As you may recall "we're all bozos on this bus"; there has never been a day in my life when it was not so.
More and more bikes are fuel injected these days. I'm going to explain with great simplicity how they work and how you deal with tuning problems with them - at the street-driver level. This is not meant to be a lesson on fuel injection for drag racers.
Fuel injection systems are simple, they are no where near as complicated as carburetors and they offer a greater range of adjustment for the street tuner than carburetors do. Not only is the range of adjustment greater but it is much easier to make changes with fuel injected engines than it is with carburated ones. In most cases you don't even need a screwdriver to tune an injected engine, your computer will do nicely.
Here are the important parts and ideas you need to understand to comprehend the inner working of injection. You have a fuel tank which holds fuel for delivery and accepts excess fuel that is being returned to it in some cases. There is a fuel pump which provides continuous pressure to the fuel in the system down stream from the tank. There is a pressure regulating device of some sort, and it may or may not return excess fuel to the tank but in every case it will precisely control the pressure of the fuel being delivered down stream of itself. You have fuel injectors at the end of the stream which are simply electronically controlled valves which allow a precise amount of gasoline to pass through them when they are opened.
Fuel injectors, the valves themselves, can not be opened part way. The valve itself is simple a metal plug with a hole in its middle. Inside the hole there is a tapered steel pin that seals the hole until it is lifted up by a small electromagnet. When an electrical charge is sent to the injector the pin is lifted up and fuel flows until the electrical power is cut, at which time a powerful spring shuts off the fuel flow instantly. So if no other changes are made the amount of fuel that flows through the injector and enters the engine is controlled by how long that electronic pulse continues. To add more fuel one needs to find a way to control the duration of that pulse length.
Obviously there is more than one way you can flow more fuel through the system. You could find a fuel pump that put out more pressure for instance. You could install injectors that had holes and pins that were larger in diameter and allow more fuel to flow through them when they were open. If the existing plumbing was too small and constricting flow you could install larger lines. You might install a regulator capable of delivering more pressure to the injectors. However if your pump, regulator, lines, and injectors are capable of delivering the maximum you need then there is no reason to install higher capacity units; enough is enough and excess is just that, a waste of your time and money.
Why would you want to change the amount of fuel flowing into the engine at all? Good question. Here is the mystery of engines, the amount of air that flows through an engine is not directly related to the speed at which the engine is running. You might think that an engine that used 100 cubic feet of air per minute at 500 RPM would suck through 200 cubic feet of air at 1,000 RPM - after all if you double the speed at which it is pumping air you'd expect it to double the amount of air it is pumping - but that is not how it works in real life. In truth the amount of air that passes through the engine varies with engine speed, air temperature, elevation, and all manner of other variables, not the least of which is the design of the curves of the tunnels it has to pass through on the way getting there. Its harder to predict than south florida weather. The point is that the amount of fuel that needs to be delivered for perfect combustion varies based on many factors and has to be dealt with.
Here is a major difference between diesel engines and conventional combustion engines and it has nothing to do with the presence or absence of spark plugs or the type of fuel used. With a diesel no attempt is made to restrict the inflow of air as a speed control mechanism. All control is exerted by fuel flow at the injector. With our gasoline powered motorcycle engines air flow is controlled by a butterfly valve. So when you twist the throttle two things happen, one is you allow more air to flow through and the second is you cause the injectors to stay open just a bit longer (nanoseconds) to allow more fuel to flow.
Control: The amount of fuel that flows is controlled by the open time of the injector and the open time of the injector is controlled by your electronic control unit (ECU). The ECU receives input from a number of sensors located around the bike. Things like engine temperature, intake air temperature, throttle position, crankshaft position, remaining oxygen in the exhaust system, and any number of other variables that the design engineers thought important (or affordable) to monitor and react to are constantly looked at and figure into how long the ECU allows the electronic signal that controls the injector to continue and in doing so controls how much fuel the engine receives.
So that's it in a nutshell. The tank holds the gas, it gets plumbed to a high but constant pressure pump, the pump's output is controlled by a pressure regulator. Fuel is delivered at exact pressure to a precise valve that is electronically opened and closed. An onboard computer monitors the engine conditions and the throttle to determine the optimum fuel flow, which it controls. All of the control is preprogrammed into the ECU. That's it.
So how do you tune one of these things? The answers are simple, you take control of the ECU and reprogram it, or you take control of the inputs the ECU receives and deceive it into doing what you want, or you take control of the output of the ECU and manually operate the injectors. The last way is impractical, the first way is expensive (new software or hardware), but the middle way is an achievable compromise.
By interrupting and modifying the inputs from the various sensors located around the bike you can fool the ECU into signaling the injectors to stay opened a bit longer to allow more gas to flow into the engine. Here is an example of how that works. With a carburated engine if its cold in the morning you probably close the choke when you start it. That is because closing the choke creates an artificial restriction to air flow creating a 'rich' situation needed to start a cold engine. In an injected engine the 'engine temperature sensor' tells the ECU that more fuel is required and so the ECU tells the injectors to stay opened just a nanosecond longer until the engine warms up. Well, as an example one way to fool the ECU is to modify the input line from the engine temperature sensor to constantly tell the ECU that the engine is running a little bit cool. In fact this is the most common way that most "controllers" actually operate.
From your point of view it works like this. There are devices that you can buy and plug into your bike that fit in between the bikes sensor inputs and the ECU itself. One such device which is commonly available for many different bikes is the "Power Commander" which is in its third generation at the moment, hence the designation PC-III. These devices, which are about the size of a cigarette pack are mounted next to your ECU and accept the plug from your bike's electrical system that supply input and they have a plug that connects to the ECU to give it modified input. You have the ability to remove the PC-III and preprogram it with "MAPs" which are available for download for free from the manufacture and many other sources as well. Different MAPs have been programmed to match various combinations of aftermarket components. Most of the MAPs available come from engine tuners who have had bikes up on dynos to develop the MAP itself. So you can get one of these sets of custom instructions for your ECU and not only that but you can then modify the MAP yourself with simple software that can also be downloaded for free. With the downloaded software a screen is presented to you that looks like a spreadsheet. You have engine RPM on one axis and on the other axis you have percentage increases in fuel flow. If you want it to run richer at 4,425 RPM you simply go to the spreadsheet cell for that RPM and insert a number which represents a percentage increase (or decrease) in fuel flow over stock that you desire. The same thing is in the software for igniting control. There is a spreadsheet showing your spark advance across the RPM band. These too are part of the MAP and these too are user adjustable. All of these adjustments can be made at various throttle settings too, it is not one size fits all. You can, for instance, increase fuel flow at zero throttle in the mid range of RPM to stop the bike from popping when you downshift but at the same time the device might increase fuel flow at the same RPM at 25% throttle and above. Although the amount of increase or decrease is limited (to 5 or 10% per cell as I recall) in effect you have infinite control over both fuel flow and spark advance. The way it is done is via a small adapter that uses one of those small square 9-volt batteries. You take the PC-III out of your bike, power it up with the little battery, and connect it via a USB cable to your computer (or take a laptop to the bike) and then do whatever you want with it.
And that's pretty much it. Here are a couple of links that probably explain it better than I do.
http://www.nightrider.com/biketech/motorcycle_efi_basics.htmhttp://www.powercommander.com/eng_default.aspx
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