Originally posted by Mister Tee
For some odd reason, I don't know why, I've been getting mileage in the high '40's (nearly 50 mpg on one long ride) on my 05 750 this last Summer, but for some reason my Winter mileage really sucks! (below 40.) I don't know if it's the gas or what - I just replaced the air filter and the plugs are fine.
Motorcycles run on an open loop system, in other words, there is no oxygen sensor in the exhaust to determine how complete the combustion was, and adjust fuel accordingly. Therefor the main determining factor in how much fuel your engine is getting is how much air its breathing.
The way the ECU measures airflow is pretty simple. A hot filament (sometimes wire, sometimes grid) is placed somewhere in the air intake. This filament has a set ammount of current flowing thru it, that is heating it up. Also, the filament is made out of a material that changes its electrical resistance based on its temperature (all materials actually do that, but thats not the point). The filament is cooled by the air thats passes over it on its way to the engine. Since the filament is heated at the same rate, if you measure its temperature (by its resistance) its possible to tell how much this filament is cooled, and therefor how much air is flowing past it. In theory. There are however some limitations. While the ammount of air is the most determining factor, the air temperature also plays a role. Since the ECU has no way of knowing what the air temperature is, it simply assumes a certain temperature. This assumed temp. is set to somewhere which is considered ideal, in other words, summer, when most people ride their bikes. When the air it too cold, the system doesnt work as well.
There are other factors as well. Fuel is denser when its cold, but ecu measures the volume of fuel injected into the cylinde (Actually it keeps a constant pressure, and measures the pulse duration). Thus the engine shoots more fuel (since its denser) to mix with colder, denser air, which is improperly measured.
One thing that cold is nice about, is the crisper throttle responce that it brings along.