This is the most precise description of the operation of the SU carb
that I have ever seen!
I started with an MGD (1932) (single SU which if I remember correctly had
NO spring) and progressed through a TC, a TD, an MGA, a 4 banger Healey,
various Rovers & L/Rovers, Jeeps etc and now to an XJS. I wish I had read
Dave Russell's description lo those many years ago.
Thank you Dave!
Robin Berens.
----- Original Message -----
From: "Dave & Marlene" <rusd@velocitus.net>
To: "Larry Colen" <lrcar@red4est.com>
Cc: <mgs@autox.team.net>; <british-cars@autox.team.net>
Sent: Saturday, March 22, 2003 12:55 PM
Subject: Re: measuring piston rise
> 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.
>
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