Dell'Orto carburetors - Tuning instructions

Tuning instructions for SS carburetors

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This information was gleaned from Moto Italia and the Australian Internet Guzzi Owners Register.

Idle adjustment

(Throttle valve opening corresponding to the Section A of Figure 1.)

It is necessary first of all to bear in mind that this adjustment must be established always when the engine has reached its normal running temperature.

• In the first idle system, where is subsisting the replacing jet and the air adjusting screw, you must operate on this air screw in order to obtain a correct carburetion, bearing in mind that by tightening it the mixture strength is enriching, while by unscrewing it the mixture strength is weakening.
• In the second system, where is subsisting the variable screwed pilot jet and the fixed air, the wished engine running will be obtained by operating only on the screw which adjusts the fuel passage; the mixture strength will weaken by tightening this screw and will enrich by unscrewing it.
  
  This second idle system, although it is not exact and sensitive like the first one (suitable to particular uses and engines as told in the general features) allows a better and quicker mixture strength change; it favours therefore easy corrections of carburetion also in the passage runnings and necessary easy enrichments in case of alcohol feed.

With both the idle systems is always advisable to adjust the mixture strength at the slowest runnings, rather about the rich, in order to have then clean passages and pickups without hesitation.

First passage adjustment

(Throttle valve opening corresponding to the Section B of Figure 1).

When obtained a satisfactory idle adjustment, one becomes to the choice of the suitable throttle valve for the intermediate runnings, proceeding as follows.

1. If opening gradually the throttle valve for a space corresponding to the part B - fig. 1 the engine running is normal, it means that the throttle valve is suitable.
2. If the engine is inclining to fall or it gives backfires for weaken mixture, it means that the fitted throttle valve has a cutaway too high and it is necessary to replace it with another one of lower size.
3. If the engine instead is emitting black smoke at the exhaust or it is giving irregular explosions with a heavy running, it means that the fitted throttle valve has a cutaway too low and it is necessary to replace it with another one of upper size.

Jet needle adjustment

(Throttle valve opening corresponding to the Section C of Figure 1).

In order to have the possibility to adjust the jet needle there are on it 5 grooves or holes (the numeration is starting from the top as follows: 1-2-3-4-5). The jet needle checks the carburetion for a throttle valve opening corresponding to the Section C of Figure 1.

If the mixture seems to be weak, the needle must be moved upwards one or two grooves so as to allow a larger flow of fuel at the exit of the needle jet.

If instead the mixture appears to be reach, the opposite must be done, by lowering the needle a few grooves (or holes where existing).

The average position of the jet needle is generally established by us at the third groove (or hole).

Main jet size

(Throttle valve opening corresponding to Section D of Figure 1).

The influence of the main jet is especially felt in the throttle valve opening corresponding to Section D of Figure 1.

It is therefore in this field that it is necessary to operate in order to establish if the main jet fitted is the most suitable one, and precisely:

1. If fully opening the gas, the engine begins to turn over with difficulty and instead of increasing in speed,it does not change or even loses and tends to backfire, and if by closing the mixture control piston, a distinct improvement in running is noted, this indicates that the mixture is too weak. In this case the main jet must be replaced by others of the next sizes up until the one, which gives the correct result, is found.
2. If the throttle valve is fully turned on, and the engine gives a muffled sound from the exhaust or is missing explosions with emission of black smoke, and by closing the air valve the defect increases, this indicates too rich a mixture. In this case it is necessary to replace the fitted main jet by others of the smaller sizes until the one, which gives the correct result, is found.

A correct carburetion at high speed must be obtained at completely opened air valve.

It must be born in mind that it is advisable to use the size of the main jet which will have given the best result in power or highest speed but that will have however kept the engine at a temperature of safety.

• Exact main jet = normal engine temperature
• Small main jet = higher engine temperature
• Large main jet = lower engine temperature

Only following scrupulously the above instructions and using a sensibility at the highest point when effecting the tests on road and at the brake, one may arrive at a perfect adjustment of the carburetor and therefore at the best performance of the engine itself.

