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BASIC BASIC BIS BIS

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What is What is BISBIS ? ?

What does BIS mean?– BIS is an acronym for

Balanced Idle System

– The BIS system is part of the idle circuit of the carburetor.

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Identifying a carburetor with Identifying a carburetor with the BIS systemthe BIS system

NO idle speed screw on the pump cover

The only adjustment screw is located very close to the throttle flange

Can be externally identified by

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WHAT is BIS? WHAT is BIS?

BIS is a circuit within the carburetor that allows for an adjustment that changes the idle speed of the engine without changing the fuel mixture.

The throttle valve is fully closed regardless of the idle speed.

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What Does BIS do?What Does BIS do?

Functionally, the Balanced Idle System keeps the air/fuel ratio at optimum as the speed of the engine changes from idle to fast idle.

This idle speed screw allows more or less air to by-pass the throttle shutter, this change in air flow makes the engine idle faster or slower

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How does BIS work?How does BIS work?

Components of the BIS idle circuit are:– idle jet– balance channel (drilled)– idle chamber (where the idle holes are)– air by pass chamber– inlet air port– outlet air port– idle speed screw (LD)

SCHEMATIC

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How does BIS work?How does BIS work?

Components of the BIS idle circuit are:– idle jet– balance channel (drilled)

_ idle chamber (where the idle holes are)– air by pass chamber– inlet air port– outlet air port– idle speed screw (LD)

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How does BIS work?How does BIS work?

Components of the BIS idle circuit are:– idle jet– balance channel (drilled)– idle chamber (where the idle holes are)– air by pass chamber– inlet air port– outlet air port– idle speed screw (LD)

SCHEMATIC

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How does BIS work?How does BIS work?

Components of the BIS idle circuit are:– idle jet– balance channel (drilled)– idle chamber (where the idle holes are)– air by pass chamber– inlet air port– outlet air port– idle speed screw (LD)

SCHEMATIC

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How does BIS work?How does BIS work?

Components of the BIS idle circuit are:– idle jet– balance channel (drilled)– idle chamber (where the idle holes are)– air by pass chamber– inlet air port– outlet air port– idle speed screw (LD)

INLETOUTLET

SCHEMATIC

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How does BIS work?How does BIS work?

Components of the BIS idle circuit are:– idle jet– balance channel (drilled)– idle chamber (where the idle holes are)– air by pass chamber– inlet air port– outlet air port– idle speed screw (LD)

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How does BIS work?How does BIS work?

To better understand the BIS system, let’s trace the fuel flow starting with the fuel in the metering chamber

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Fuel Flow through idle circuitFuel Flow through idle circuit As the engine idles, fuel flows from the

metering chamber through the fixed idle jet.

IDLE JET

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Fuel Flow through idle circuit Fuel Flow through idle circuit continued….continued….

The fuel then flows via a drilled hole into the idle chamber. (This idle chamber is covered by a welch plug)– Inside the idle chamber are the idle holes. As

the engine idles, fuel flows through the 1st idle hole. Air through the air by pass chamber and out the outlet port into the throttle bore.

IDLE JET

FuelAir

At Normal Idle speed

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How does BIS work?How does BIS work?

NOTICE A very small hole is drilled from the idle

chamber into the AIR By Pass Chamber. This small hole is called the balance channelThis channel allows sometimes air and

sometimes fuel to flow between the chambers.

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How does BIS work?How does BIS work?

Changes in Idle Speed The idle speed screw, functionally allows

more or less air to by-pass the throttle shutter.

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NOTICENOTICE

The threads on the screw are left hand or “reverse thread”.– WHY ARE THE THREADS on the speed screw LEFT

HAND THREADS?

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Adjusting the idle speedAdjusting the idle speed At idle speed, the throttle shutter is in full closed

position. To arrive at a low idle speed the idle speed screw is

turned IN (counter clockwise) . The engine speed decreases.

