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

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D14/4 CARBURETTER C7
PARAGRAPH “C” — Main Jet
The main jet controls the fuel supply when the
throttle is more than three-quarters open, but at
smaller throttle openings although the supply of fuel
goes through the main jet, the amount is diminished
by the metering effect of the needle in the needle jet.
Each jet is calibrated and numbered so that its
exact discharge is known and two jets of the same
number are alike. Never ream out a jet, get another
the right size. The bigger the number the bigger the
jet.
To gain access to the main jet, the concentric float
chamber must first be removed (two screws).
PARAGRAPH “D” — Needle and Needle Jet
The needle is attached to the throttle valve and being
taper — either allows more or less fuel to pass
through the needle jet as the throttle is opened or
closed throughout the range, except when idling or
nearly full throttle. The taper needle position in
relation to the throttle opening can be set according to
the mixture required by fixing it to the throttle valve
with the jet needle clip in a certain groove, thus either
raising or lowering it. Raising the needle richens the
mixture and lowering it weakens the mixture at
throttle openings from one-quarter to three-quarters
open.
PARAGRAPH “E” — Throttle Valve Cut-away
The atmospheric side of the throttle is cut-away to
influence the depression on the main fuel supply and
thus gives a means of tuning between the pilot and
needle jet range of throttle opening. The amount of
cut-away is recorded by a number marked on the
throttle valve,
viz., 389/3½ means throttle valve type
389 with number 3½ cut-away; larger cut-aways, say
4 and 5, give weaker mixtures and 2 a richer mixture.
PARAGRAPH “F” — Air Valve
This is only used for starting the engine, and for
experimenting with air supply. It must be fully open
when the engine is running.
PARAGRAPH “G” — Tickler or Primer
This is a small spring-loaded plunger, in the float
chamber wall. When pressed down on the float, the
needle valve is allowed to open and so "flooding" is
achieved. Flooding temporarily enrichens the mixture
until the level of the fuel subsides to normal.
TUNING THE CARBURETTER
Tune-up in the following order
Read remarks in sections above for each tuning device
and get the motor going perfectly on a quiet road with
a slight up-gradient so that on test the engine is pulling
under load.
FIRST — Main jet with throttle in position 1, Fig.
C3. If at full throttle the engine runs "heavily", the
main jet is too large. If at full throttle, the engine
seems to have better power when the throttle is eased
off or the air valve is slightly closed, then the main jet
is too small.
With the correct sized main jet, the engine at full
throttle should run evenly and regularly with
maximum power.
If testing for speed work, ensure that the main jet
size is sufficient for the mixture to be rich enough to
maintain a cool engine. To verify this, examine the
sparking plug after taking a fast run, declutching and
stopping the engine quickly. If the sparking plug has a
cool appearance the mixture is correct; if sooty, the
mixture is rich; if however, there are signs of intense
heat, the plug being very white in appearance, the
mixture is too weak and a larger main jet is necessary.
SECOND — Pilot jet (Fig. C3) with throttle in
positions 2 and 5. With engine idling too fast with the
twist grip shut off and the throttle shut down on to the
throttle adjusting screw, and ignition set for best slow-
running; (1) screw out throttle adjusting screw until
the engine runs slower and begins to falter, then screw
pilot air adjusting screw in or out, to make engine run
regularly and faster.