Villiers MARK 12D, Operating Instructions And Spare Parts List

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When the throttle slide
is
o
pe
n
ed
a
small amount
beyond
that
required
fo:r
idlin�,
the suction_ on the pilot. o
u
�
�t hole
is. r
e?;
u
c
ed
,
but at the· same time, the suction on the p1lot progressiOn · hole
(G) increases. A further supply of petrol is, therefore, drawn
through the "progression" hole,
and
prevents the weak spot which
would otherwise occur due to the fall off in supply from the pilot
hole before the main jet comes into full operation.
It follows from the preceding remarks that whenever the throttle
is shut off whilst the engine speed is high (such as on long downhill
sections), the pilot system is subject to the full engine suction, and
petrol will flow into the engine from the pilot outlet hole. As the
engine is not firing under these conditions, this fuel supply will
tend to build up in the crankcase and cylinder and cause seve're
" four-stroking " or " eight-stroking " when the throttle is opened
again. To overcome this fault in the present range of Villiers
Carburetters, an automatic air bleed to the pilot has been incorpo­
rated, which relies upon the matching of two slots, one in the
throttle slide and the other in the carburetter body. When the
throttle slide is shut, these two slots line up and air can flow from
the front of the Carburetter through the throttle slide and down
passages H and J into the pilot system. The high depression on
the pilot system is then destroyed. In all other throttle positions,
the two slots do not line up, and no air can pass to the pilot system
through these passages.
1
(b)
Main
Jet System.
As the throttle slide is opened further beyond the idling and
progression positions, the engine suction has its effect upon the
main-jet system, and petrol is drawn from the float chamber
through the calibrated main jet (K) and the needle-jet (L) and
into the small pre-mixing chamber
(M).
There the petrol !s atom­
ised by the filtered secondary air which is drawn from the mouth
of the. �arburetter along passage (N), and which enters the centre­
piece (0) through four small holes (P). The rich petrol-air mixture
then flows from the pre-mixing chamber into the main mixing
chamber, where it meets the main air stream. The effective size
of the needle-jet (L) depends upon the throttle slide position (as
the taper needle is fixed to the slide), and the sizes of the needle­
jet and the needle are chosen to give correct carburation over the
range.
·
8
Fig. 3A.