CARBURETOR
Influence
of
air
den
si
ty
on
th
e air-fuel ratio.
Besides lhese lhr
ee
factors, there
is
one mo
re
that makes it ve
ry
difficult to maintain a consistent oir·fuel ratio: changing
air
density. The air density,
or
actual
weight
of
the air, is
one
of
the factors mentioned earlier that oHec
ts
the compression pressure
of
on
engine. The density is dependenl
on
lwo
rhings:
air
lemperolure;
and
air
pressu
re
.
As
the
air
tem
pe
rature increases, lhe
air
mol
ec
ules become spaced farther
aport,
causing a smaller
amo
unt
of
ai
r Ia
oc
c
upy
the some amount of spa
ce
t
ha
t o larger vol
ume
did
at a lower tempera
tur
e.
Th
is is
why
obo
ve
·normol air temperatur
es
may require leaning o f the fuel mixture.
Conv
ersely,
as
the temperature decreases, the
air
molecules press closer together. causing
on
increase
in the density
of
the air. The mcreose
in
density
of
the
air
must
be
compensoled lor
by
on
increase in
fuel
to
main
ta
in the some oir·fuel rat
io
.
HIGHER AIR TEMP.
LOWER
AIR
TEMP
.
- >
LESS
FUEL
REQUIR
ED
->
MORE
FUEL
REQUIRED
The
o
th
er
lec
tor lhot a
ff
ec
ts a ir de
nsi
ty is a
ir
pressure Air
pre
ss
ure
is
dete
rm
in
ed
by
both oltilude
and
wea
th
er
conditions.
Air
becom
es
l
ess
den
se
as the altilude increases because air is compr
es
sed
by
the
weight
of
the remaining
air
above
it.
For
example. at sea level.
air
is compressed
by
the weight
of
the entire atmosphere
abov
e
rt.
AI an oltilude
of
10.000 leet, however, the air
is
compressed only
by
thai port
of
th
e atmo
sp
here
above
1
0,000
feet. Because of changes
in
air density, it is necessary to modify
th
e
am
o
unt
of
fuel supplied to the eng ine.
HIGHER
ALTITUDE
LOWER AlTITUDE
->
lESS FUEl REQUIRED
->
M O
RE
F
UEL
RE
QU
IR
ED
It
is imporlont
lo
realize, though,
1ho1
rejetting
of
corburelors
ot
hrgher altitudes only restores the air-fuel rolio
lo
lhe proper selling, il does nol replace
lhe loss
of
powe
r lhol accompanies on increase in oltilude
Weather
co
nditions also
aff
ec
t
th
e air pressure, but not
to
the some
ex
tent
as
a chan
ge
in alti
tu
de.
O
nl
y
th
e most finely tuned
engin
es
requrre rejetting
for changes in
weolher
condilions.
REVIE
W
1 )All engines r
eq
uire
on
air·fuel ratio within o specific
range
to
maintain combustion. Too leon o ratio causes poor combustion (commonly called
"leon misfire")
and
poor
performance. Too rich o rat
io
not only causes poor
co
mbustion, but also
res
uhs
in exc
ess
ive emissions.
2)The air-fuel requiremen
ts
of the eng
in
e ore constantly cha
ng
ing wi
th
varia
ti
ons
in
eng
ine speed, l
oa
ds,
and
temperatures. Rrcher mixt
ures
ore
used
of
low
and
high speeds as
well
as under h
eavy
l
oads
. Leaner mixtures ore used at cruising speeds
and
under light
loads
.
3)The
air
density,
or
wei
ght, ploys o major pori in determining the amount
of
fuel
required for efficient burning
of
the air-f
uel
mixture. The
lwo
items
which determine
th
e air densily
or
e
air
tempera
tur
e
and
air
pressur
e.
HI
GH
ER
TEMP.
LOWER TEMP.
H
IGH
ER
AL
TI
TUDE
lO
WER AlTI
TU
DE
- >
LESS
FUEl REQUIRED
->
MORE
FU
EL REQUIRED
- >
LE
SS
FUEL
RE
QU
I
RE
D
->
MORE
FU
EL
RE
QU
IRED
From
this discussion
of
air
-
fu
el rolios, il should be
ob
vious that precise metering is e
sse
ntial to meet
oi
l
th
e
re
quirements
of
o
mo
torcycle engine.
The
next section introduces lhe circui
ts
necessary to meet these requireme
nts
.
The air-fuel mixture in modern
motorcycles
is
very imporlonl f
or
overall perfo
rman
ce. The mixture ne
eded
for slorting o
cold
engine is much
ri
cher
th
an the mixtu
re
needed for cruis
in
g.
Th
e mixlure n
ee
ded Ia start o cold engine is obaul l 0 ports of air to 1
por
t
of
fuel,
by
w
eig
ht.
A
worm
engin
e,
al
cruising spe
ed
, should
hov
e o mixlure
of
abo
ut
15
: 1
to
1
6:
l.
Under lull-lhrottle operation, the mixture should
be
enriched
lo
about
12
: 1.
Air-fuel mixtures aff
ec
t en
gi
ne
co
ndition
s.
Fue
l mix
tu
r
es
that
or
e too
ri
ch
co
n contribute to fouled p
lu
gs, carbon
dep
osits in
th
e combustion chambe
r,
poor
mileage,
and
high emissions.
Fue
l mix
tur
es
th
at
ore
too
leon con cause misfiring, burned valves, and overhearing. lnc
orrec
l fuel mixlures could
lead to engine
damage.
g
WARNING
: Gasoline
is
extremely flammable.
To
avoid severe injury,
do
not
allow
open flames, sparks, cigarettes,
or
any
other
[.lJ
sourc
es
of ignition
near
the
area in which you
ar
e working.
0.12
·-
......
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