Hi Dave. When you can't answer the hard question, answer an easier
one. See my comments below.
David Littlefield wrote:
> Alright, all you SU experts out there...
This should cause some grief. ;-)
> I'm rebuilding the carbs on my racing Midget. It is a 1974 with the AUD
> 549 carbs. I've been told to use a "rich" needle for racing (engine is
> mostly stock, slightly hotter cam, according to Spec-Sprite specs).
Conventional thinking is that all fuel:air ratio settings are a compromise
as most cars get used for a variety of purposes and under varying
conditions. Your application is probably different. And while it is true
that a perfect 14.7:1 ratio is not correct for any application, most argue
that a slightly rich mixture is desirable for optimum power at high revs.
Perhaps a 12:1 average ratio. The power you enjoy at red line is paid
for with fouled plugs if you let it idle.
> The Victoria British catalog lists a "weak (lean)," "standard," and
> "rich" needle for earlier carb models, but for the AUD 549 it lists only
> two needles: the AAT and ABC. Neither are designated as "std" or "rich"
> or anything else.
IMHO, VB and Moss generally supply parts to folks who wouldn't
have a clue what rich and lean means beyond their investment portfolio.
They certainly don't supply distributors with any insight concerning advance
curves. My guess they mean 'lean' if you live in Vail and rich if you're
racing in Miami. Perhaps I'm wrong. It happened once before.
> The Moss catalog lists only the ABC, and calls it "spring-loaded." I'm
> not sure what the latter means since I haven't got the carbs apart yet.
> The only carbs I've rebuilt in the past are my MGA carbs, which are from
> a simpler time.
The spring loaded needle is just that; it is not firmly mounted in the
air
piston but rather held roughly in place by a spring. It can move around a
little and won't get scraped up my a misaligned jet. From what I can see,
that is it's only value -- except that it may be the right profile to give
you
optimum fuel:air ratio for your application.
> Questions:
>
> 1) Can I use an earlier carb model's "rich" needle in the AUD 549, or
> does the needle have to be "spring-loaded?"
> 2) Do I even need to worry about changing out the needle because the
> later ones worked better over all ranges (or some such)?
> 3) What's the difference between the AAT and ABC?
THIS is where you need the SU Expert requested above. I'm just
a physicist.
Best luck,
rick
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