered with the pilot j~t (6) is mixed .... ith a ir adjusted in a propEr amount by the air $Crew 16) snd is

broken in to fine particles.

Fig. 2 Fig. 3

The mixture i ~ again mixed with air <;oming from th" bypass and is drawn into t he pi lot outlet to mix

with air flow in!} throU!Jh the main bore 0), The fuel mixed with air in thl' optimum ratio at this sta!}l'

th<;tn goes into thi an9ine, Wh9n the throttle va lve opening gtH wider for low EpItKl operation, the pilot

out let alone cannot supply the required fuel and the shortage has to be made up wit h fuel injected from

the bypass. The adjustment of the mixture ratio during th is su.ge is rTIOIde by the pilot jet and the air

screw, in the C~$C 01 a tWO·hole type fuel ~stcm. There is also a one-hole type low speed fu el system

- 2-

mainly used for carburetors having It

YTlaII main bore. The Pfocess of pro­

ducing thf air-lut! mbrture and 01

adjurting the mix:ure rat io .e the

$Olmt as in a two-hole tYpe low ~

3-2. On Mikuni VM-tYPtI carbureton.

thl pilot $y$t9m and thll m~in synem

,r1I o f independent cO<'Inruction.

The f ... eI flO" ... in th_ two cystem: is

lhown in Fig. 5. Thu. are two types of

the m.in fuel system; O'-I! il It pri m~ry

type used widely for 2-cycle engines

and the other is a IJI~ lYlle which

rNy be used for 'H::yc l ~ qnes !l~

well as for rOtary viillve type 2-cycle

engin~.

-,-

Fig. 4

TImmie ~.j .. ~""'" 1%1-

Fig_ 5

3·2·1 . Primlry type

Wherl me throttle valve Is opened about 1/4 or more, the velocitY of air flowing through the rlf:If:Id le

jet increases and also the negative pressure increases to the point where fuel can be sucked in . When the

opening of throttle val~e is between about a Quarter and three quarters. fuel passes through tl1e main jet

and, aita- being metered in the clearance between the

neoclle jet Ind the jet needle, it is mixed with air that is

metered in till air jet and atomization of the fuel is

3ccclcr~tcd.

The mixture is then injected, lifter mixing with ~i r flowins

through the mai rl bOl"e, to the engine in the optimum

llir· fuel ratio. During this process of operation, the

cutaway of the tl1ro«le valve SefVes to controL the negative

pressu re on the needle jet, thf:lfeby regulating the amount

of fuel that is Injected to the engine. When the throttle

val~e is opened more than three quarters for high speed

operation, fuel is chiefl y metered with the main jet.

3-2·2. Bleed type (Fig. 7)

The construction of the bleed·type main fuel system

is the !.8m~ as that of the primary type, except for the

bleed hole that ill provided in the needle jet (F ig. 71.

In the case of the primary type, air that comes from the air

jet is mixed with fuel that i$ metered with the needle jet

and the jet needle. T he bletld type, an the other hand, i~

designed to hald ~II In [he OOdy section of the need le jet

and then to have the air and fuel melered.

-.-

Fig. 6

Fig. 7

3-3. F lo.1tlyst1lm (Fig. B)

The float $'(3I:em Sl!l'Ves to maintain a ~nltant level of ruel in the bowL Fuel flows betw,.n the

needle valve a nd the valve seat and enters the float chamber. As the fuel enter, thll float chamber, the

II000t mQIIC$ l.IPl'IIOrd tQ the pro-detormined 1CV(l1 boc.ouSQ

of the action of buoy~ncy. When the buoyancy mnd the

fuel pressure balance, tile ne!ldle ~alve and the valve

draw dose to each other, shutting off thB wpply of fuel

from the pump.

The fuel level in the bowl controls the arrKlunt of fuel in

the fuel mixture, Too high a I~el alloW'! more fuel than

necessary to leave the noZZII, enriching the mixture.

Too Iowa level results in a l(lOner mixture, 011$ not

enough fuel l ea~s the noule. Therefore, the pre­

determined luel IlIVel should not btl ~h~ng~ IIrbitrllrily.

3-4. Slartililldevice btarter Wltem) (Fig. 9)

In plac~ of the choke, the starter system

15 employed for Mlle.unl carburetOfi. In me

starter type, fuel and air for starting Ihe

engine are metered with entirely independent

jets. The fuel metered in the stnlet jet is

mixed with 3i r and is broken into t iny

partie l" in the emulsion tube. The mixtl.ln!

