NB: These instructions are given for guidance only and cannot engage the author's liability in the event of misinterpretation, failure to assess the vehicle's actual condition and/or non-conformity, unsuitable mechanical practice or the use of inappropriate tools. This is a translation of the original Dell'Orto manual; for practical application, rely on the original version.
Two- and four-stroke engines run on a fuel (normal petrol, competition petrol, or sometimes methyl/ethyl alcohol) volatile enough to be pre-mixed with the combustion air before ignition. When fuel is pre-mixed this way, the air flow — and, indirectly, the fuel flow — must be controlled. On most motorcycles this is the carburettor's job: it introduces fuel according to the depression generated by various jet circuits.
The carburettor performs three functions: adjust the admitted air flow according to the rider's demand; meter the fuel into that air flow while keeping the optimal air/fuel ratio across the whole range; and homogenise the mixture for good combustion.
The mixture ratio (AFR)
The stoichiometric air/fuel ratio (AFR = air mass / fuel mass) is the one allowing complete combustion, with no excess air (lean mixture) or unburnt fuel (rich mixture). For commercial petrol it is about 14.5 to 14.8; it drops to 6.5 for methyl alcohol and 9 for ethyl alcohol.
The mixture the carburettor delivers is not necessarily stoichiometric: depending on rpm and load, some fuel may not burn. In general the mixture should be richer at idle, on acceleration and at full power, and can be leaner at constant load.
The constant-level float bowl
Fuel is held in a constant-level float bowl, keeping the pressure on the jets relatively stable. A float follows the fuel surface and operates a needle valve: as the level drops, the float lowers, opens the needle and lets fuel in. The bowl level is thus a calibration element: a high level enriches, a low level leans out. It depends on the float weight (a heavier float raises the level, so enriches) and on the angle of the lever operating the needle. Too low a level can cause dangerous leaning — notably off-road, in corners or under braking, when fuel moves in the bowl — hence the "bowl-bottom traps" that keep fuel around the jets. With alcohol-bearing fuels, check seal compatibility.
The venturi and air flow
The venturi is defined by its diameter (in mm), chosen according to the engine's needs then refined by testing. A large-diameter venturi offers less resistance (favourable to power); a smaller one gives higher air speed and depression, hence a stronger signal at the jets and better response. For smooth modulation, Dell'Orto uses elongated ("oval"), or even "shield" (insigne), venturi shapes. The slide has a chamfered edge (in tenths of a mm, e.g. 0.30) that strongly influences carburation at small throttle openings: a small chamfer enriches up to 1/4 throttle.
The slide
The slide, connected to the throttle by a cable, moves across the venturi and sets the passage section. On the PH series (Piston Horizontal) it is cylindrical; on the VH series (Valve) it is flat, with edges designed to limit air leaks. These parts receive a surface treatment (chromed brass) to reduce wear and seizing. A flat slide reduces turbulence but requires careful design of the progression holes needed to feed the transition from idle to main circuit.
The idle circuit and progression
With the throttle closed, the air flow at the main jet is too weak to draw fuel: a second circuit (idle) takes over via a port just downstream of the slide, where a strong depression exists. An idle jet calibrates this flow; its choice is crucial, including for transition response.
Choosing the idle jet: too large, the engine tends to stall and responds sluggishly; too small, it responds better but the rpm drops slowly when the throttle is closed (and, on a two-stroke, risks seizure on deceleration). The idle fuel is emulsified with a little air; a calibrated "idle air bleed" can set this contribution.
Air screw / mixture screw: depending on the model, the screw acts on air alone (near the front cap, filter side) or on the already-emulsified mixture (on the side, engine side). They therefore work in opposite directions: to enrich, close the air screw or open the mixture screw; to lean out, open the air screw or close the mixture screw.
Transition circuit: as the slide lifts (up to ~1/4 throttle), the depression at the idle jet falls, but the progression port, still fed by the idle jet, takes over. This port is first crossed by air (towards idle), then, with the throttle open, by fuel — hence the importance of the idle jet even at the start of throttle opening.
The main circuit: needle and atomiser
The tapered-needle system meters fuel from partial opening up to full opening. At low lift, the needle restricts the passage in the atomiser: despite a strong depression, flow stays low and the ratio correct. At large opening, the thinner tapered part frees the section and keeps the ratio optimal.
Needle adjustment: the needle is held by a clip in a notch on its shaft (notches numbered from the top). Clip in a high notch = low needle = leaner mixture; clip in a low notch = high needle = richer mixture. If position alone cannot achieve correct tuning, change the needle (taper, cone length).
The atomiser contains a calibrated diameter: increasing it enriches, decreasing it leans out. Depending on the model, the atomiser is of the "two-stroke" type (edge protruding into an annular chamber) or "four-stroke" type (series of emulsion holes). On the two-stroke type, a "low" atomiser gives more immediate delivery (typical of competition); a "high" atomiser leans out on acceleration. The layout and diameter of the four-stroke type's holes optimise the mixture and the acceleration transition.
The main jet
The main jet calibrates the flow at full power and large opening. It sits at the lowest point of the bowl to stay fed. It is chosen by testing (dyno or acceleration runs): deliberately start larger for safety (a rich mixture does not give the best efficiency but avoids the seizure or piston holing caused by too lean a mixture).
Spark-plug reading: after a full-throttle run, the central-electrode insulator should be light brown (dark = too large; white = too small). On the earth electrode (usable with a new plug), the base should be at least half black near the bend; fully black and fouled = rich; perfectly clean = too lean (dangerous).
On sudden throttle opening a "lean spike" occurs: the mixture leans out for an instant before returning to the optimal value.
The starting device
When cold, some of the mixture condenses on the cold intake walls, leaning out the useful mixture: a separate starting circuit enriches the mixture during warm-up. The simplest is the "mixer" (the rider lowers the float to raise the level). More advanced circuits have their own jet and a regulating device, manual (small piston valve) or automatic ("wax motor", whose thermal expansion opens/closes the valve according to temperature).
Accelerator pump
Also called the acceleration pump, it compensates for the sudden leaning-out on a fast throttle opening (common on four-strokes) by injecting a calibrated amount of fuel directly into the venturi. Diaphragm-type on the Dell'Orto PHF and PHM, it is operated by a lever following an inclined ramp on the throttle body. The injected quantity is set by the diaphragm stop (screw in = less); the spray duration is set by the downstream jet (large jet = short spray).
Power jet
On some carburettors, a power jet allows the use of a smaller main jet (good response at small/medium openings) while enriching at full throttle: its sprayer, placed upstream of the throttle and exposed only to high depression (full load, wide-open throttle), then delivers fuel. Full-throttle tuning then acts on both jets at once.