Bill Babcock wrote
>
> My experience with constant velocity carburetors is all motorcycle based,
> but I expect the basic information remains valid. At no load, the carbs
> should always remain mostly closed, regardless of the engine RPMs. The
idea
> of a CV carb is to provide a relatively narrow range of air velocities
> across the jets so that the mixture can be consistently and precisely
> controlled. The only way to maintain similar velocity when the engine
needs
> less air (no load) is to reduce the size of the carb inlet. In other
words,
> that's what they are supposed to do.
>
>
> The drop rates and small changes in the opening at no load conditions are
> probably insignificant. When I used to dyno a lot of bikes with CV carbs
I
> was always surprised to see how load-sensitive the carbs were. Load up
the
> dyno at even moderate RPM and the carbs pop open.
Thanks
The motor cycle experience is just what I need.
OK I'll except this and figure a way to look at my carbs under load. It has
been a nagging concern in the back of my mind. We have made a few open hood
runs with popsicle sticks in the carbs, but only in first gear, with an on
board observer, and only at 3000 to 4000, carbs pop open with a throttle
change, then seem to settle back down to about the same height regardless
of constant engine speed (low load).
However, I am not sure I understand all of this.
If I look at the engine as an air pump, the volume of flow must be
proportional to the rpm. Double the RPM - double the volume pumped through
the engine.
Therefore, for a constant cross section (holding the piston still), the
flow velocity must double, ignoring turbulence and friction. Since the
velocity went up, Bernoulli's theorem indicates there should be a pressure
drop across the base of the piston and the holes in the piston will
propagate the pressure above the piston and raise it against the spring and
the piston weight. How does load, rather than demand, effect this?
The transient behavior, under load change, I have attributed to a sudden
change in pressure causing acceleration of the piston and over reaction.
Seems like I may again be wrong. Perhaps it is the load, not the change.
Need to find that chassis dyno.
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