Ihen flows into the plunger aree, miKeS egain

wllh air coming I rom the air Inlake port for

starting ~nd is delivered to the engine in the

optimum iir·fuel ratio through tl'le fuel dis·

charge nozzle. The starter is opened and

closed by means of the !tarter plunger. Since

th~ starter type i$ constructed ,so as to utilize

the negative pre:lsure of the inlet pipe, it is

impOrtant that the throttle valve is closed,

-.-

Fig. 8

when startirtg the engin~. At high temperatures (approx. above 68· Fj, the engine can be stllrted without

usi ng the starter.

4. Tuning up of the engine and selection of the aperture of carburetor

Tuning up normally m~an~ a process 01 I!l:&Urale Bnd &Breful ~diunmenllO obtain ITlIIximum engine

performance, although It means in a broad sense an economical improvement in fuel consumption.

Improvement ot power output of the engine depends on the amount of air drilwn into the cylinder per

unit time. A practice generally followed tor engine tune·up includes:

(11 To imprOY9 suction IIfficilHlCY and exhaun efficiency by remodelling the intake and m<haust

syst9m

121 To impr()Ye combustion efficiency by r;)ising the oompression ratio

(31 To incrQ;lsc the number of revolutions by adjusting the ignition timing

Just as we need to take balanced meals of high calories in our da ily life, the engine cylinden need to

t<lke sufficient air and fuel mixed in proper amounu. The functiort of a Cilrburetor is to prePilre and

supply a mixture of fuel vapor and air to the engine cylinders in proper proportion for efficient

combustion.

4·1. Selection of the ap~rtule of <:.Ilrburetor

One ul the prt!"equisit~ for improving mil output i. to use a carbtlretor with as larye art apenure as

possible. However, a lergs aperture alone does not improve the output necessari ly . As shown in Fig. 10, it

is true that a large aperture improves the powar output in the high speed range. In the slow speed range,

on the oth"r hand, the output ramer drops. The aperture of a carburetor is determined by variou!

factors. These factors irclude 111 whether the vehicle is intended for racing. 121 tile design of the ell9ine.

131 driving technique of the driver, (4) the driver's preference. etc. In addition, the maKimum output, the

ITlIIximum torque and mil minimum numl:J.er of revolutions for stable engine operation must also be tak~n

imo ar:count. Fig. 11 shows the values which we have obtained throU(}h OLl' e)(perienCf' ov~r the year!:..

Since the engine com~ in 3 wide variety of tvpe~, the values giv9n in Fi9.1 1 should be taken only as refer_

ence ,·;:IIucs.

-.-

I~ , , :/

//1 1;/

Rnolutoon. pow """"Ie (Rpmi

Fig. 10

4·2. S",," of Miku"i carlPurelon

1.49 " >.34 34

• , 1.16 " , j , 1.!l2 " • • ... l " § .,.

" ." "

!!roch) 1m",>

, , , , , 10 2O '0 .0 50 i>S Pee Cv1in "'"

Nt>!8 ' Th",,~ e rR tnII'~ly ,~I~r.""'" ~.It ... s on Int b,$ls 01 one e~liodar and one COl rQurlttO<.

Fig. 11

Mikuni VM type carburetors eome in various si:ttl$, with the main bore ranging from 39"(10) to

1.73" (44) (in wen numbers lor the mo$l port), Tile body 01 carburetor' i. made of aIYminum?' zinc.

In S()I11e carburetors, the main iel i, in$ulIed In .. cr. (I manner as to plt.m;! qu ick and IiI"Y r.pl~n'lQnt

frem Ina outside by mt:rt:ly letllOV int; the jet holtSM. For iurtt.er informetion, tee OIJ' <;<It3109.

5. CerbYre10r H!tti!lg

Once Ihe aperture of the carburetor is determin~, a test (norm~lIy .eferred 10'1S sett ing or matC h­

ing) to select the proper jet or setting part should be made. The size of the iet is determined by measur·

ing the output in ~ bench or in a (hassi5dvn~mo test. For racing. it is ben to ':ete rmine the prooer size

of 11'1, jtt on the racing coune. because th, followins poinTs muSi be uken irlto .ccoont.

Ill. The altitude (alml»pller ic pressure) , tempe'rature ~nd humid ity 01 ttl , 'King OOU'5e

b . The operation of tile engine based OIl the topography of the rac ing cw.se

-,-

5-1 . Selection of the mlln jet

First. dn ttv! fol lowing on a flat 'Glld.

i 1) Select the largest main je t (the lim;t of i rictl mixtl,lr.) which effords the rnaKimum I'8VOlut ions

per mirull! (the m;lx;rnum speed)_ In this eaJII, $11llet the engine 'P8IId eeeording to the dimensions of the

If!St course.

