Ask Lemmy

Constant velocity carburetors blew my mind when I learned how they worked, and I got the funniest introduction to them.

I had an Aprilia RS-50 motorcycle which had a slide-type carburetor. Instead of a coin-in-a-pipe throttle, this thing basically had a portcullis across the intake. Pulling on the throttle cable pulled the slide upwards making the aperture/venturi larger, allowing in more air, while also lifting a needle up out of the jet to allow more fuel in. It's a 2-stroke race bike, so you could easily bog down the engine if you opened the throttle too fast.

Then I bought a Ninja 250F, which has constant velocity carbs. Which also have a slide, AND a butterfly valve. The butterfly valve is operated by the throttle cable to control power. The slide is vacuum powered from the engine, and opens and closes the venturi to keep the air velocity through the carburetor constant, in order to keep the suction at the jet constant. It also has a needle in the main jet which it lifts along with the slide, so the needle's taper meters the fuel mixture for the amount of air going through the carb. This inherently compensates for air density; if the air is less dense the vacuum mechanism can't pull the slide open as far so the slide doesn't open as far, and neither does the needle valve. So it automatically maintains the mixture.

Which is why using constant velocity carburetors on the Rotax 912 engine is such a brilliant idea. A carbureted airplane engine with no cockpit mixture control.