Finding someone familiar with your fuel injection system is going to be hard to do because there weren't that many of them even when it was new.
If all four plugs are sooty, then it's not an injector. If one is sooty but the others are ok, then it's likely a dribbling injector. the best way to clean them is to use an ultrasonic jewelery cleaner, using rubbing alcohol in it and immersing the tips only. I run mine for three, 3 minute cycles.
If it's not an injector, then there are several things it could be. A bad sensor as you say, the throttle position sensor needs to be adjusted, the pressure regulator is bad or the check valve in the return fuel line at the bottom of the tank.
It could be but I don't suspect it's the atmospheric pressure sensor. If it's the ambient temp sensor, it will run better when the weather if hot or cold but not both. If the coolant temperature sensor, it will either run good when the engine is cold but bad when hot or vice versa. There are resistance checks you can make to test these components. However, since the DFI computer doesn't show any fault codes, II suspect it may be oneof the next few things. Just remember that the engine must be running for the computer to display the fault codes, as soon as you shut the engine off, they stop displaying. Unlike modern vehicles, the computer doesn't save the codes.
The fuel regulator sets behind the cylinderblock and has three hoses attached. You need a pressure tester to check the pressure. The pump itself will produce around 50-60 psi, but past the regulator is should run approximately 33 psi at idle and 36 psi with the throttle open. With the engine off, it should show 36 psi. If it considerabaly more or less, it's bad. You can buy aftermarket adjustable regulators for $35-$45, much less than the OEM.
If the pressure is ok, it may be the check valve in the return line. If it's restricting flow, that will increase the pressure, increasing the flow through the injectors. Take it off and clean it well with carb cleaner
The TPS need to be adjusted periodically, but finding the factory test "checker" is hard to do. If you want to try to adjust it yourself, remove the "DFI" cover to expose the TPS. Spray paint the ears where the Phillips head set screws are, this it to accurately mark the present location. Turning it just a tiny bit counterclockwise will lean it out, clockwise will richen the mixture. It takes only a very tiny bit of movement. Too lean and when you whack open the throttle (but not actually moving the bike) and the enigne will hesitate. Too lean on the road and when you chop the throttle, it will die. Painting the ears will allow you to return it to the orginal position if necessary. When someone added the less restrictive K&N filter, they may have adjusted the TPS to richen up the mixture a bit.
Many people removed the DFI system and replaced it with carbs bach then. You're lucky to have yours unmolested. Hang on to it and take care of it. If you need more info, email me.
Oh, DO NOT run 12V to the injectors to "check" them, you'll fry them. They run on 3V.