12) Compan the gain in speed that you can obtain by ~uk:k 1ICC!1..,.~tjon from 0 coru tant speed

of 25-30 M·P· H [41l-S0 km!h), by udng cllfferent sizes of main jete.

i3) Check th l Ixheust fu m" a nd road t he spark plug (Selection of me S!J,Jfk p lug should be

made based on lh1 thermal value that would bu t 5Uit power outpul of the engine).

Nut, camp . ... , on th. r1tCing OOUIW. tho te1;t rCN!ti you obl.illed from m e above. The points to lre

checkwd, among o ther" ere:

(1) Smooth end st~lJdy operation o f tho engine n (I~ hiSh (I 'poed al possible under v.rying

o perating condit ions such ;)s shifting of the geor, ch.,.~ in rood col'ldi lions,ascending a nd de:il.""eIlIJing

slopes, etc.

(2) Scsuined operat ion al low $peedsand e t heovy engine loed

(3 ) Su.Uined op!ration at high $pCoc:h (without knocl<ing or selture)

As shown In Photo 1, there are three Iypes of main jet, Model A, B mllcl C. Th_ mode l. come in

the following si~".

ModelA&8

No. 50, 55, eo ..... up to No. 95 (in multiples of 5)

No. 100, 110, 120 ••.•. up to No. 500 (1 n multiples of 10)

Modele

No. 50, 55, 00 .. . .. I,Ip to No. 195 (I n multIp les of!51

No. 200, 210, 220 ..... up t o No. 300 Un multiples of 10)

-.-

Fig. 12 shows comparlwn of fuel flow curves.

The mai~t li ne is for Model C main jet and the

donedllne fOf Model A and B main jets. In eitCh

model 01 main jet, different siZes wi th in the

range of i 10% were t ried.

(4/0421 (MtOlt4) IN100.604)

A B c

5-2. SeillCtion of the nNdle jet and mejer needle

1\ CarbUN!10r Wilh a pi 5ton-tl'~e throttle v~lve is ~bo called a

variable ~enturi-type carburetor. In thi, type of CElrburelor, th~ needle

jet snd the jet necdlc serve 10 control e proper air-fuel mixture ratio

50 75 1001%) Throttl .. vol"" OlMninO

Fig. 13

-,-

Fig. 12

H Zli 50 ThroWe ~"v. q!enlng 1'i'I)

Fig. 14 '"'

during the so-called medium throttle valve opening (between 1/4 and 3/4 opening). Whether or not the

right size of these parts is uSBd gives a big beilting on the engine performance at partial load. The jet

neEdle tapers off at one end and the c learance between the jet needle and the needle jet increases as the

throttle valve opening gets wider. The air·fuel mixture ratio is controllEd by the height of "E" r ing that is

inserted into one of the five slots provided in the head of the jet needle. Thevllriation of fuel flow based

on the height of "E" ring is shown in Fig. 13.

5-3. Pilot Jet 31'ld the slow fuel system (Fig, 15 & 16)

In the slow sYStlml of The carbLiretor, the pilot out let lind the hypass have holes whose size matches

the aperture provided to the body. Hence, the sele(:tion of the opening of the pilot jet and the airscrew

is important. Turn the grip a little at no-load operation and see if the engine revolution increasf'$

$mooth ly_ lf the pilat jet is too srm.ll, inereise in the engine speed is ~Iow and i rreoul~r. Too big a pilot jet,

on the other hand, would give rise to heavy exhaust smoke a5 well as a dull exhaust noise. If you cannot

maintain the speed in the range of 12-25 M·P· H (30---40 km/h) with the grip held constant, the pilot

jet is too small.

Selection of the opening of the air screw should be made in the following manner. First, warm up

the engine adequately and set the throttl e stop screw so that the engine re~olLJtion at idling will be about

10· 20% higher than th( number of revolutions you ilre aiming at. Then, turn the ilir screw left and right

(between 1/4 and 1/2 turn ) and select the position where the engine revolution reaches the maximum.

Adjust the throttle stOp screw to bring down the engine revolution to your target speed for idl ing. After

th is adjustment of the th rottle stop 5Crew is made, select om;e mort: tht: plJ5ith.m wht:rt: tilt: eflyillt:

I ! , , , , , 0 - II !

1

I ! I

,

--

// rJ f/ ~

r% 1If$! """I"{I "n~ .lir .. , ..... op.~;",

Fig. 15

- -

I . •.