As approximate adjustment data please see at the adjustment key for gasoline.

Base adjustment for gasoline

Use of alcohol fuel

All our carburetor types of the series SS-A and SS-I are very well suitable also for running with alcohol fuel provided that their general adjustment is revised in the following points.

1. Throttle valve: The size generally used for normal super fuels must be replaced by one immediately lower.
2. Needle jet: A needle with calibrated hole of higher size must be fitted, bearing in mind that the increase of this hole will be directly proportional to the alcohol quantity contained in the fuel that one is intended to use.
3. Jet needle: For fuels with much than the 50% of alcohol end up to the use of methanol, it will be necessary else to replace the needle used for super fuels by one with a thinner end, foreseen by us for this use. The average position advised by us for alcohol fuels is at the 4th groove (or hole).
4. Main jet: A main jet oversized from 25 to 50 points about must be fitted according to the carburetor diameter and to the alcohol percentage of the new fuel.
5. Pilot jet: Also for the running in the idle area it will be necessary increase the mixture strength, operating as follows.
   1. Idle with rechangeable jet and air adjusting screw:
      • for fuels with alcohol up to 50% it will be enough to tighten the air screw, in comparison to the adjustment used for super fuels, just to its complete tightening.
      • for fuels with alcohol over the above percentage it will be necessary also to replace the pilot idle jet by a larger one from 5 to 15 points and adjust comparatively the air screw.
   2. Idle with variable screw and air fixed:
      • with the idle system it is enough to open the screw at pleasure just when a correct idle running is obtained and in the first throttle valve openings.

Of course all these instructions are not rigorously fixed but partly entrusted to the judgment and to the sensibility of the experimenter who only by meticulous tests and exactly knowing the features of his engine and of the fuel used, will have the possibility to realize a perfect carburetor adjustment.

As approximate adjustment data please see at the adjustment key for alcohol.

Base adjustment for alcohol (upper the 50%)

Figure 1 - Proving diagram of the different running phases.

Figure 2 - Proving diagram of the exact level position.

Table A - for the choice of carburetor diameter

Proving section of SS-A carburetor with idle system of rechangeable jet and normal float chamber.

1. Fixing float chamber top screw
2. Tickler
3. Float chamber cap
4. Mixing chamber top
5. Mixing chamber cap
6. Carburetor body
7. Air valve
8. Air valve channel
9. Jet holder
10. Holding bolt for float chamber
11. Main jet
12. Filter
13. Banjo nut
14. Banjo
15. Needle seating
16. Float needle
17. Float
18. Float chamber body
19. Throttle valve
20. Choke adapter
21. Throttle stop screw
22. Throttle cable adjuster
23. Outlet clip
24. Outlet cable clip
25. Idle second hole
26. Idle air channel
27. Air adjusting screw
28. Idle first hole
29. Pilot jet
30. Idle fuel channel
31. Jet plug
32. Needle jet
33. Spray tube
34. Jet needle
35. Air tube

Proving section of SSI-B carburetor with idle system of rechangeable jet and remote float chamber SS-I.

1. Jet needle
2. Float
3. Banjo
4. Banjo nut
5. Jet needle seating
6. Jet needle
7. Tickler
8. Rocking lever
9. Clamp ring for float chamber
10. Clamp ring gasket for float chamber
11. Float chamber body
12. Petrol outlet connection
13. Banjo
14. Carburetor body
15. Choke adaptor
16. Air valve channel
17. Air valve
18. Throttle valve
19. Mixing chamber top
20. Throttle cable adjuster
21. Throttle stop screw
22. Outlet clip
23. Outlet clip pin
24. Pilot adjuster
25. Pilot needle insert
26. Idle air channel
27. Idle fuel channel
28. Idle outlet channel
29. Needle jet
30. Main jet
31. Banjo nut
32. Jet holder
33. Air channel for emulsioning
34. Air tube
35. Spray tube
36. Mixing chamber cap

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