Carburetor Adjustments & Tuning Tips; Carburetor Components; Carburetor Circuit Functions - Honda CR85 Expert Owner's Manual

Carburetor Adjustments & Tuning Tips
Refer to Safety Precautions on page 19.
The carburetor on your CR should
perform suitably with the standard recommended
settings under average load, climatic, and
barometric conditions. However, to fine tune the
engine's power output, the carburetor may
require adjustments for specific competition
needs.
Optional main jets and slow jets are available for
your motorcycle. See your Honda dealer.
Any engine or airbox modifications or the use of
an aftermarket exhaust system may require
jetting changes.

Carburetor Components

Starting Circuit
A very rich mixture must be delivered to the
cylinder when the engine is being started. When
the choke knob (1) is pulled up, fuel is metered
by the starter jet (2) and is mixed with air from
the air passage (3) to provide a rich mixture for
starting. The mixture discharges through the
orifice (4) into the cylinder.
(1)
(4)
(2)
(1) choke knob (3) air passage
(2) starter jet (4) orifice
88
Adjustments for Competition
Slow Circuit
Fuel is metered by the slow jet (5) and is mixed
with air that has been metered by the air screw
(6). The mixture enters the venturi through the
bypass (7) and slow jet circuit outlet (8).
(8)
(5)
(5) slow jet
(6) air screw
Main Circuit
Fuel is metered by the main jet (9), jet needle
(10) and needle jet (11). It is then mixed with air
coming from the air jet (12) and enters the
venturi at the nozzle (13).
Float Chamber
The float (14) and float valve (15) operate to
(3)
maintain a constant level of fuel in the float bowl
(16) .
(7)
(9) main jet
(10) jet needle
(11) needle jet
(12) air jet
(13) nozzle

Carburetor Circuit Functions

(6)
The carburetor has several major circuits, each
providing the fuel/air mixture over a given range
of throttle valve opening.
(7) bypass
(8) slow jet circuit outlet
These major circuits overlap as shown below.
Main Jet
The main jet affects air/fuel ratio from half to full
throttle. The size should be reduced at higher
altitudes.
Honda carburetor jet sizes are numbered in
increments of 2or3. When changing the main jet
size, increase or decrease it gradually until the
desired setting is obtained. Because your CR's jet
size numbers do not correspond with other
carburetor manufacturer's jet size numbers, use
only Honda jets.
(10)
(12)
(13)
(11)
(16)
(15)
(14)
(9)
(14) float
(15) float valve
(16) float bowl