Airbox

[Bixxer Bob]

Just picking up on a question I had from a forum member and recent comment in here; here's my theory as to why the airbox integrity is important.   I'm assuming here that you already know about the stociometric value of 14.7 to 1 air/fuel ratio and how that is affected by temp / pressure.

At first look, you'd be forgiven for thinking the airbox seals don't matter that much, as there's a socking great hole at the front where the air rushes in; then it goes through the filter and down the throttle bodies into the engine.  I disagree, but as always, the point is open to discussion. 

Upstream of the air filter the air pressure is whatever is the ambient around the bike.  All that is important there, as far as the ECU and working out how much fuel to inject is concerned, is the temperature - as that affects the oxygen content.  Downstream, however, the pressure is monitored by the baro sensor inside the ECU which is connected to the airbox by a long plastic tube. 

While you're riding, the ECU looks at the throttle position sensor to see how fast you want to go, but also the engine temperature and air temperature to work out how much oxygen there is available.  In that calculation it also takes into consideration the air pressure, but uses the drop in pressure when the throttle is opened, to work out the engine load - which is another important part of the fuel calculation.

If you have an air leak around the oil vent tube at the base of the airbox, or the tube isn't connected, or there is an air leak around the baro tube elbow on the rear right of the airbox, or in the tube going to the ECU, the pressure drop will not occur or will be reduced, and so the engine load calculation will be wrong, and so will be the final fuel value because it's looking at the wrong part of the fuel map.  In short,  you could have the throttle wide open accelerating hard, but the ECU will think you are stationary revving the nuts off it but not actually going anywhere.

So, to sum up, next time you are in there, make sure all the seals are good.  But don't blow down the tube to the baro, it's a sensitive piece of kit and could be damaged.     You could try sucking though...