USA ZAMA BASIC BIS

USA ZAMA What is BIS ?  What does BIS mean? – BIS is an acronym for B alanced I dle S ystem – The BIS system is part of the idle circuit of the carburetor.

USA ZAMA Identifying a carburetor with the 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

USA ZAMA 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.

USA ZAMA 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

USA ZAMA 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

USA ZAMA 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)

USA ZAMA 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

USA ZAMA 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

USA ZAMA 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) INLET OUTLET SCHEMATIC

USA ZAMA 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)

USA ZAMA How does BIS work?  To better understand the BIS system, let’s trace the fuel flow starting with the fuel in the metering chamber

USA ZAMA Fuel Flow through idle circuit  As the engine idles, fuel flows from the metering chamber through the fixed idle jet. IDLE JET

USA ZAMA Fuel Flow through idle circuit 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 Fuel Air At Normal Idle speed

USA ZAMA 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 channel  This channel allows sometimes air and sometimes fuel to flow between the chambers.

USA ZAMA How does BIS work?  Changes in Idle Speed  The idle speed screw, functionally allows more or less air to by-pass the throttle shutter.

USA ZAMA NOTICE  The threads on the screw are left hand or “reverse thread”. – WHY ARE THE THREADS on the speed screw LEFT HAND THREADS?

USA ZAMA Adjusting 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)

USA ZAMA Adjusting the idle speed 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.....

USA ZAMA Adjusting the idle speed 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.

USA ZAMA Adjusting the idle speed 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.

USA ZAMA 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.

USA ZAMA Question??  When idle speed screw adjustments are made  Is the amount of fuel fed to the idle circuit being changed?

USA ZAMA ANSWER NO

USA ZAMA Question?? Is the high speed fuel mixture affected by the speed screw adjustment?

USA ZAMA ANSWER NO

USA ZAMA High 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

USA ZAMA High Speed Fuel Circuit continued...  At full throttle conditions fuel flows through the main nozzle, through a check valve and then into the venturi of the carburetor

USA ZAMA Does any other power equipment manufacturer use the BIS system?  NO  Only Stihl Products use the BIS system

USA ZAMA Troubleshooting

Trouble Shooting a BIS System 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.

USA ZAMA Trouble Shooting a BIS System 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

USA ZAMA Trouble Shooting a BIS System continued... – muffler clogged – throttle cable not connected – choke linkage not assembled correctly – various engine problems  mechanical problems  gasket leaks

USA ZAMA Trouble Shooting a BIS System continued...  Restricted fuel at high speed results in: – lower high speed – less power – poor acceleration

USA ZAMA Solutions to restricted fuel flow at high speed  Replace the high speed jet  Flush out the areas of the carburetor where dirt is suspected

USA ZAMA Trouble Shooting a BIS System 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

USA ZAMA Solutions to restricted fuel flow at 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

USA ZAMA Trouble Shooting a BIS System 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.

USA ZAMA Trouble Shooting a BIS System continued...  Run the engine at high speed  If the performance at high speed is still unacceptable, replace the high speed jet.

USA ZAMA R.P.M. TIME Lean Come Down Excessive Rich Come Down

USA ZAMA The end of the BIS mystery

USA ZAMA