When the idle speed screw is turned IN, THE AIR OUTLET PORT IS RESTRICTED and the balance channel acts as an air bleed which allows air to flow into the idle chamber. (because the vacuum is higher in the idle chamber)

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Adjusting the idle speed Adjusting the idle speed continued...continued...

As the idle speed screw is opened, (turned clockwise) the balance channel will begin to feed fuel into the air by pass chamber because.....

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Adjusting the idle speed Adjusting the idle speed continued...continued...

Because....– there is a higher vacuum in the air by pass

chamber and it pulls the fuel out of the idle chamber.

– The head of the idle speed screw is out of the air stream.

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Adjusting the idle speed Adjusting the idle speed continued...continued...

With the engine at idle speed, Turning the speed screw in, will decrease

the idle speed of the engine. WHY?

– Less air enters the engine and the idle speed decreases.

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Adjusting the idle speed...Adjusting the idle speed...

Turning the screw out will increase the idle speed of the engine.

WHY?– More air leans out the engine and the idle speed

increases.

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Question??Question??

When idle speed screw adjustments are made

Is the amount of fuel fed to the idle circuit being changed?

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ANSWERANSWER

NO

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Question??Question??

Is the high speed fuel mixture affected by the speed screw adjustment?

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ANSWERANSWER

NONO

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High Speed Fuel CircuitHigh Speed Fuel Circuit

The main jet in the BIS carburetor is a “fixed jet”– Fixed jet means that it is not adjustable

The main jets are either pressed or threaded into the carburetor body

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High Speed Fuel Circuit High Speed Fuel Circuit continued...continued...

At full throttle conditions fuel flows through the main nozzle, through a check valve and then into the venturi of the carburetor

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Does any other power Does any other power equipment manufacturer use equipment manufacturer use

the BIS system?the BIS system?

NO

Only Stihl Products use the BIS system

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TroubleshootingTroubleshooting

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Trouble Shooting a BIS Trouble Shooting a BIS SystemSystem

REMEMBER A carburetor with a BIS system is a fixed

jet carburetor. Dirt is the biggest enemy of any fixed jet

carburetor. Dirt will restrict fuel flow to the idle circuit and

main circuit.

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Trouble Shooting a BIS System Trouble Shooting a BIS System continued...continued...

Before we dig into the carburetor, what items outside of the carburetor can cause problems?

– “bad fuel” old fuel , water, etc– dirty air filter– restricted fuel filter– fuel cap not working correctly

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Trouble Shooting a BIS System Trouble Shooting a BIS System continued...continued...

– muffler clogged

– throttle cable not connected

– choke linkage not assembled correctly

– various engine problemsmechanical problemsgasket leaks

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Trouble Shooting a BIS Trouble Shooting a BIS System System continued..continued....

Restricted fuel at high speed results in:– lower high speed– less power– poor acceleration

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Solutions to restricted fuel flow Solutions to restricted fuel flow at high speedat high speed

Replace the high speed jet Flush out the areas of the carburetor where

dirt is suspected

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Trouble Shooting a BIS Trouble Shooting a BIS System System continued..continued....

Restricted fuel in the idle speed circuit results in:– unstable idle speed– no idle speed– poor acceleration– engine dies at idle– can’t adjust carburetor to get engine to idle

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Solutions to restricted fuel flow Solutions to restricted fuel flow at idle speedat idle speed

Flush out the areas of the metering chamber where dirt is suspected

Remove the two welch plugs– inspect for dirt– flush out these chambers

Install new welch plugs Seal the area around the welch plug with

ZAMA glue

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Trouble Shooting a BIS Trouble Shooting a BIS System System continued..continued... .

After the carburetor has been cleaned reinstall it on the engine.

Run the engine. If the performance at idle speed is still unacceptable, replace the carburetor.

If the performance at high speed is still unacceptable, replace the high speed jet.

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Trouble Shooting a BIS Trouble Shooting a BIS System System continued..continued... .

Run the engine at high speed If the performance at high speed is

still unacceptable, replace the high speed jet.

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Lean Come Down Excessive Rich Come Down

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The end of the BIS The end of the BIS mysterymystery