Pistons - BSA B31 Instruction Manual

performance
to
suit
a
wide variety
of
sporting
events.
These arc
indicated
in
Table
I.
Cams.
various
cam combinations
are available
for
the
cold
Star engine,
and the
sele.tion
of
these
depend$
upon the desir€d
performance, They are shown id
Table
II,
and
as
a
general rule
the
pair of
cams
fitted
to
an etrgine
at
the
factory during maNfacture and
special
tuning,
is
selected
as being
the most
suitable
for
the
specification
ordered
by the prospective owner.
Thus,
if
a low
com-
pression
pistor
is fitted, and the machine is
to
be used
for
touring,
the
cams
in
the
first line
will
be
employed. The
reminader
of
the table
is
devoted
to
competitive events
where
the
highest possible performance is
required,
and
for
these
the
owtrer
is
advised
to follow
the recomrnen-
dations exactly as
given.
The
main
jet
size
for
alcohol fuels
depends largety
upon
the
actual
fuel
used, as there arc one
or two
pro-
pdetary
blands available
at
the
present time in
addition
to pule
methanol,
and the composition
of
these
varies
somewhat.
It
is
for this
and
other
rcasons that the figutes
given for alcohol
and dope are subject to
alteation,
If
the machine has been ordered
in
the first place with
the
T.T. carbuetter
specially
tuned for
alcohol fuel
it will
already
have a
suitable main
jet,
but
if
the owner
is
making
the
oonversion himself,
he
should take
as a
rough guide
the
fact that
the main
jet for
alcohol fuel should be at
least
two
and a
half
times as
big
as
that
fot petrol.
For
certain
fuels, however,
an even
larger
size
may
be
found
necessary,
and
the owner
is
advised
to
experiment
with this point,
bearing
in mind that
a
careful
test
of
the
mixturc
strength
with
rcspect
to perfomance should
be
made
as
quickly
as
possible,
as
full throttle
work with
a weak
mixture
is
liable
to
cause
rapid seizurc.
It
is advisable when using alcohol
fuel to run with
the
mixture slightly on
the rich
side. Any
published recommendation for a given
fuel should, there.
fore,
be
looked
upon ss
a minimum.
In
the
case
of
the
832 for J.A.P. mcing fuel, No.
800
or No.
850 should be
suitable, while
for pure
methanol
ajet
size
up to No.
1300
may
be
found necessary.
For
the B34
Gold
Star the
sizes
will, of course,
be
propodionately
larger and
will
probably
be
up to 1700.
The throttle valve and
needle
position
may
vary slightly to suit individual
cases.
Petrol
Tånk.
A
two or four
gallon
tank
can b€
fitted
if
speÆially
ordered,
Ignition
Timing.
The
best
timing
for
the B32 ol!
dope is
$
inch
advance,
and fr
inch
advance
on petrol or petrol-
benzole,
39'(lå
inch) Clubman's.
On
the B34 use
*
inch
advarce
for
dope and
+
inch
advance
for
petrol or pehol-
benzole
({d inch
for
sc@mbles with engine
prefix
No. BB),
36'
(l*
inch
Scrambles and Clubman's).
Vålve
Timing. Tiniing procedure is
as
described
for
other
'B'models. (See
page 46.) Theactualvalvetimingfor Gold
Stars
differs
frcm
Standard
'B'
models,
but
the
pinioN
are malked
in
the
same
way
as
the standard pinions to
facililate assembly.
In
addition,
the idet ard
exhaust
pinions are marked 'IN'
and 'EX'
respectively.
TABLE
I
PISTONS
.
These Pistons
are
for
engines
number
with
prefix CB
832 Gold Star B34 Gold
Star
Component
No.
Complete
Com.
Ratio
Component
No.
Complet€
Com.
Ratio
65.1681
6.5 65-1245 6.8
*65-1921
7.25
*65-1930 't.25
65-t265
65-1616
8.0
65-1285
8.0
*65-2254
r65-2260
8.5
65-1614
*65-2287
9.0
65-1532
. *65-2300
9.0
65-1687
13.1
65-1279
1t.0
52
and
after englne
53
l