I thought I'd touch on this subject as many riders that I talk to get a glazed look on their face when we get on to the subject of air-bleeds & air jets. I do not profess to be the expert on the multiple styles of carburetors but it might help with this basic outline on the subject, any with more knowledge, please feel free to add :-)
Let me start by giving a fuel curve situation, I'll talk 'main fuel jet / main air jet circuit', I imagine the pilot circuit would have to operate the same?
Imagine your engine running at 6,000 rpm at full throttle, you select the main jet for the right air/fuel ratio, now that is great at 6,000 rpm but what mixture is coming into the engine at 10,000 rpm? At the higher engine speeds the drop in atmosphere pressure in the venturi (because the faster the air moves, the lower its atmospheric pressure) allows the constant 15 psi in the fuel bowl to push more fuel into the venturi enrichening the mixture more than is liked by the engine.
Remember, you are still at WOT (Wide Open Throttle) and the size of the main jet cannot change as the rpm rise. You have WOT at 6,000 rpm and WOT at 10,000 rpm.
To reduce the mixture going excessively rich, air is bled into the circuit, after the main jet but before the venturi. In my younger years I only ever thought it was to turn neat fuel into a mist to help with combustion.
Now I only know part of the story but I guess as the rpm rise, the size of the main air-bleed interrupts the variance in the drop in atmospheric pressure in the venturi verses the 15 psi in the fuel bowl? So juggling the size of the fuel-jet verses the size of the air-jet will alter how rich/lean the mixture starts out at low rpm and how rich/lean it ends up at a higher rpm (for a set throttle opening).
I only discovered this after learning the basics in an old English Performance Bike's magazine and watching an exhaust air/fuel reading on a dyno as my bikes were revved-out.
So, your bike is running ok, you decide to take out the 'main air jet', what happens to the fuel curve? You'd find in the very high rpm, or an 'over-rev' situation that the engine would be leaner, whether you noticed it was too rich or lean in the first place I don't know but if was already lean you have just made it leaner. While many of us respect our engines enough not to hold the high rpm for very long, we don't suffer the issues that would come with a much lean mixture at that point.
