e) Cover cap
(4)
at the face cracked
f) Screwed connection loose [2 nuts
(5)
to be locked
!l
Dirty paper air filter to be cleaned by tapping or re-
placed (every 30,000 km on an average).
6.1. Carburetter
BVF
22
N
1-3
and
24
N
1-1
6.1.1.
General
The correct tuning and proper functioning of the
carburetter is not only decisive for a good engine out-
put, a reasonable fuel consumption, and excellent
startability, but also for reliability in operation and
minimum wear. If, however, with an adjustment that
provides too lean a mixture or secondary air, the op-
erating temperature rises considerably, the mainte-
nance of a
minimum wear rate is doubtful.
The phrase "to step on the gas", which is used in some
countries, is incorrect. The carburetter is not used to
"gasify" anything because this would require heat from
\\hi& the carburetter must be protected (thermal in-
sulation flange). Heated air has a larger volume than
cold air and would impair the charge of the cylinder
and thus reduce the power output. (That is \vhy the
engine lacks power in the heat of mid-summer. whereas
the full output is given in the cool morning.)
For the internal combustion of 1 litre of petrol about
9.300 litres of air are required. This enormous quan-
tity of air (with only about 20 per cent of osygen)
must be
mixed \\.ith fuel by the carburetter so thor-
oughly that a fine fuel "mist" is produced. Everything
depends on the correct "density" of this vapour for the
engine to produce the rated output.
In this connection the terms "fuel-air mixture too
lean", and "fuel-air mixture too rich" are used. Hcw
can this be distinguished? Which effects are produced?
For the ratio of air to fuel in the mixture the unit
1
(Lambda) is used. The mean value adopted for
1,
=
1
(S
13.8 parts by weight of air and 1 part by weight of
fuel). Values over 1 have a surplus of air, values below
1 lack air. [l m3 of air has a weight of 1.2 kg at 10
"C
and a pressure of 0.1 MPa (1 kp/cm2)].
The tuning of the carburetter must be correct at an
ambient temperature of plus or minus 20
"C,
there-
fore, the standard tuning provides for a slightly richer
mixture
(?.
=
0.95 to 0.90).
This small lack of air ensures:
a)
a
good full load performance (full load operation
requires a slightly rich mixture because the suc-
tion at the needle jet is silghtly reduced due to the
completely open carburetter passage).
b) good cold starting and proper transition (the slight-
ly rich mixture provides for a compensation for
the fuel drops condensing in the cold intake pipe
and crankcase).
The permissible carburetter tuning range (partial load
needle) is given with
A
=
0.95
to
1.0.
Since neither a
repair shop nor an amateur constructor normally has
an engine test bench with the required measuring
equipment, a test run over a distance of least 10 km
is the only alternative for adjusting the carburetter.
The engine must have reached normal operating tem-
perature, otherwise misinterpretations of the engine
behaviour will be inevitable.
Engine operating behaviour and the appearance of the
sparking-plug are the only criterions for evaluating
the correctness of the carburetter tuning.
A
short trial
run is useless because a change in the carburette:
tun-
ing.
A
short trial run is useless because a change in
the carburetter adjustment will not be mirrored in the
appearance of the sparking-plug after a very short
time:
Mixing Ratio of Rich Air-fuel Mixture
Below
?,
=
0.9
Since the mixture is too rich a lack of oxygen is localiy
given, consequently "retarded combustion"
=
poor
engine output! Due to the incomplete combustion, not
only the relatively harmless carbon dioxide (COP) but
also the colourless but poisonous
(!)
carbon monoxide
(CO) are produced. The latter is combustible, that is
to say, fuel eqergy is wasted!
Especially in short-trip driving hydrocarbon particles
remain in the engine which cause corrosion at the
big-end bearing. cylinder liner and piston. This is the
cause of premature wear.
A
certain quantity of oil found in the crankcase after
its dismantling is no sign of normal operating con-
ditions. This enlulsion is engine oil which is "saponi-
fied" \vith hydrcgen and has no lubricating pi-operties
but should
L-e
considered an acid.
S
>.
m p t o m
S
:
Engine starts from cold even with-
out the choke pulled. The output of the engine is satis-
factory as long as the engine is cold, but decreases
with increasing engine temperature. Inclination tor
"lour-stroke cycling". Black exhaust gases, high con-
sumption. sparking-plug of correct thermal value is
oiled up.
C
a u s e
s
:
Dry filter dirty, has become wet or is too
old (over 15,000 km). Float needle seat pocketed. Nec-
dle jet loose or worn (with partial load needle). Central
float bent so that float needle valve does not close
prcperly. Main jet too large (rebored?).
Mixing Ration of Lean Air-fuel Mixture
Over
1.
=
1.0
The air percentage in the mixture is excessive, the
speed of combustion is extremely high, the exhaust
gases are not visible and contain only little poisonous
gases, there are no deposits in the engine which in-
crease the rate of wear.
S y m p t o m s
:
Engine starts well with correct set-
ting of the slow running air screw, however, it must
be
run with a more or less pulled choke for a longer
period. The engine output is satisfactory up to about
half of the throttle valve opening, with increased throt-
tle opening drop in output. If the engine is run within
the range from half to full load, the operating temper-
ature will raise abnormally.
The engine causes the carburetter to "splash".
Pinlr-
ing and even inclination to seize. Due to overheating
the sparking-plug shows blue-grey deposits, beads,
and heavy consumption of the electrodes.
.
C a
U
S
e
S
:
Air leak in induction system or directly
at the engine, central float bent
-
fails to open suffi-
ciently, float needle sticking, fuel feed obstructed (tank
cap, fuel shut-off cock?). Silencer inserts removed.
therefore, back pressure too low (losses of fresh gas).
6.1.2.
Description of the
BVF
Carburetters
22
N
1-3
and
24
N
1-1
These tow types of carburetters are starting carbu-
retters which are basically of the same design. They
are only distinguished by different throttle opening
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