Hey Bilger,
Most KX 5's came stock with a #7 slide.
The Slides aren't cheap, the last new one I bought was about $100 shipped.
They are just another tool used to tune your bike,
Here's a little bit about slides I got from an article in MXA
Tuck\o/
When you spray a bike with a garden hose, you use your thumb to modulate the spray. If you squeeze your thumb over the tip, the water shoots out harder, faster and farther.
A carburetor?s slide works just like a thumb over the open end of a hose. Just as you increase the force of the water by decreasing the size of the opening, the same can be done with the fuel flowing through a carb.
HOW'S MY CARB LIKE A GARDEN HOSE?
From the crack of the throttle through 1/4 throttle openings, the slide cutaway determines how much fuel flows into the engine. It doesn't do this by allowing more air into the engine--that duty is handled by the straight, front edge of the slide. It does it by changing the velocity at which the air streaks under the slide; just like you do with your thumb over the hose.
AND THE VELOCITY DOES WHAT?
The velocity of the air draws gas through the pilot and needle circuit. Since there isn't much air velocity at small slide openings, a cutaway is used to create more air force.
A large cutaway results in less velocity, less fuel and a leaner mixture.
A smaller cutaway increases the velocity of the air and pulls more fuel producing a richer mixture.
DOES THE SHAPE OF THE SLIDE MAKE A DIFFERENCE?
Pre-'85 vintage bikes and a few of the new minicycles, 60s and 50s use carburetors with round carburetor slides. All current, full-size motocross bikes use D-, flat- or elliptic-shaped slides.
ARE ELLIPTICAL SLIDES BETTER THAN ROUND SLIDES?
Yes. Why? Here are six good reasons:
(1) Smoother venturi shape: Shortening the fore-to-aft thickness of the slide reduces the amount of the sharp machined edges where the slide and carburetor bores intersect.
(2) Improved slide physics: Turbulence is reduced by shortening the distance the air travels as it slingshots around the rough bottom edges of the slide.
(3) Lighter action: The new slide shapes are lighter and have much less surface stiction.
(4) Improved seal: Less stiction allows for a tighter fitting slide. A tighter slide reduces blow by and effects more precise jetting.
(5) Improved jet layout: A more harmonious relationship between the position and the flow of fuel from the mixture screw, pilot, needle and main jet circuits.
(6) Reduced carb length: The squashed slide valve results in a slimmer carb that positions the fuel circuits closer to the cylinder's induction system.
All six aspects add up to a less traumatized fuel flow and a bike that accelerates harder.
More than you probably wanted to know,
Tuck\o/
