The
carburetter proportions and atomises the right amount of
petrol with the air that is drawn in by the engine because
of the correct proportions of jet sizes and the main choke
bore. The float chamber maintains a constant level of fuel
at the jets and cuts off supply when the engine stops.
The throttle control from the handlebar controls the volume
of mixture and therefore the power, and at all positions
of the throttle the mixture is automatically correct. The
opening of the throttle brings first into action the mixture
supply from the pilot jet system for idling, then as it
progressively opens, via the pilot by-pass, the mixture
is augmented from the main jet, the earlier stages of which
action is controlled by the needle in the needle jet. The
pilot jet system is supplied by the pilot jet (30) which
is detachable on removal of the float chamber. On most 4
stroke models no pilot jet is fitted, but a pilot bush is
inserted in the continuation of the pilot air adjusting
screw passage. The main jet does not spray directly into
the mixing chamber, but discharges through the needle jet
into the primary air chamber, and goes from there as as
a rich petrol-air mixture through the primary air choke
into the main air choke.
The carburetters usually have a seperately-operated mixture
control called an air valve, for use when starting from
cold, and until the engine is warm; this control partially
blocks the passage of air through the main choke.
| 1. |
Mixing
Chamber |
 |
19. |
Float |
| 2. |
Air
Valve Spring |
20. |
Mixing
Chamber Top Screws |
| 3. |
Air
Valve |
21. |
Throttle
Valve |
| 4. |
Float
Spindle |
22. |
Jet
Needle |
| 5. |
Float
Needle |
23. |
Choke
Tube |
| 6. |
Needle
Seating |
24. |
Needle
Jet |
| 7. |
Filter
Gauze |
25. |
Tickler |
| 8. |
Banjo |
26. |
Throttle
Adjusting Screw |
| 9. |
Banjo
Bolt |
27. |
Float
Chamber Washer |
| 10.. |
Cable
Adjuster (Air) |
28. |
Jet
Holder |
| 11. |
Cable
Adjuster (Throttle) |
29. |
Main
Jet |
| 12. |
Cable
Adjuster Locknuts |
30. |
*
Pilot Jet |
| 13. |
Carburetter
Body |
31. |
Pilot
Jet Feed Passages |
| 14. |
Throttle
Valve Spring |
32. |
Feed
Passage from Pilot Jet |
| 15. |
Jet
Needle Clip |
|
33. |
Pilot
Air Feed Passage |
| 16. |
Pilot
By-pass |
|
34. |
Primary
Air Adjusting Screw |
| 17. |
Pilot
Outlet |
|
|
|
| 18. |
Float
Chamber Body |
|
|
|
