Larry,
You are correct, the top of the piston gets vacuum from between the
butterfly & the piston.
It is designed to maintain a CONSTANT velocity through the carb throat.
This velocity is determined by the spring pressure balanced against the
vacuum. As the engine tries to pull more air through the carb the
velocity would increase through the throat, except that the vacuuum on
the piston top increases & raises the piston so that the velocity
remains the same as it was before. The spring weight is set just enough
to provide an adequate fuel flow vacuum signal & still not restrict air
flow too much at wot.
The spring weight is adjustable through a few ounce range to permit fine
tuning. Typically 2.5 oz for the blue spring to 12 oz for the green spring.
The spring weight should not have much effect on acceleration response.
The damper controls how fast the piston moves. When the butterfly is
opened the damper causes a delay of piston rise. This causes a momentary
higher vacuum on the main jet to give a richer mixture for a short time.
Just like an accelerator pump on some other carbs. If you are having
acceleration stumble try a heavier oil in the damper chamber. (Delays
piston rise for a longer time & provides a richer mixture for a longer
time.) The damper only provided resistance on opening. There is no
damping when the piston closes.
People have tried all manner of oils in the damper. The SU recommended
oil is SAE 20. The oil viscosity can be varied from light to heavy as
needed to obtain good throttle response.
Dave Russell
Larry Colen wrote:
> I just realized that the HIF 6 in this application seems to have a
> vacuum port that is plugged off. If it leads to where I think it does,
> it gets the vacuum from between the butterfly and the piston, which
> means that it would "measure" the amount of vacuum that the piston is
> exposed to. Am I reading this correctly?
>
> I've been experimenting this week with using the green spring rather
> than the yellow (12 oz rather than 8oz), it seems to work pretty well,
> except for sometimes when trying to accelerate from a very low load
> situation, in which case the car just doesn't want to do anything,
> except possibly die. If I were a little less restrained on my
> sanity/time/research budget I'd experiment with a progressively wound
> spring on the carb piston. But if I were going to that much bother,
> I'd probably do better with a closed loop fuel injection system.
/// british-cars@autox.team.net mailing list
/// or try http://www.team.net/cgi-bin/majorcool
/// Archives at http://www.team.net/archive
|