Dellorto Motorcycle Carburetor Tuning Guide - home

2.1 Carburetor diagram and principal parts

    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

fig. 1

2.2 Operating ranges. Scheme of phases while running

fig. 2

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.

In the idle stage, the idle circuit and idle adjustment is set with the mixture screw and idle-speed screw.

In the "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.

In the "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.

In the "D" period of full throttle and, with all the circuits of the earlier periods operating correctly, the size of the main jet is now finally selected.

2.3 Installation angles

The tapered-needle-type Carburetor s 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.

fig. 3

2.4 Engine connections

The Carburetor is usually connected to the engine with one of the following:

fig. 4
Male clamp fixing

the male clamp connection used for the flexible fixing of the Carburetor 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.

fig. 5
Female clamp & Flange fixing

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.

fig. 6

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 Carburetor :

  • 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 Carburetor s for two-stroke engines and longer ones on Carburetor s for four-stroke engines.

    For particular requirements, such as on some racing engines, Carburetor s 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 Carburetor .
    Any change in the filter-silencer may produce a change in the carburation and consequently fresh adjustment and tuning of the Carburetor 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 Carburetor bodies are diecast in aluminium or zamak alloys.

    For special weight-conscious requirements, there are some small-volume Carburetor s in elektron magnesium alloy.

    All the setting parts such as the jets, atomisers, needle-valve seats, etc are made of brass.

    B A C K
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