Changing the jet size as you go up in elevation helps to maintain the correct fuel/air ratio, so the engine does not run too rich. With the rise in elevation comes less dense air (less combustible oxygen) being drawn into the engine. Without a jet change, the engine would flow too much fuel and possibly foul a spark plug or have other running problems.
Modern fuel-injection systems use an oxygen sensor to provide feedback to a computer that can adjust the amount of fuel on the fly, and maintain the proper fuel/air ratio.
There will still be power loss on pretty much any engine with an increase in altitude, but by adjusting the fuel flow, the engine can at least operate with the proper fuel/air ratio.
A little off-topic, but maybe some of the aviation-types/pilots out there can describe how the mixture control works on small aircraft with carburetor-style engines? I'm guessing it allows for manual control of the amount of fuel flow to best match the altitude?
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I work for Honda, but the preceding is my opinion alone.
Robert@Honda
