It's a cheeezy way to richen up the fuel mixture
Think about a carburetor - when the engine is cold the choke comes on, the choke plate closes up and reduces the amount of air coming into the engine. By effect, this makes it richer. The engine needs to be richer when it's cold.
So with EFI you need to do the same thing. The computer (ECU) needs to know if it's cold. The way it determines this is primarily through the coolant temperature sensor. The secondary way it changes air/fuel ratio is through the IAT (Intake Air Temperature) sensor.
But, the main way is through the coolant temperature sensor. The coolant temperature sensor is a NTC resistor. This means it changes resistance depending on temperature. The ECU has an algorithm built in that says, 'if resistance =x then I'm cold, if resistance =y then I'm warm and if resistance =z then I'm hot'
Based on the resistance of that sensor, it checks it's program and makes the fuel richer or leaner depending on whether it thinks the engine is cold or hot. It does this by leaving the injector open longer which lets more fuel in.
So if you install a resistor in-line on that sensor then it will cause the ECU to always think it's cold. If the ECU always thinks it's cold it's going to always richen up the Air Fuel mixture.
It really is a hack. The commonly used device to do this more 'properly' is a Power Commander which is a piggy-back ECU that sits between the stock computer and the wiring harness. It works much the same way (a hack) by intercepting the injector signals from the stock ECU and then altering them as you see fit. This does the same thing but in a better way in that you can adjust it over the full range of operation.
When you really boil it down to basics you're fooling (fueling?
) the ECU into leaving the choke on - on a permanent basis You can do this, it's certainly not expensive, but you have to cut your wires and experiment. Another way to skin the cat is to install the injector from a CBR150r. This injector flows more fuel then the CBR125r injector. I've done this and it's really remarkable how much power it makes on the high RPM side. The stock ECU is designed to use the 125cc injector so it just operates as if it has one. When the engine is hot it uses the oxygen sensor to adjust the fueling so it will trim the open time of the injector as required. However, when the throttle is wide open the stock ECU goes open-loop and ignores that sensor and defaults back to 'max performance' mode and the bike really flies.
I have one here that I will be putting up for sale at some point. It's new OEM with only about 1 month of runtime on it. If you want to try it, let me know, otherwise I'll be putting it up pretty soon in the store. They're hard to get. I need a bigger one yet as I have a need for more fuel flow during nitrous operations
