Joe Random net.harley.guy asks:
"Would one of you netters be willing to explain what a 'constant velocity carb' is as opposed to another type of carb."To which cheezie@ezl.com replies:
I'll try...
There are two types of CV carbs that I know of for HD's and they are the SU type and the present day stock HD type. I won't go into the SU types....(personal opinions deleted)
Basically the CV (also called a variable venturi) carb's air flow is controlled by a throttle plate (butterfly) just like the fixed venturi carb (S&S and earlier HD carbs)....the major difference is that a CV uses a slide that varies the size of the venturi and a fixed venturi has a fixed venturi. This prevents feeding more fuel than the carb can use which in turn leads to better emissions and fuel economy but personal opinions again deleted.
The slide is controlled by positive and negative pressure differences. A rubber diaphragm above the slide seals the two pressure chambers away from each other...positive atmospheric pressure in the bottom chamber and negative (vacuum) in the top chamber. The negative pressure reaches the top chamber (above the diaphragm) by a hole drilled thru the slide. Air moving past the hole creates a negative pressure above the slide. The positive pressure pushing towards the negative pressure (against the downward spring pressure) raises the piston further up in the venturi.
This is done in proportion to the pressure differences between the upper and lower chambers of the diaphragm and automatically keeps the air at a constant velocity. As the slide raises so does the jet needle in the needle jet cause it's attached to the bottom of the slide...this controls the fuel that gets out of the jet into the motor.
The throttle plate controls the amount of air and the slide controls the velocity of the air.
Keep in mind that the throttle cable is attached to a butterfly and not to the slide itself...if the cable were attached to the slide this would be termed a "mechanical slide, variable venturi carb" (like a Mickey Rooney).
Thanks to "Happy Motoring from cheezie@ezl.com" for the answer.