Gregory Bender


Tuning instructions for Dellorto SS carburetors

Detailed technical information regarding Dell’Orto motorcycle carburetors, how to tune, parts diagrams, slide specifications, needle specifications, jet specifications, and atomizer specifications.

Created:

Updated:


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.

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.

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

Carburetor type Throttle valve Jet needle Needle jet Main jet Pilot jet
SSI-C
18-20-22-23
Cat. Nº 1916
70
Cat. Nº 2289
R2 at 3 groove
Cat. Nº 1805
260
Cat. Nº 1126
85-95-105-110
Cat. Nº 1159
50
SS-A, SSI-A, SSF-A, SSFF-A
24-25
26-27-28
29-30
Cat. Nº 2384
90
100
100
Cat. Nº 1824
M7 at 3 groove
M13 at 3 groove
M13 at 3 groove
Cat. Nº 1805
260
265
270
Cat. Nº 1126
115-120
125-130-135
140-145
Cat. Nº 1159
50
50
50
SSI-B, SSFI-B
32
Cat. Nº 3466
110
Cat. Nº 1141
N1 at 3 groove
Cat. Nº 1121
315
Cat. Nº 1126
155
 
SSI-B, SSFI-B
35
Cat. Nº 3686
120
Cat. Nº 1900
P1 at 3 groove
Cat. Nº 1121
320
Cat. Nº 1126
170
 
SSI-B
36-38
40-42
Cat. Nº 4545
130
150
Cat. Nº 2470
S1
S1
Cat. Nº 1121
325
330
Cat. Nº 2475
180-200
220-240
 

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%)

Carburetor type Throttle valve Jet needle Needle jet Main jet Pilot jet
SSI-C
18-20-22-23
Cat. Nº 1916
60
Cat. Nº 2289
R4 at 4 groove
Cat. Nº 1805
275
Cat. Nº 1126
130-135-140-145
Cat. Nº 1159
60
SS-A, SSI-A, SSF-A, SSFF-A
24-25
26-27-28
29-30
Cat. Nº 2384
70
80
90
Cat. Nº 1824
M3
M3
M3
Cat. Nº 1805
280
280
280
Cat. Nº 1126
150-155
160-170-180
190-200
Cat. Nº 1159
70
70
70
SSI-B, SSFI-B
32
Cat. Nº 3466
80
Cat. Nº 4338
N2
Cat. Nº 1121
350
Cat. Nº 1126
220
 
SSI-B, SSFI-B
35
Cat. Nº 3686
90
Cat. Nº 4339
P2
Cat. Nº 1121
360
Cat. Nº 1126
250
 
SSI-B
36-38
40-42
Cat. Nº 4545
100
110
Cat. Nº 2470
S8
S8
Cat. Nº 1121
370
380
Cat. Nº 2475
250-280
310-340
 

Figure 1 - Proving diagram of the different running phases.

Photo courtesy of Australian Internet Guzzi Owners Register.

Figure 2 - Proving diagram of the exact level position.

Photo courtesy of Australian Internet Guzzi Owners Register.

Table A - for the choice of carburetor diameter

Cylinder displacement Choke adaptor diameter for the undermentioned motorcycle classes
Sport Supersport Racing
125 cc 4 st. 20 22 28
125 cc 2 st. 22 24 30
175 cc 4 st. 24 26 30
175 cc 2 st. 26 28 32
250 cc 1 st. 26 28 32
250 cc 2 st. 28 30 35
350 cc 4 st. 28 30 35
500 cc 4 st. 30 32 38
600 cc 4 st. 32 35 40
750 cc 4 st. 35 38 42

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

Photo courtesy of Australian Internet Guzzi Owners Register.

  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.

Photo courtesy of Australian Internet Guzzi Owners Register.

  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