<VV> Carburetor Mystery

[Kenneth Schifftner]

Brian:

Some thoughts. Having designed venturi type gas scrubbers for decades, some of the “mysteries” of the function of a venturi became evident. The venturi physics are the same in a gas scrubber as they are in a carburetor.

The venturi’s restriction causes an increase in gas velocity that produces a reduction in the static pressure (creates suction). The larger the venturi throat, the lower the gas velocity, and the lower the produced suction. The idle circuit in a carburetor requires a certain amount of suction to deliver the fuel. Once the fuel is in the throat, the intent is to break apart the fuel by a shearing action. If the throat velocity is lowered by enlarging the throat, you get less suction and less shearing both at the same time.

If the suction (actually atmospheric pressure pushing the fuel through the carburetor fuel regulating circuits) is lowered, the margins for error based upon the fuel bowl level becomes critical. The carburetor become extra sensitive to fuel level.

Now a subtlety. The fuel viscosity is also an issue. Fuels with alcohols or additives also affect the fuel delivery. The fuel shears at different rates.

I’d guess (am I allowed a couple?) that you have a combination of more sensitive fuel level and to a lesser extent, some change in fuel viscosity.

Ken Schifftner