BSA D14/4 BANTAM SUPREME D14/4 BANTAM SPORTS, Workshop Manual

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D14/4 CARBURETTER C5
Eliminate by fitting new joint washers and tightening
the flange nuts evenly to a torque wrench setting of
10—12 lb./ft.
Also check that the rubber sealing ring in the
carburetter flange is undamaged and located correctly.
On much used or old machines look for air leaks
caused by a worn throttle.
Banging in Exhaust
This may be caused by too weak a pilot mixture when
the throttle is closed or nearly closed. It may also be
caused by too rich a pilot mixture and an air leak in
the exhaust system. The reason in either case is that
the mixture has not fired in the cylinder but has fired
in the hot silencer.
If the banging occurs when the throttle is fairly
wide open, the trouble will be traced to ignition, not
carburation.
Excessive Fuel Consumption
If this cannot be corrected by normal adjustments, it
may due to flooding caused by impurities from the
fuel tank lodging on the float needle seat, so
preventing its value from closing. The float needle
should also be checked for wear or damage.
High consumption can also be caused by a worn
needle jet and may be remedied or improved by
lowering the needle in the throttle. If this method is
unsatisfactory, then a new needle and needle jet will
have to be fitted.
There are many other causes of high fuel
consumption and it should not be assumed that the
fault lies in the carburetter alone.
Air Filters
If a carburetter is first set with an air filter and then the
engine is run without, the jet setting may be affected
and care must be taken to avoid overheating the engine
due to a weak mixture. Testing with the air control
will indicate if a larger main jet and higher needle
position are required.
Air Control
The air control should at all times be kept open except
when starting from cold. When the engine fires, the
control must be opened.
Repeated operation of the kickstart pedal with the
air valve closed results in an accumulation of liquid
petrol in the crankcase and until this has been drained
away, it will be quite impossible to start. The
crankcase drain plug is the smaller of the two plugs
under the crankcase. If poor starting re-occurs, then
the fault will most likely be found in the ignition
system.
Effect of Altitude on a Carburetter
Increased altitude tends to produce a rich mixture; the
greater the altitude, the smaller the main jet required.
Carburetters ex-works are suitably set for use in
altitudes up of to approximately 3,000 feet.
Carburetters used constantly in altitudes of between
3,000 to 6,000 feet should have a reduction in main jet
size of 5 per cent. A further reduction of 4 per cent
should be made for every 3,000 feet in excess of 6,000
feet altitude.
No adjustment can be made to compensate for lost
power due to rarified air.
TRACING FAULTS
Faults likely to occur in carburation can be placed in
one of two categories; either richness of weakness of
petroil/air mixture.
Indications of Richness
Black smoke in exhaust.
Fuel spraying out of carburetter.
Two-strokes, four-stroking.
Heavy lumpy running.
Sparking plug sooty.
Indications of Weakness
Spitting back in carburetter.
Erratic slow-running.
Over heating.
Engine goes better if throttle is almost closed.