® The Venturi Effect Revisited

Everyone has heard of the Venturi effect. We were all taught it in high school, and most of us link it to how a garden plant sprayer works. We may even have vague ideas that there is something having to do with a venturi going on inside our carburetors. And of course, there is. Eighteenth-century Italian physicist Giovanni Venturi revealed to the world that a medium moving through a confined area loses pressure in proportion to any increases in speed. The carburetor uses this princple, as does the plant sprayer, to pull fluid toward the area of reduced pressure. Neat and simple.

An area where we don't naturally expect the Venturi effect to show itself is at the carburetor's float bowl. But it is potentially there nonetheless. A classic example of an unwanted Venturi effect is in first-generation Honda V4 carburetors, whose float bowl vents were designed routed to the airbox, for evaporative emissions reasons. The problem is, even the first year (1982) models exhibited surging at highway speeds due to the resulting engine's direct effect on the float bowl's fuel height. The fuel's height actually oscillated up and down at high engine speed, producing the surging. Honda directed field agents to make corrections at the dealership level that first year with inserts pressed into the bowl vents designed to dampen, but not totally remove, the engine's vacuum influence on the fuel levels. These inserts subsequently were installed during production in all models through 1985. More than ten years after that first fix, the real solution finally appeared in a handful of factory Honda bulletins in which it was encouraged that the float bowl vents be disconnected from the airbox for improved engine performance.

It's interesting that Honda had to relearn this lesson, for they went through this same thing several years before on the 1977 CB750K. This machine's float bowl overflow/drain hoses were made too long and hung down so far below the engine that they were in the under-bike airstream. At very high speed a Venturi effect occured at the hoses, negatively affecting the fuel levels in the bowls, resulting in inconsistent levels and thereby engine performance. Venturi strikes again!

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