The following Weber carburetor types have been fitted:
- New 500 ——————— 26 IMB 1
- 500 Sport ——————– 26 IMB 3
- 500D————————– 26 IMB 4
- Station Wagon ————- 26 OC
- 500F ————————– 26 IMB 6
- 500L ————————– 26 IMB 10 (incorporates CO limiter for idling)
The weber 26 IMBl carburetor is a downdraft, single body carburetor with 1.0236” (26 mm) diameter throat at the height of throttle shaft.
The fuel mixture rating system consists of a throttle valve controlled by accelerator pedal through a cable which operates a lever secured to throttle valve shaft. This carburetor is fitted with a progressive-action starting device which enables the driver to suit the mixture richness to the most varied conditions of starting, until the engine has reached the rated operation temperature.
The dampened needle valve ensures a smooth engine running since, thanks to its dampening device, it is not affected by vibrations and, therefore, keeps steadily constant the level in carburetor bowl.
Operation.
Diagrammatic section of Weber 26 IMB1 carburetor
1. air corrector jet 2. air inlet 3. idle speed mixture duct 4. idle speed jet holder 5. idle speed air orifice 6. filter cover 7. Filter 8. fuel inlet connection 9. needle valve seat 10. Needle 11. float pivot 12. Float 13. idle speed jet 14. main jet holder 15. main jet 16. Bowl 17. idle speed mixture adjustment screw 18. idle speed mixture orifice 19. Throttle 20. transition hole 21. primary Venturi (not interchangeable) 22. emulsion orifices 23. emulsion well 24. secondary Venturi (not interchangeable) 25. main nozzle
Referring to the diagram. the air from above, flows through Venturi (24) where it mixes with the fuel issuing from nozzle (25) and is then conveyed to cylinders through primary Venturi (21) and throat, where throttle (19) is adjusting the flow.
From fuel line, joined to carburetor by connection (8), the fuel flows across filter gauze (7), through needle valve (10) into bowl (16) where float (12) hinged to pivot (11) controls the opening of needle (10) and maintains a constant fuel level.
From bowl (16) – via metered main jet (15)– fuel reaches emulsion well (23) whence, after having been mixed with the air coming from metered air corrector screw (1), through emulsion orifices (22) and spray nozzle,(25)it finally reaches the Venturis where it blends with the air stream promoted by engine suction and is then drawn into cylinders.
The purpose of secondary Venturi (24) is to increase the vacuum around nozzle (25) and to carry the air/fuel mixture to the center of primary Venturi.(21)
While idling, through an appropriate passage fuel is carried from well (23) to idle speed jet (13) where it is mixed with the air coming from air inlet (5). Through duct (3) and idle speed orifice (18) – adjustable by taper-point screw (17) – the fuel reaches carburetor throat past throttle (19) where it is further mixed with the air stream drawn in by engine vacuum through the gap around the throttle in idling speed position.
From duct (3) the mixture can also reach the carburetor throttle chamber through transition hole (20), located in exact relation to the throttle; the purpose of this progression hole is to permit a smooth acceleration of engine from idling speed, proportionately to the increase in throttle opening.
Starting Device. (figure below)
This device has the function of ensuring proper engine cold starting. It is controlled by means of the lever placed behind the gearbox lever and must be progressively set back to rest position as engine is reaching the rated operation temperature. The starting device is made up of valve (33) actuated by the lug of rocker (36) connected, through a suitable shaft, to control lever (38). By pulling the device control to stroke end through lever (38) and rocker (36), valve (33) is lifted from its seat and brought to the fully open position (diagram A). Under these conditions valve (33) closes air hole (27) and mixture hole (29) and uncovers mixture orifices (30) (32) [which communicate with starting jet (46) through duct (26)] and air holes (35). With valve (33) partially opened, hole (29) may communicate with carburetor throat, through the valve central slot, duct (28) and hole (31) drilled in Venturi (21) in correspondence with the restriction.
With throttle in idling speed position, the vacuum of engine cranked by the starter causes the fuel contained in the recess of jet (46), in the jet and in reserve well (45) to be emulsioned with the air coming from holes (43) and (44). Through duct (26) and holes (30) and (32) the mixture arrives – simultaneously with air from holes (35) – past the throttle through duct (34), thus permitting prompt starting of the engine.
After engine firing the device delivers a mixture whose fuel/air ratio is such as to permit regular running of engine while still cold. But, as the engine warms up, this mixture would be excessive and too rich; therefore, it becomes necessary to exclude gradually the device as the engine is reaching the rated operation temperature. During this maneuver, valve (33) slowly uncovers hole (27) which permits a greater amount of air to enter through spring guide hole (42) (to weaken the mixture) while, by closing progressively holes (30) and (32) and air holes (35) also the amount of mixture is reduced. (see diagram B)
Hole (29), duct (28), and hole (31), drilled in Venturi (21), have the task of permitting a regular progression of acceleration also with cold engine. By opening throttle (19) to speed up the engine, the vacuum acting on duct (34) is reduced. This would cause a reduction in the amount of fuel delivered through said duct (34), with consequent irregular running of the engine, but, through hole (31), duct (28)and hole (29) (from which air is drawn when throttle is closed), some mixture is sucked in by the vacuum formed in the restriction of the Venturi consequent to the opening of the throttle, and this compensates for the reduction in delivery through duct (34).
When the starting device is excluded, valve (33) covers also hole (29) and prevents the entrance of mixture.(see diagram C)
Starting device (choke) diagrammatic section
<<A>>: Device fully inserted. - <<B>>: Device partially inserted. - <<C>>: Device didinserted
2. air inlet 16. bowl 19. primary Venturi 24. secondary Venturi 26. mixture duct 27. mixture leaning air orifice 28. transition duct 29. transition mixture orifice 30. starting mixture orifice 31. transition orifice 32. starting mixture orifice 33. starting valve 34. mixture duct 35. starting device air orifices 36. rocker 37. lever return spring 42. spring casing 43. starting jet emulsion air orifice 44. air emulsion reserve well orifice 45. starting reserve well 46. starting jet
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