How carb diaphram works  By  jjoshua20213 (Read 1,743 times)

The "CV" in a "CV carburetor" stands for "Constant Velocity". This type of carb design more precisely controls fuel flow over varying throttle by attempting to maintain a constant airflow velocity across the narrowest portion of the venturi... which is right at the bottom of the slide, where the needle and needle jet reside. Maintaining a constant airflow velocity provides a steady reference of pressure/vacuum over the fuel delivery ports - that steady reference aiding in more precise control of fuel delivery over the carb's operating range (i.e. - lower emissions, lower fuel consumption).

To do that, CV's incorporate the flexible diaphram (seal) as a means of moving the slide up/down, via the pressure differential between the (more or less) atmospheric pressure in the airbox and the low pressure (vacuum) that exists right there, at the very bottom of the slide (narrowest point of the venturi).

The top of the diaphram "sees" that vacuum via a hole (port) in the bottom of the slide - and that vacuum is thus applied to the entire (otherwise sealed) topside diaphram/slide chamber. That vacuum is what lifts the slide against the slide's spring, and gravity.

In an enclosed tube, as airflow velocity increases, air pressure decreases proportionally (Bernoulli's Principle). So as you open the throttle (butterfly valve), the airflow velocity through the carburetor increases, pressure at the bottom of the slide drops (vacuum), and that vacuum is applied to the top diaphram chamber - lifting the slide, and pulling the tapered slide needle further out of it's opening (needle jet) to deliver appropriately more fuel.

Of course, the bottom chamber side of the diaphram must not be sealed off, in order to allow it to move freely and find it's steady, proper "place". So the lower chamber has to be open to somewhere - and since the engine's entire process of "breathing" is dependent on the environment's atmospheric pressure in which it's operating... then atmospheric it is. That's what the elliptical port is that Ally circled in the pic above - the lower-side diaphram's reference port to the atmosphere.

So the slide's position at any given engine intake demand is determined by the balance between vacuum at the bottom of the slide and atmospheric pressure at the carb mouth inlet - and to a lesser degree, the weight of the slide assembly and the slide return spring. That balancing act of pressures is what maintains something close to a constant airflow velocity across the narrow point of the venturi. A simple feedback-based governor system, really.

For most CV carbs, a quick and easy check of the slide diaphram for sealing integrity is to gently push the slide all of the way up with an index finger, and then tightly close off the elliptical port with the thumb of your free hand (lick your thumb first, for a good seal ). Then release the slide - it will drop part way, but then should hang partially raised, more or less until you remove your thumb to uncover the port. A leaky diaphram will continue to drop when the slide is released.

Bob