UK: 01564  783  192
Int: +44 1564 783 192

How It Works & Part Names

A. Mixing Chamber O. Needle Jet
B. Throttle Valve P. Main Jet (see sheet 6)
C. Jet Needle and Clip above Q. Float Chamber Holding Bolt
D. Air Valve R. Float Chamber
E. Mixing Chamber Union Nut S. Needle Valve Seating
F. Jet Block T. Float
G. Cable Adjuster (Throttle) U. Float Needle Valve
GI. Cable Adjuster (Air) V. Float Needle Clip
H. Jet Block Barrel W. Float Chamber Cover
J. Pilot Hole (see sheet 6) X. Float Chamber Lock Screw
K. Passage to Pilot   Tickler (to left of W.)
L. Pilot Air Passage Y. Mixing Chamber Top Cap
M. Pilot Mixture Outlet Z. Mixing Chamber Lock Ring
N. Pilot By-pass ZI. Security Spring for above

The carburetter proportions and atomises the right amount of petrol with the air that is sucked 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 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. This primary air choke has a compensating action.
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.
This design of carburetter offers perfectly simple and effective tuning facilities.

Fig 3.
This section view does NOT apply if your carburetter has FOUR EXTERNAL primary air holes at the base of the mixing chamber. It is for carburetters with the primary air inlet in the main air intake

Diagrammatic section of carburetter showing only the lower half of the throttle chamber with the throttle a little open and the internal primary air passages to the main jet and pilot system.

If the carburetter should flood whilst the engine is not running, the overflow from the main jet will run into the primary air passages and trickle out from there through a small hole seen at the side of the carburetter body
Opening times: Monday - Friday 9am - 6pm | Closed: Saturday and Sunday
Units 7 & 8, Rosemary Court, Oldwich Lane West, Chadwick End, Solihull, B93 0EY, UK

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