.

-10-

.

, ,

\(, ._ ..

, I" ' r--I \ -

-. -

_ ... ...L. I

"

. -"

Air II''' ..... <>P<In;'g Fig. 16

. ._-_.- ,

.

_. _.-

-.-

-F

"

revolution reaches the maximum, by turning the ~ir screw left and r ight (between 1/4 and 1/2

alternatelyl. At this point. attention should be paid to the following points.

(1 f If ti1ere il " certain rllngo in the opening of the air screw where the fut ellyine revolution can

be obuined (for instlll'1ce, the number of revolution! dOe$ not chaonge in the range of 1·1/2 to 2.0 turn ),

it would be better for ~e1eration to select 1·1/2 tu rn.

(2 f To determine the "fully cl~ed" posi tioo of the air :;crew, turn the air IiCrew slightly. E)(ce"ssive

tightening of the air screw would damage the seal. The pOiiilion when:. the air ,;:rew comes 10 ~ liIop

should be oonsidered the "fully closed" posit ion. The m(l)(irnum number or turn~ irl the opening of the

ai r :;crew muSI loe lirni led to 3.0. If thed;r 1I\;r~ i5 UJ.)ened over 3.0 turn, the 5prinS will not work and the

~ir screw CCln come olf durlrlg operation of the vehicle. Fig. 16 shows the fuel flow curve In relation to

the openirlg of the air screw.

5-4. The cutaway size of the throttle valve (Fig. 17)

The size of the cutaway of the throttle valve affects the aiduel mixture ratio when the degree of

the throttle valve oruming is between 1/8 and 112. especially in the ranoe of 1/8 and 1/4 opening. As the

Clttaway gets larger in lizt!, with the throttle

valve opening kept unchanged, air inflow

resi$tance i~ redueoo ~nd cauws the amount

of air inUlke to inercoso, rcsullin9 in a leon

miJ<turc, On the otc.cr hond, the smaller thfl

,iz:~ of the cutaway, the richer the air·fuel

mixture will become. Intercllange of the

cutiM'ay is made, wtlen the low speed fuel

svstem is out of balarlce with the m~in fu~1

svstem.

Fig. 17 show$ the fu~1 flow curve in re lation

to the size of th~ wtawi!Y,

- 11 -

CA3.0

- B-lO%

- 9-1 A'*'

5 1!\ 25 50 Throtlle valve

F ig. 17

75 ' 00

6. Maintenance

A carburetor consists of variou1 precision.machined parts such as jets. Therefore, care should be

cxercillCd, when removing jets or di$olsscmbling the carburetor for cleanin9.

(1) Proper tools sho~ l d be used for disassembling and reassembling of iets. Handle each part

carefullv to avoid scratchl'S. bending. etc.

(2) Wash the jets and the carburetor proper in solvent and blow them out with compressed air .

(3) For carbureton whose main jet oon bc replaced from the cutside, an "0" rinl) is used to

prEWent leakage 01 fu~1. When you fi l the "0" ring. apply ~ little lubricant or fuel to the "0" ring.

(4) It is important to maintain the fu€1 level in the carburetor. Do not touch the float arm, when

disassembling the carburetor. If the float arm is bent accidentally, adjust the height of rib to the

1pedfied mEa~U rMlenl (refer 10 Fig. tSI

Rib)O.3(o.,,)

Twin float type Independent float type

Mooel VM26 VM28 VM30 VM32 VM34 VIJI36 VM38 VM40 VM44 -74 - 49 -¥ -3' -20 -, - 3 - 1 - 1

H {Irn:h) .59-- .66 .59- .66 .86--.94 .86- .94 .86-.94 .66-.74 .66-.74 .66-.74 .66- .74 {mm) 15---17 15-17 22-24 22-24 22-24 17-19 17"' 19 17- 19 , 17-19

Fig. II!

-12-

7. Tuning up of o;.1rburttor for ....,ing

The maximum output of theengina depends on:

(t) The amount of air dr_n into the cylindel'S

(2) Whether an a ir-fuel mixture is delivered to the cylinders in a proper ratio

Sino;e thl: amount 01 air th~t Ii drlwn in to tile carburetor v. rlllS with the temperature, me

atmospheric pressure, humidity, etC., the mlx1Ur~ ratio is ijlso chiln!l'iKl. It i~ irnponilfll, lheref0f"8, tha'

the luel fl ow be ~jU$ted in BCCotdance with the a ltitude 01 the n c lng coone and meteorological

cooditil)rlS orevailing at a givan tima..

