agoblez wrote:
>
> I've had my Mini Cooper for 3 yrs now, I've bought every book that I
> could find on S-U carbs and still can't get mine tuned to what I
> think is correct and proper.
> They have been completely rebuilt.
> Could anyone give me that magic formula?
> The raising and lowering of the jets to effect rich/lean makes sense
> to me, but, the adjustment of the butterflys effecting the amount
> of air into the combustion chamber remains a great mystery.
>
> Any replies would be greatly appreciated!!!
Here's the scoop on balancing twins; this should only be done after
the mix is balanced and correct.
Let's review the mix settings: first one must make sure the jets are
perfectly centered; I prefer to do this upside down on the bench,
since it's often damned hard to get them centered, since when you
tighten the locknut, the jet assembly often nudges off to one side.
To test centering, turn the carb right-side up and lift the piston
and let it fall. It should fall freely, all the way to the carb
body, and make a sharp little metallic clunk as it hits the carb
body. If it stops with the piston above the carb body, it usually
means the jet assembly is off-center and the needle is sticking on
the side of jet or the jet is sticking up above the carb body.
To test the mix and get ready to balance the carbs, loosen one of
the little pinch nuts on the wavy-metal linkage between the throttle
shafts (butterfly, not jets), so the throttle shafts are no longer
linked. Back one carb's idle screw all the way off so there's no air
flow or at least *very* little. Set the idle screw on the other,
working, carb to a low idle (700 to 1000 rpm) Then test the working
carb's mix by lifting the piston (use the little peg, or a flat
screwdriver) about 1/8 to 3/16". The idle speed should increase
slightly, then fall off. If it falls off immediately, it's too lean;
if it raises and stays raised, it's too rich, since raising the
piston leans the mix slightly. The mixture should be just a bit
richer than the perfect stochiometric mixture, to protect the
pistons and valves and seats from melting. That's why when you lift
the piston and the mix leans, it initially speeds up just a bit, but
then it falls off, since it has become too lean.
Once that's all sorted, shut off the idle screw on that carb and set
the same low idle on the new working carb and do the same test by
lifting that cylinder.
Once this is all perfect, set the idle screws on the two carbs so
the same amount of air is being pulled into each carb. This can be
determined by using airflow sensors, like Uni-Syn (tm) or a
listening device. I use a small rubber tube, one end in my ear and
the other held to the mouth of each carb, trying to listen to the
same place on both carbs, adjusting the idle speeds until the flow
is the same in each carb as evidenced by the sound through the tube.
Now, here's the part often neglected: you must tighten the little
pinch nut in the throttle shaft link to lock the shafts together. If
one side of this linkage has become worn and you don't compensate
for it, your carbs will be badly unbalanced when above an idle,
right when you need them to work together. Using the throttle cable,
open the butterflies very slowly and make sure both throttle shafts
begin to open exactly together.
The very best way to set the balance, in my biased opinion, is two
Uni-Syns.
When I raced an 850 Mini years ago, and also the MGA, I would take a
whole weekend to tune them, setting and rechecking valve clearances
hot and cold, fiddling with mix and balance. Sometimes we would do a
short run with each plug wire pulled off, one at a time, to see if
there was a power imbalance anywhere. Quite frankly, it was great
fun.
(SU Carbs:) Be thankful you aren't driving a big Jag with three of
the damned things!! ;)
I still have twins on the Victor, but the Cooper S now has one big
HIF6, so there's no balancing and no jets to center. :)
I hope to upgrade the Victor to a pair of HIFs; not sure what size
to start with.
-Rock http://www.rocky-frisco.com
--
Red Dirt Rangers (Rocky on piano): http://www.reddirtrangers.com
JJ Cale Live (w/Rocky): http://www.rocky-frisco.com/calelive.htm
The Luggage Fan Club: http://groups.yahoo.com/group/luggage-fans
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