Tuning the Carburetor

When you open the throttle in a round or flat slide carburetor, in which the slide is raised by the throttle cable, the engine draws more air through the carburetor.   Since a large quantity of air must move faster than a small quantity of air through the same diameter venturi, the air speeds up and slows down in the venturi as the engine demands more or less airflow.   And when you "whack" the throttle wide open, you get a sudden, radically lean condition.

In a constant velocity carburetor, however, in which the slide is raised by airflow, the venturi automatically gets smaller when the engine isn't drawing much air, and gets larger when the engine draws more air.   This keeps the speed of the air close to the same value at all throttle openings, and provides a more consistent metering of fuel in the venturi.

When you "whack" the throttle wide open in a CV carb, the slide can't come up any faster than engine vacuum will allow, so you don't experience that lean bog.   Since a CV carb must "wait" for engine vacuum to pull the slide up, you'll get a delay in the engine's response when you twist the throttle open, like the throttle is "disconnected" from the engine.   A pumper carb, however, shoots a stream of gas when you twist the throttle, so you feel the "hit" immediately -- the "squirt" not only offsets the lean condition that occurs in slide carbs, but gives you a strong, immediate response.

The stock KLX-300 has a constant velocity carburetor.   You get better fuel economy, cleaner emissions, less sensitivity to changes in altitude, and some hesitation but no lean "bog" when you whack the throttle.

Although a CV-equipped bike doesn't go rich at altitude the way a fixed jet carb does, it will lose power due to the lower air pressure, which is reduced about 10% for every 3,000' gain in elevation.   With 10% fewer oxygen molecules going into the engine on each intake stroke, we should reduce the fuel by 10% to get back to that "ideal" fuel-to-air ratio of 12.5 to 1 for maximum power.   But how do you translate these "numbers" to a change in needle position or a different main jet?   And there are lots of variables: how the reduced air density affects the venturi, how the lower air pressure affects the fuel in the fuel bowl and so forth.   So perhaps we are left with a simple rule of thumb...

Expect rich running, stalling, hard starting and irregular idling at slow engine rpms when you take your KLX on that Colorado trip.   Screw in the idle screw, lower the needle and fit a smaller main jet...

The pilot circuit mixture adjust screw on the Keihin CV carburetor is properly termed a idle gas screw rather than an idle air screw.   An idle gas adjustment screw is located "downstream" in the pilot circuit from the idle/slow gas jet.   An idle air adjustment screw is located "upstream" in the pilot circuit from the idle/slow gas jet.   Most carbs use only one or the other but not both.

An idle gas screw regulates how much air/fuel mixture is allowed to enter the mostly pure air flowing in the carburetor throat at the intake manifold side of the carburetor.   By backing the screw out, more air/fuel mixture is allowed to enter, which results in a richer idle mixture.   The air/fuel mixture flowing within the pilot circuit is set by the pilot/slow air jet and pilot/slow gas jet when a gas screw is used in design.

An idle air screw regulates how much air enters the pilot circuit.   By backing the screw out, more air is allowed into the pilot circuit, which leans the air/fuel mixture in the pilot circuit.   This in turn leans the resulting idle mixture when the pilot air/fuel mixture enters the mostly pure air flowing in the carburetor throat at the intake manifold side of the carburetor.   Because of it's location, the idle air screw does not appreciably change the over-all amount of air/fuel mixture flowing in the pilot circuit as the idle air intake is only very slightly modulated by the screw.   It only regulates the mixture.

Idle gas screw in front => Turn it out for enrichment.
Idle air screw in rear => Turn it in for enrichment.