7·1. Tha arnQUnt of the ilWlominl.i. in rll.ion 10 m8tClOr010llic:a1 condition,

The :;moon! of air drawn into tht c\l lindeo is influenced by such factors as the altitude, the

1&mperature, the humidity. etc. Suppose that the amount of air sucked into the cylinders at an elevation

of lEro is taken s. 1()O (The temperature Ind humidity in this Cti$t .re con~idered constant!. T he

amount of ai r in qu~tion decreaun in proportiOfl to, rise in ~ll!\IatiDn as $hown in Fig. 19. Reduction in

the amo ... n, of .. ir dr~wn into ,hi cylinder. c:tllnge!: the air·fuel mixture r' t iD. with the resul t ttl .. ! ttle

power o ... tput drOjn mJrkedly. Fig. 20 ~()W$ the rolatlOO$ ~ I ri~ In twnplr.lt ... re and the amount

of ~i l drawn into the cylinden lin \:tIis e.se, the IItmo.,hflic prenUI"I! (elevcl1ionl llnd the humidity are

(XJ11~idt!nld und,anyed and the arnoom or . if going into the cy~nders '[ 32" F (O"CI ;, IlIken lIS l00).

In the CHe 01 the engine for racing where the ma~imum wlp ... t Is coll$tfnt ly ~1 1t!d fQl", i l is Wsl 10

tune up the engine by makl~ ! match ing test 01 the carburetor In accordance with the te/Tllefll tu re

and other condi tions on the rac ing course.

4 t OO

I 90

~ 8

o 500 1000 t 500 2000

Altitude (m)

Fig. 19 -

i 100

90

# '0 ! -• <

, "

'" '" '" 40 14 C) .. 68 .. 104 I·F)

T"""P .... !Uf. -Fig. 20

En,ine Trouble·shooting

When th, catb\lr2tor Sl'tting ;s not (XIrtP.t.t for the e~joo. various ilTEguluilies l r@ noticecl. These

can be tr3Ced to two eauW$ as: a whole.

111 WhP.f'l1in ai r· fuel miNture is too rid!:

(a) T he e nginp. nniSl! is dull and intermittent .

(h) The condition grows worse, wt1en the stertel" is npened.

Ie) The condition grows warSI', when the en~ine !Jets hot .

Id ) RlImOYili of the .ir el(qn~r win improve the cond ition ~mewluu .

Ie) Exhaust 91_ oro hc;wy.

(f) Spark p lYg II fouled.

(2) Whe n an air-fuel mixture is too lean:

Ill) The engine 9011 overhllElted.

(h) Thecondit ion impnwe:I, """en the St.lrlCf is opened.

Ie) A:;:celer~lion Is poor.

(d! 5p<Irk. p luyloums.

Ie) The revolutiun I) r th~ t!llyirlll n uctuates and lK:k of power is noticed.

- 14 -

CARBURETOR DIMENSIONS

~d VlI<IJ-~1 VlDO-~% 'M.;I ~-~' V/G'-l0 --, 1'>O8-l 11)140-.

• " 00 .. " ,

" " " , " " " n " " '" , "

, , , , , , , , , , , , , , ,

" 4~.5 4 •• ! .4 .5 " " " DI.m ... don

, .. .. 51 .8 .. " " " (- ) , , , , , , , , , • • • • , • • • .. 57.5 51. ; 17.5 " " " • " 83.4 83. " 13.4 80.4 80.4 /~. 4

, 66.5 " " " " "' " , " " .,

" " n .. ?odt l "" 01 l"b.roUh L.U ,u. ~" Lott L.,I~ I;t1g~~ ~h

T~"o<tl" ·acl jutU IdJUlt.~

'0' Ut" sorev .ur ,.uv lUR~~ 1I1gll" 111511< 1l'5h< US'" 1I.J5hL \l.I~."

"ela~t (,r) '" ." no n. ." ". "w

mUUal ~ '" " ~ " " "

RUBBER FLANGE DIMENSIONS (Millill\et.eT!I)

r-VM38-Z00-1

t-VI14\l-ZOo-l

• • , • • IS 37

YH44-3

, " " , , " " , , " n .•

" "

Le( "

uih <

". "

F undional Range Effectiveness of Tuning Parts In relation to the Thronle Valve Opening (Approximation)

full 0..-,,-

Pilot Air Screw & Jet Throttle Vall,e __ -.l

Jet Needle ------' Needle Jet _____ ...J

Main Jet --.:::======~J Air Jet -

-