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.
These carbs reduce the necessity for the approximations of multiple air/gas
circuits at various throttle opening (like Weber carbs have) that aim to
compensate for the effects of broad variations in throttle air velocity.
They also compensate for variations in air density due to temperature or
altitude changes.
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. One time in a fit of
mathematical weirdness I measured the differential pressure across the
diaphragm and calculated the full load, full RPM force on a Mikuni CV carb
on a Yamaha XS1 (650 cc twin) because I thought the closing springs for the
diaphragm might be too stiff. I was surprised to learn that the force was
potentially more than a few pounds. I don't recall exactly what it was, but
it's substantial--figure a three inch piston diameter gives you seven square
inches of piston area, even at one pound differential pressure that's seven
pounds of opening force.
|