Originally posted by: Doctorturbo
I don't know the quad or the engine but if I had to take a wild guess I'd say:
EFI cars have a lot of sensors. These sensors give the puter feed back to tell the total management system what to do. The EFI on quads are not like that on modern cars. They are very crude.
To give the puter feed back on a car, you need a water temp sensor. Some even have air intake sensors and more. These sensors will tell the puter to tell the injector that it is cold out and to give the engine more fuel. When the engine is cold it goes into "open loop" which just means the puter does not listen to the sensors and it just deflaults to a pre-written program.
When all warmed up the O2 sensor and many more will tell the puter that the injector is giving the engine too much fuel and to cut back. When all sensors are in the game that is called "closed loop".

I know of no quad (yet) that has an O2 sensor. (there may be but I don't know of one) Just a guess, but maybe the Suzuki 450 does not have some of these sensors because they are trying to cut down on weight? Just simpler to add an enrichener valve?
Maybe they are worried about reliability? Even on relatively clean and vibration free roads, car sensors go out from time to time. Leading a race would not be the time to have a sensor go out.

The way some of the people on this board use their quads as boats, they are going to be in for a shock at the cost of a fix if silt and water ever gets into the fuel system. It won't be cheap!!!!!