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Cutaway Drawing

Use the cutaway drawing (above) to learn about the inner

workings of the Super BN carburetor. Starting with the fuel pump, fuel

enters through the fuel inlet, being drawn in by the engine’s pulse acting

on the pump diaphragm. The fuel passes through a series of check

valves and then the fuel required by the engine passes through a fuel

ilter. Excess fuel is diverted through a restricter and then back to the fuel

tank via the fuel return. The purpose of the fuel restricter is to act as a

fuel pressure regulator for the fuel pump.

After the fuel passes through the ilter it goes directly to the

needle valve assembly and then into the chamber. Fuel low into

the fuel chamber is controlled by the regulator diaphragm, arm spring,

arm and vent to the atmosphere. These components, along with the

needle valve, make u to the regulator portion of the Super BN. As you

will learn, the Super BN is part carburetor and part fuel regulator. The

regulator portion controls fuel low at lower speeds, idle to 1/4 throttle,

and the carburetor portion controls fuel low from about 1/4 throttle to full


At idle, fuel is drawn through the low speed jet to the low

speed jet to the speed outlet, via the low speed adjuster, and to the

bypass holes. The low speed adjuster controls fuel low for a

smooth idle and initial throttle response. As the throttle valve is

opened from the idle position, the bypass holes are increasingly

exposed to the air low. Their function is to help the carburetor

“transition” from low speed to high speed operation. The size of the

low speed jet directly affects fuel low through the bypass holes for

low speed performance. the other function affecting low speed per-

formance is the regulator portion of the BN. the regulator portion

can be tuned by changing the arm spring tension of the size of the

needle valve, or both. More of that later.

As the Super BN transition to its high speed fuel circuit,

at about 3/8 throttle, fuel is drawn through the check valve is to

prevent the low speed circuit from sucking in air through the high

speed jet passes through the inner venturi then into the enine.

The high speed adjuster controls the maximum amount of fuel low

for full throttle performance from about 3/4 throttle opening.

Super BN

Internal Components