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2. DIAPHRAGM CARBURETTOR
There are 2 types of diaphragm carburettor :
- Standard carburettor
- Carburettor with primer
a)
ADJUSTMENT OF DIAPHRAGM CARBURETTOR
For carburettors with variable main and idle jets, proceed
as follows (see Fig 3).
A.
High speed adjusting screw (See E & Text)
B.
Low speed adjusting screw
C.
Idle speed regulating screw
D.
Choke lever
E.
Fixed main jet (high speed)
- Tighten the main adjustment needle A and idle adjustment
needle B. (Do not use force or the needle seats may
be damaged).
- Turn main adjustment needle A (hex head) 1.1/4 turns
anticlockwise. Turn idle adjustment needle B (knurled
head) 1 turn anticlockwise. Back off idle speed
regulating screw C so that it is out of contact with the
throttle stop lever (fully closed throttle), then screw in
one full turn to hold throttle slightly open.
- Move control lever to full choke, or actuate primer. Start
engine and allow to warm up. Make sure choke, when
provided, is fully open after warm up.
- Run engine at full operating speed and, by final careful
adjustment, set main adjustment needle A to position
that will give smoothest performance. Final adjustment
should range between one and one and a half turns
open.
- Run engine at idle speed and adjust idle screw B to
position that will give the smoothest idle operation.
- Adjust idle speed regulating screw C to give an idling
speed of 1.800 rpm. Use a tachometer to check speed.
Carburettors fitted with a fixed main jet E are now
standard, and the only adjustments possible are to the
idle jet and slow running screws.
NOTE - The final setting should give a slightly richer
mixture.
b)
DIAPHRAGM CARBURETTOR WITH THE MAIN
JET INSIDE THE FUEL CHAMBER
From Fig 4 it can be observed that the main jet, instead
of being located in its normal position, i.e. at the side of
the idle jet, is fitted inside the fuel chamber. This is a
fixed main jet and thus does not require any adjustment.
In the case of engine running at a fixed speed (for example
Flymo) the jet will not have or require other parts. On the
contrary, for engines running with variable rpm, a nylon
ball valve will be located under the main jet.
Fig. 3
Fig. 4
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