03-71
SECTION 03 - ENGINE PREPARATION
Pop-off Pressure Chart
Fuel Flow Chart
NOTE:
This chart depicts the approximate
amount of fuel that each circuit contributes to the
total fuel flow of the Super BN’S. Fuel flow is in
percentage.
1. % of total fuel flow
2. Throttle position
3. High speed screw
4. Main jet
5. Transition circuit
6. Low speed screw
Low Speed Circuit
Low speed circuit
contributes fuel all the way to
full throttle. All fuel circuits within the carb work in
this manner: as the throttle opens, each circuit in
turn contributes to the total fuel requirements of
the engine.
Transition Circuit
Transition circuit
controls the fuel flow for a
smooth “transition” to the high speed circuit. If
the transition circuit is far enough out of adjust-
ment, it may become difficult to get to the high
speed circuit and the engine will not rev up.
High Speed Circuit
High speed circuit
controls the fuel flow in addi-
tion to the low speed and transition circuits. The
high speed screw can control up to as much as 1/2
of the fuel delivered by the high speed circuit, de-
pending on the relative size of the high speed jet.
This adjustability allows for a wide range of tuning
without changing an internal jet.
Jetting
The oxygen content of air varies with different at-
mospheric conditions. Cool, dry air at a high baro-
metric pressure has more oxygen molecules per
ft
3
than does hot moist air at low barometric pres-
sure readings. Hot air expands; therefore, there
are less oxygen molecules per ft
3
. At low baro-
metric pressure also allows air to expand. An in-
crease in altitude results in a decrease in
barometric pressure. Air with high humidity has
more water molecules which take up space so
there is less room for oxygen molecules. Since
oxygen is what is needed to mix fuel to provide a
burnable mixture, changing atmospheric condi-
tions will require a change in fuel flow.
Pilot and main jets are replaceable. Different jet
sizes are available to suit altitude and/or tempera-
ture conditions. Different jetting may be required
as engine performance is increased.
Always inspect spark plug and piston dome color
before selecting jetting options. Spark plug or pis-
ton dome color is an indication of carburetion mix-
ture conditions.
Dark brown or black coloration indicates a rich
condition, while light tan or white indicates a lean
condition. Medium brown coloration may indicate
a correct jetting range.
Needle Valve
Size
Arm Spring
115 g
95 g
80 g
65 g
1.5
55 PSI
43 PSI
38 PSI
32 PSI
2.0
32 PSI
25 PSI
21 PSI
18 PSI
2.3
22 PSI
20 PSI
17 PSI
15 PSI
2.5
19 PSI
17 PSI
14 PSI
12 PSI
1
2
3
4
5
6
F01D71A
Full
3/4
1/2
1/4
Idle
1%
33%
33%
100%
Summary of Contents for Bombardier
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