Dellorto Motorcycle Carburetor Tuning Guide - home
3.3 Starting from cold

Although there are normally no difficulties starting the engine when it is hot, it is necessary to alter the carburation somewhat when the engine is cold.
When starting from cold, the Carburetor has to deliver a fuel mixture rich enough to produce in the cylinders a mixture ratio very close to the stoichiometric ratio; due to the low engine temperature, a large part of the fuel does not atomise completely or condenses on the cold portions of the in let tracts and the cylinders themselves.
It should therefore be clear that, at the moment of ignition, it is the actual fuel-air ratio which reaches the cylinder that is important and not the amount of fuel, atomised or not, delivered by the Carburetor .

3.3.1. Independent starting circuit.

It is called independent because the starting device operates with its own circuit including a starter jet, emulsion tube and a starter valve (fig. 15)
Start the engine from cold with the throttle closed (7) and the starter valve (2) opened by pulling up the lever (1). If a remote cable control is fitted in stead of a lever on the Carburetor , the lever should be operated fully.
Vacuum present in the barrel (8) downstream of the throttle valve (7) draws mixture to be delivered through passage (9) from the duct (4) and then it further mixes with the main airflow drawn from the intake (3). This mixture is formed by fuel metered through the starter jet (6) mixed with air from channel (10) and drawn through the emulsion tube holes (5).

fig. 15

3.3.2. Selection of emulsion tube and starter jet


The operation of the independent circuit starting device can be divided into two parts:

Initially when starting, during the first few turns of the crankshaft on the kick-starter or the starter motor, the device delivers a very rich mixture.

Figure 16 shows the mixture ratio depends entirely on the variety of drillings in the emulsion tube, because air passing through holes (2) draws up fuel which is standing in the jet well (1). In this period, the mixture strength is not determined by the starter jet size but only by the amount of fuel contained in the well above the holes located below the float-chamber fuel level.

After this, a mixture leaner than previously is delivered and this mixture reaching the combustion chamber produces the first proper running of the engine.

Figure 15 shows the mixture strength delivered through the emulsion tube depends on the size of the starter jet (6) and on the size of the air duct (10).

The channel size (4) is such that it creates an optimum vacuum in the starter valve chamber, at the emulsion tube outlet both for starting up and for the mixture required by the engine for its running and warming up. Therefore, varying the position or the size of the starter emulsion tube holes will change the amount of fuel delivered; the mixture ratio is controlled by the starter jet size and therefore a larger jet causes enrichment and vice-versa.

Difficulties in starting the engine can occur when this mixture is too rich or too lean and you can see this from the spark plugs. After some starting attempts, remove the spark plugs and, if these are wet, the mixture is too rich and you will therefore need an emulsion tube with holes higher up.

Conversely, if the spark plugs are found to be dry, the mixture is too lean and an emulsion tube with holes lower down is therefore needed.

If the engine stalls when the engine is first started from cold before it has been running for at least a minute with the starting device on, you will need to reduce the starter jet size because of an over-rich mixture or increase it if the engine stalls because of a lean mixture.

Check that the starter valve closes completely afterwards to avoid any mixture blow-by which may later disturb the carburation.

Therefore check that with the starting device off, the control lever is free to move a little on its pivot pin or that, where a remote cable control is fitted, the cable has at least 1-2 mm of free play.

3.3.3 - The flooding-plunger cold starting device

The starting device with a flooding plunger, or tickler, is shown in figure 17 and uses the normal main and idle circuits.

It is composed simply of a push button (1) which, when manually operated, holds down the float (2).

This forces the fuel inlet valve open causing an influx of fuel which raises the float chamber fuel level above normal and consequently enriches the mixture. This enrichment gradually decreases as the fuel is used up and stops when the float chamber level has returned to normal.

This device requires quite a lot of care from the operator because if the chamber fuel level is raised insufficiently, the engine may not start because the mixture is still excessively weak; alternatively, if the chamber level is raised too much, the resulting over-rich mixture may also prevent the engine starting.

fig. 17