Decarbonising The Engine; The Standard Carburetor - BSA B31 Instruction Manual

Carburetter.
So
long
as
the engine contidues
to perform
satisfactorily, the
carbuetter
fu
best
left
alone,
pa.ticularly
by
the experienced
dder.
The only adjustment
in
which
the owner is
lik€ly to
be
interested
is
the
pilot air
søew
(Fig. l6a), (Fie.
16b
'Monobloc') which controls
the slow
running mixture. This is
set
before the machine
leaves
the
works, but tbe
best
setting may vary
slightly to
suit
rider's
requircments,
or different iocalities.
Tuming the
pilot
air
screw
inwards enriches the mixture
and unsøewing weakens
it.
A
richer
mixture gives
a
slower
and
mole reliable tick-over, but
it
is
naturally
undesfuable
to
have
the mixture
too rich.
even
with
the throttle
practically closed-which
is, of course, the
throttle
position
for
tick-over-so
the best
way
to
set
the
pilot
arl
sqew is
to
screw
it
in
until
the
mixturc
is
obviously too rich,
and then
unscrew
it
until
the setting
is just right,
atrd further
unscrewing increases the
9icEqy[spggd unduly.
This
may
seem
complicated,
but
the efrect
of
alteritrg the setting is
vely
noticeable,
and the dd€r
will
easily detect
the
symptoms.
It
is
doubly important
to
avoid
over-richness
of
the
tick-over setting
if
much ddiDg
is
done
on
small
throttle
openings
for this is
what
is kao\tn
as 'ruDning on
the
pilot'
and
if
the
pilot
setting is
too
ch, the
getreEl petrol
consumption
will
be
advenely
affected.
The
running mixture can only be altered by
adjusting
the
height
of
the needle
in
the
throttle
valve or by
fitting
a
main
jet of diferent
size.
Such
alterations should not
be made except
for
very special purposes, as dictated by
abnormal requirements
and only
then
on
expert
advice.
DECARBONISING
TIIE
ENGINE
Decarbonising
and
'top
overhaul'
of
an
engine
is
exfemely simple,
but
it
should
only
be carried
out
wben
the engine
reålly rceds
it.
The usual symptoms are
an
increased
tendency
to 'pink'
(a metallic knocking
when
under
healy load)
due
to
the building-up of
carborl
on
the
top
of the piston and
inside
of
cylinder head;
a
general falling-off
of power
noticeable mainly on
hills,
atrd
the engine inclined
to run hotter
than usual.
Befote
cor nencing geDeBl djsmantling,
rotate
the engine
until
cilp
t
N0Tcl{
AIllusIMElfI
NEtDtE
p0slTloll.
CANBUIEMN
fip0mE
AIR
su0E.
AN
siuDE.
MAtN
l,,l$BIu"o*''
Fig.l6a. Th. standad carbtlrettet
39