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Combustion process Timing Fuel Management HP & Problems

Fuel management is one of the most important factors in the combustion process.  Maintaining a proper Air to Fuel ratio is crucial to getting the most out of your motor and also to prevent damage.   It is important to understand what the proper Air/Fuel ratio is and how it needs to change when you do different modifications. 

Air/Fuel Ratio:

The "Perfect" Air to fuel ratio is 14.8 parts air to 1 part fuel.  That is the point at which perfect combustion happens.  This ratio is also called the Stoiciometric ratio.  In the real world this ratio is too lean for normal combustion engines and produces way too much heat which does nothing for efficient production of power.  The heat produced by a lean mixture can also cause engine damage.

The "Optimum" Air/Fuel ratio is somewhere right around 13 parts air to 1 part fuel (Between 12.5 and 13.8).  This mixture produces the highest horsepower with a safe amount of heat.   The richer the mixture (lower air number) the lower the heat production.  If the mixture is too rich, too much cooling takes place which reduces horsepower as well.

So in an "Optimized" engine the fuel would be completely mixed with approximately 13 parts air to 1 part fuel.  That is our goal complete mixture of the optimum A/F ratio.

[Insert A/F curve here]

How to manage the Air/Fuel Ratio

Whenever you change the amount of air flowing through the engine in the combustion process,  you must change the fuel being sent to the engine as well to maintain the A/F ratio.  Carburetors or Fuel injectors send atomized fuel into the air being drawn into the cylinders.  


Air flow:  A carburetor works kind of like an air control valve.  As you open the throttle,  you open a butterfly valve that reduces the resistance to the flow of air.  The cylinder is at a vacuum so it is trying to draw as much air in through the carburetor as it can.  As the butterfly is opened the resistance to flow goes down and more air flows into the cylinder.  Another phenomenon occurs too.  The pressure in the throat of the carburetor gets lower or closer to the vacuum in the cylinder.  

Fuel flow:  Carburetors normally have a fuel reservoir that is connected to the fuel pump.  The reservoir is maintained at a constant level by a float arrangement.  (Exactly like the fill float in a toilet tank).  The reservoir is called the bowl.

   There is a series of tubes which dip into the bowl.  Within those tubes are metering devices called jets.  The tubes/jets connect from the bowl to the throat of the carburetor.  Each jet is designed to work at a different range of demand by it's sizing and placement.   The jets in a motorcycle carburetor are generally called the Pilot Jet, the Needle Jet, and the Main Jet.  

  The Pilot Jet (also called the idle speed jet)  is the smallest of the jets.  It connects the bowl to the throat of the carburetor on the engine side of the butterfly valve.  This allows the engine to idle without any throttle applied.  The pilot jet accounts for approximately 0 to 1/4 throttle.  The pilot jet is normally adjusted by a screw type adjustment.

  The Needle Jet is the moving jet.  It is operated by a slide mechanism which is either controlled by vacuum or directly by the throttle.  The needle jet connects to the throat of the carburetor on the air box side of the butterfly valve.  The needle jet controls fuel in the throttle range of 1/4 to 3/4 open.  The needle is tapered and acts as a throttle valve for fuel.  It will operate at the same rate as the butterfly to meter fuel proportionally with air flow.  The needle jet is normally adjusted by changing the needle to a different taper needle and/or shimming the height of the needle using clips or shim washers.

  The Main Jet (Also called the full power mixture) sits deep in the bowl of the carburetor.  It's inlet to the carburetor is near the throat entrance.  This location makes fuel flow through the main jet totally dependent on air flow through the carburetor.    As the throttle butterflies are opened less throttling of air occurs which allows more air to flow from the air box to the cylinders.  As the air flows across the main jet,  a proportionate amount of fuel is drawn up through the jet and atomized into the air.  The process is known as the Venturi Effect.  The main jet operates in the throttle range of 3/4 to WOT (wide open throttle).  The main jet is adjusted by changing the size of the main jet.

All of the jets work in conjunction to give relatively smooth power delivery while maintaining the correct air to fuel ratio.



Fuel Injection:

Fuel injection is relatively new on the scene of motorcycling.   Fuel injectors, managed by the motorcycles electronic control module (ECM),  provide a much more accurate control of air to fuel ratio.  This allows a much smoother delivery of power.  

Fuel injector theory is much the same as carburetor theory except that instead of jets there are fuel injector nozzles.  The amount of fuel sent through each injector is governed by a preset map in the ECM.   At a specific throttle position, gear, and RPM the injector is directed how much fuel to "squirt" into the throat of the throttle body.  This "squirting" is known as an injector pulse.  The duration of the pulse determines how much fuel is "squirted" in.   The injector map in the ECM is usually compensated by sensors such as atmospheric pressure, air box pressure, and sometimes engine temperature.  The injector nozzle serves as a fuel atomizer and helps to ensure complete air to fuel mixing.  


[Insert Fuel Map example here]

If modifications are done to the bike which change the air flow,  the fuel flow must still be changed to maintain the optimum A/F ratio.   To change the fuel flow on a fuel injected motorcycle, the fuel map in the ECM must be changed within the ECM or altered after leaving the ECM.   There are a number of methods and products on the market for accomplishing this.  Some may require a dealers assistance in remapping the ECM and others allow the user to set the map themselves.

Power Commander

Yoshimura EMS



Combustion process Timing Fuel Management HP and Problems


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