Post by Mx Life286 on Sept 27, 2005 21:11:22 GMT -5
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.
QUESTION ONE: 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.
QUESTION TWO: 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 (and vice versa for a small cutaway).
QUESTION THREE: 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.
QUESTION FOUR: ARE ELLIPTICAL SLIDES BETTER THAN ROUND SLIDES?
Yes. Why? Here are six good reasons:
(1) Smoother venturi shape: Compacting 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.
QUESTION FIVE: ARE THERE TWO MIKUNI TMX CARBS?
Yes. The Honda CR250 comes with the original TMX carburetor--with a flat slide. The CR125, KX125, RM125 and YZ125 use a TMX with a slide that has a convex curve on the airbox side and a concave curve on the intake side. Mikuni claims that this new elliptic shape improves air flow on smaller displacement engines. By improving air flow, it's possible to run a carburetor equal in size to what is used on a 250.
The new Mikuni slide is similar in concept to the D-shaped slide used on the Keihin PWK carb. The two companies go about it with two completely different sets of curves.
QUESTION SIX: WHY DON'T MOTORCYCLES USE BUTTERFLY VALVES?
When you bury your foot into your Chevy’s floor boards it opens a butterfly valve. A butterfly valve looks like a flat circle that is centered over a pin. When you stomp on the gas pedal, the pin rotates and the flat valve turns on its edge and opens the throat of the carburetor.
Why don't they use butterfly valves on motorcycles? Because the current arrangement of modern motorcycle carburetors has more to do with happenstance than a plan. Motorcycles have plenty of free space above the carburetor. It's much easier and more contamination free to pull a slide open than it is to arrange the linkage it would take to modulate a butterfly system.
While the throat of a butterfly carburetor has much smoother sides than a motorcycles', you can't get past the fact that at full throttle there is a big cross-shaped obstruction spanning the carburetor opening. For the size of a motorcycle carburetor, a carburetor with a butterfly valve could never flow fuel as well.
QUESTION SEVEN: DO FOUR-STROKES HAVE DIFFERENT SLIDES?
Yes. Four-strokes demand tremendous air velocity. That strong suction force can easily override the carb’s slide spring's ability to return the slide from an open position. To keep the throttle from sticking, Keihin rolls their slide on wheels positioned on the top and bottom corners of the slide. Slide friction is virtually eliminated with this bearing-assisted system. It allows Keihin to get by using a lighter return spring and a throttle that has light action.
To absolutely guarantee that the slide will not stick, the Keihin FCR also incorporates a push-pull cable system. One cable is used to open the slide; the other forces it shut.
QUESTION EIGHT: ARE SLIDE CUTAWAYS MORE IMPORTANT ON FOUR-STROKES?
No. The slide cutaway isn't as essential to how responsive a four-stroke is. Since four-strokes create much less vacuum at lower rpm, they tend to rely on accelerator pumps to squirt gas into the air stream as the throttle opens.
QUESTION NINE: SO THE SAME CARBS ALWAYS HAVE THE SAME SLIDE?
No. Even though two bikes may use the same carburetor, they could have two completely different slides. The 38mm Mikuni TMXx 38 used on the 2003 CR125, KX125, RM125 and YZ 125 comes with 5.75, 5.75, 5.0 and 4.0 cutaways respectively.
QUESTION TEN: WHY DON’T MORE PEOPLE CHANGE THEIR SLIDES?
Cost. A Keihin FCR slide costs $120.
QUESTION 11: IS THERE A CHEAP WAY TO CHANGE A SLIDE?
Yes--by cutting the cutaway. It is easy to change a round slide, but much more difficult on the other slide shapes. Additionally, you can only make the slide cutaway larger (leaner). Once a slide has been cut, it's impossible to go back to the original (richer) cutaway.
QUESTION 12: HOW ARE SLIDES CALIBRATED?
Slides are calibrated in millimeters. A 4.0 slide has a 4mm cutaway, and a 5.75 slide has a 5.75mm cutaway. If you modify a slide, it will take a half millimeter cut before you feel a noticeable difference.
QUESTION 13: HOW IS THE CUTAWAY MEASURED?
It's measured from the highest centermost point of the cutaway down to the bottom of the slide. It's easy to do. Place the bottom of the slide flat on a smooth surface. Then measure the cutaway off the straight surface. This applies to round slides only.
QUESTION 14: WHAT ABOUT THE OTHER SLIDES?
You'll find that it's impossible to measure an elliptical slide’s cutaways using the aforementioned procedure. Flat, elliptic and D-shaped slides are all calibrate in millimeters, but the point at which the measurement is taken from isn't that clear. It's a measurement that is derived from a computer CAD/CAM design and not a machinist's measuring table.
QUESTION 15: WHICH BIKES NEED A DIFFERENT SLIDE?
There is only one production bike that comes stock with the wrong slide. The 2003 Suzuki RM250 is drastically rich from zero to half throttle opening. No pilot or needle jet and air screw adjustment could overcome the rich 6.5 slide. It takes a leaner 7.0 slide (with a leaner main and needle) to make the jetting work.