This article concludes in this thread. Tuning The CV
Carburetion on a Harley-Davidson is all about compromise. It’s rarely perfect because of barometric influences and the overlap of various fuel circuits, each with a specific primary function. The enrichener circuit richens the air/fuel ratio (AFR) on startup to aid in cold starts and engine warm-up. The accelerator pump squirts fuel into the carburetor throat/venturi when you first twist the throttle for acceleration and the transition off idle.
Let’s start with the idle and low-speed circuit. The throttle is closed at idle. Fuel passing through the slow-speed jet and idle mixture screw controls idle speed when the throttle plate is barely open. At low speed, when the throttle opens the plate additional fuel is directed into the carburetor throat venturi. The lowered position of the vacuum-operated slide/piston reduces the size of the venturi opening. The idle and low-speed circuit controls idle and assists the transition into midrange. This circuit controls 100-percent of idle and about 25-percent of right-off idle low-speed operation. However, the primary function of this circuit is the idle.
The midrange circuit slide/piston position is higher than its low-speed location. This movement occurs as the throttle plate opens and creates a stronger low-pressure area in the crab’s throat, which moves the piston upward. There is a tapered needle jet attached to the bottom of the piston that protrudes down into a tube located in the floor of the carb’s throat. As the piston rises, so does the needle jet, which allows fuel to rise up from the float bowl and into the air stream flowing through the carb’s throat to be atomized and mixed with the air. The higher the needle rises out of the tube, the more fuel rises up to mix with the air in the carb’s venturi. The midrange circuit controls steady throttle opening and lighter acceleration.
The high-speed circuit is activated by fully opening the throttle plate, increasing the low-pressure differential even more. This raises the piston slide fully and opens the venturi to its full size. Vacuum controls the slide that lifts the needle with wide-open throttle during hard acceleration. The high-speed circuit controls the amount of gasoline passing by via a main jet that the tapered needle slides up and down inside of. The needle jet opens to its maximum and allows in enough fuel to meet engine demands. When at wide-open throttle during hard acceleration, the main jet is the primary controller of fuel delivery. However, the needle controls fuel delivery for everything in-between idle and full-tilt boogie. It will start to meter fuel right off idle with light throttle. As the throttle opens, air velocity increases under the slide, and fuel wells up around the needle, even before the slide begins to lift.
Beware a Sportster CV Keihin carburetor on a Big Twin and vice versa since the stock jetting may be too rich or too lean if carburetor’s intended use is mixed up. This will not be a problem on a new or close-to-new stock Harley-Davidson. However, as carburetors wear or ground-up engine builds occur, used carburetors may be bolted onto the wrong engine. Some models received the CV Keihin carburetor mid-year in 1989. Use 1990 jetting specifications for these bikes.
Jet sizing identification leaves out the decimal point in most literature but this is not correct. The size of the jet is the diameter of the hole (orifice) in the jet that the fuel flows through. If the reader compares US jetting chronologically by year, it is generally evident that emissions controls get more restrictive as the years roll by, causing engines to run leaner with more air relative to fuel in order to burn the highest percentage of emissions possible. In 1994, 49-state and CARB bikes began to almost always use the same size jets, since jetting on all models became as lean as possible while still allowing the bike to function satisfactorily. As the year 2000 approached, the jetting richens a little, as it appears that carburetor jetting became too lean for the oppressed 45-degree, air-cooled, two-cylinder V-twin to run properly.
Throttle Setups
Attached to the carburetor is the throttle and idle cables bracket that holds the cables’ outer shielding in place. The bracket is necessary so that the inner cables can respond to the rider’s opening and shutting of the throttle without the outer cables moving. The inner cables mount in the round spindle below the bracket. This spindle is attached to the carburetor’s internal throttle butterfly plate via the throttle plate shaft. The CV Keihin throttle/idle cable spindle has a strong return spring between it and the carburetor body. Up to the middle years of the Shovelhead, H-D used a single throttle cable and depended solely on a return spring to always shut the throttle off as the rider manually shuts the throttle. The addition of the idle cable is a safety feature that ensures the throttle will respond to rider demands and close. Previous to the spring return or the CV automatic return action, H-D utilized the dead man’s throttle. This single throttle cable was a stiff solid wire with no return spring. The rider manually opened the throttle and manually closed it. If the rider fell off the bike, the throttle would stay in the last position, hence the name. Late Twin Cams eliminate throttle cables altogether in favor of an electronic activation system called fly-by-wire throttle. The rider still twists the handlebar throttle grip, but now electric pulses are sent to a servomotor on the side of the electronic fuel injection module. This sends a signal to the ECM to activate the fuel injectors.
Conclusion
Next month we’ll finish our discussion of the Keihin CV carburetor and start on the Magneti-Marelli EFI.
Donny Petersen
Tattoo Tony’s Heavy Duty Cycles
Toronto, Canada
www.HeavyDutyCycles.com Sidebar Honda Parts On Harleys
Keihin carbs are named after their Japanese manufacturer, which is a subsidiary of Honda, Harley-Davidson’s archenemy. This is not the first or last time a subsidiary of Honda or another Japanese company will supply Harley-Davidson with components. A few years earlier (1973), H-D began to use suspension parts, such as front forks, manufactured by another Honda subsidiary named Showa. A company named Kayaba made the narrow glide forks in 1973. Then there was the Hitachi starter motor that was superseded by the Nippodenso starter motor in 1989.
Sidebar Harley Davidson International
Different countries have regulations, such as varying emissions controls, that may differ from those in America. Harley-Davidson has always accommodated (reasonable) specific requests for production runs for a particular city or country orders, especially for police or military purchases. It was not until 1988 that the Motor Company began making explicit bikes for California to comply with CARB (California Air Resources Board) regulations. Camshaft configurations (duration and overlap), mufflers, carburetor jetting, and ignition fuel/spark maps will usually be different. For example, the Harley-Davidson International (HDI) carburetor for 1988-89 Big Twins (#27026-88A) are specific for the Swiss. Other countries like Canada use the 49-state bikes. The two main differences for a Canadian H-D and an American one is that the speedometers will be in kilometers and miles respectively. All late foreign Harley VIN numbers will start with the digit 5 while American ones start with the digit 1. HDI motorcycles and component parts are those sold everywhere but the US.
Feb.2010-Part V: Wrapping up the butterfly Keihin and starting the CV (concludes) 
Linear Mode
