2.FEATURES
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2.2
Operating ranges
2.5
Air Intakes
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fig. 1 |
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1
- air intake
2 - throttle valve 3 - tapered needle 4 - atomiser and needlejet 5 - main jet 6 - starting device 7 - venturi 8 - idle speed adjusting-screw |
9
- idle mixture adjusting-screw 10 - starter jet 11 -idle jet 12 - float chamber vent 13 - fuel inlet banjo union 14 - needle valve 15 -float 16- float chamber |
2.2
Operating ranges. Scheme of phases while running
| fig. 2 |
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Figure 2 shows the section of a venturi according to the operating periods regulated
by the throttle valve opening. In every phase of operation, it is possible to
vary and select the optimum setting.
(A) Idle stage, the idle circuit and idle adjustment is set with the
mixture screw and idle-speed screw.
(B) Progression phase, fuel mixture delivery from the idle hole is steadily
replaced by mixture delivery from the progression hole, drawing emulsion mixture
from the idle circuit, and in this range, choosing the correct idle jet and
throttleslide cutaway is necessary. The throttle valve cutaway slightly affects
the carburation up to about half throttle.
(C) High-speed period, mixture delivery from the idle circuit and from
the progression hole is replaced by mixture from the main circuit and selection
of both the atomiser and the tapered needle should then be made.
(D) Full throttle and, with all the circuits of the earlier periods operating
correctly, the size of the main jet is now finally selected.
The
tapered-needle-type carburettors with concentric, central float chambers have
a horizontal main barrel and can be mounted up to a maximum inclination of 40
degrees from the horizontal (figure 3).
For applications on motocross and trials engines, etc, this inclination should
be 30 degrees or less.
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fig. 3 |
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The carburettor is usually connected to the engine with one of the following:
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A-male clamp fixing (fig. 4) |
B-female clamp fixing (fig. 5) |
C-flange fixing (fig. 6) |
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the male clamp connection used for the flexible fixing of the carburettor
to the engine is usually recommended on motorcycles for motocross, trials,
etc or fitted to engines which run to high rpm or those which produce
strong vibrations. | -
the female clip connection and the flange connection, with a rigid
fitting to the engine, are usable on road motorcycles or fitted to engines
which do not generate very strong vibrations. Note that with the female clamp fixing and the flange connection, as you can see in figure 5 and 6, there is also the need to provide both effective heat insulation and a perfect airtight seal. | |
2.5 Air intakes
Different air intake arrangements are possible for each type
of carburettor:
Open air intakes; Trumpets of various shapes and lengths; Aircleaners and
filter-silencers
As far as the lengths of the trumpets is concerned, remember that short trumpets
are usually used on carburettors for two-stroke engines and longer ones on
carburettors for four-stroke engines.
For particular requirements, such as on some racing engines, carburettors
with air intakes having a special shape are available eg PHBE H and PHM H
models.
On motorcycles with simple aircleaners or air filter-silencers, it is extremely
important to check on the efficiency of the filter and for perfect sealing
of the filter box to prevent damage to the engine and to the carburettor.
Any change in the filter-silencer may produce a change in the carburation
and consequently fresh adjustment and tuning of the carburettor may then become
necessary.
Remember also that replacing the filter or silencer with a trumpet usually
results in an increase in the amount of air drawn into the engine and consequently
there should also be a suitable increase in the size of the main jet fitted.
2.6 Construction
materials
The carburettor bodies are diecast in aluminium or zamak
alloys.
For special weight-conscious requirements, there are some small-volume carburettors
in elektron magnesium alloy.
All the setting parts such as the jets, atomisers, needle-valve seats, etc
are made of brass.