Fuel System Types - Gunson COLORTUNE G4074 Mode D'emploi

The following description of fuel systems is intended purely to identify the various types
which may be encountered and to highlight the basic features. While this may be
sufficient description for some simple adjustments to be performed, a comprehensive
vehicle manual or the General Fuel System Manual should be referred to for further
detailed information.
The information is in two main sections, one covering the various types of carburettor
and the second dealing with fuel injection systems. In each case the least complex
systems are described first for reasons of clarity.
NOTE: Many twin and multiple carburettors require flow balancing before mixture is set
- see the end of this section
1. There are four basic elements to a simple "fixed choke" carburettor which might be
fitted to garden machinery or small power plant engines
2. A throttle to open or restrict the main passage of fuel / air mixture to the engine,
this usually has an adjustable throttle stop to control idle speed.
3. A float and valve arrangement to maintain a steady fuel supply level (or a sensitive
pressure regulating diaphragm)
4. A slow speed fuel discharge and adjustment (idle jet and idle mixture screw) this is
found close to the engine mounting flange and the throttle plate. The throttle
uncovers extra passages (progression holes) for feeding fuel as it begins to open.
5. A high speed fuel feed and adjustment (main jet and main mixture screw) normally
located closer to the inlet air cleaner mounting flange. The main fuel discharge feeds
into a venturi shape, whose restriction (a fixed size choke) gives a low pressure to
pull the fuel in.
6. Note: a cold start device is also sometimes called a choke - it also gives a restriction
to draw in extra fuel.
Carburettor fuel systems as fitted to motorcycles
1. Variable choke carburettor. This usually has the same parts as described in items 1-3
for the basic fixed choke carburettor but the main discharge has fine tapered needle in a
jet to meter the fuel accurately. The variable choke size is formed by a rising piston
arrangement attached to a flexible diaphragm. When the throttle is opened, the engine
draws in more air and the diaphragm pulls the piston and the needle further open,
maintaining a constant low pressure (constant depression) in the carburetter. This gives
precision control of the air and fuel over a range of conditions. Enrichment for
acceleration conditions may be provided with a separate pump device or by controlling
the rate at which the piston rises. Needle / jet calibration size controls mixture except
at low speed / light load which is governed by the adjustable idle mixture control.
2. Slide type carburettor. The slide carburettor has been used on motorcycles and
other small engines for very many years. Fuel feed is from a chamber with float and
needle valve and an idle fuel system is provided with an adjustable mixture screw. There
are no progression holes in the idle system as the throttle plate is replaced by the slide

7. Fuel System types

A basic Carburettor fuel system
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