First off:
>Now you know why I want to dump my SU's for a weber setup :-)
The only advantage that jumping from the frying pan into the fire
actually gives you is that you aren't likely to increase your
cholesterol level any further... I'd guess that if a 1275 gets
15 mpg with two SUs, you're simply running so rich that the Colortune
can't ignite the mixture at idle.
>I'm assuming that the air leaks decrease as the carbs get heated up and the
>mixture richness rises correspondingly and therefore the idle goes up...
In the words of an unremembered Usenet wag, "Close, but exactly backwards."
As has been reliably demonstrated by Mark Bradakis et al., holes get
bigger when heated up (unless you actually melt the medium in which
the holes are located, as in the case of broiling a slice of Swiss
cheese). Besides, idle speed never goes up when the mixture gets
too rich unless you're getting MORE air, not less air. Your poor car
is running so rich that as the leaks let more air in, the idle is
climbing.
Here's what I recommend:
Start from zero. This means screw the mixture adjusting nuts ALL THE
WAY UP to the bridge -- just for yuks, count the turns. I'll bet you're
at around 16 or 18 flats down. Screw the nuts till the spring between
the mixture adjusting nut and the jet retaining nut is fully compressed.
Oh, yeah, loosen the 5/16" nuts that are holding the choke lever so that
the carbs aren't connected to one another.
Now, mark the flat of the nut that's pointing straight out toward the
driver's side fender so that you can recognize it. It's only bronze, so
you can scratch it with a screwdriver. Do this to both carbs.
Once marked, turn each nut EXACTLY two full turns down from the bridge --
that is, bring the marked flat to its same orientation but after it's come
around twice. This is known as "twelve flats" (sounds like the musical
key used for a Bartok piece) and it the standard default starting position
for carb tuning.
Now start the car. Blip the throttle by pushing down on the linkage a
few times, to clear the plugs. When the car settles down to something
approaching a rough idle, loosen the 5/16" nuts on the throttle linkage
and check flow with a Unisyn.
Now use the lifting pin to push the dashpot pistons up about 1/16" -- just
a hair. Lif it only for a second, then let it go. Listen to the idle speed
while the piston is up:
- if it drops, that carb is too lean. Lower the adjusting nut one flat
and repeat test.
- if it rises and stays risen, that carb is too rich. Raise the adjusting
nut one flat and repeat test.
- if it rises and then settles back to the earlier level, you're cool.
Do this for both carbs, then check the airflow again. It usually changes
a teeny bit but not much. When it's all as happy as it's going to get,
tighten the 5/16" nuts (taking care to get the orientation of the throttle
linkage pins in each carb correct, see your manual for details, California
clearances will probably be less) and test drive the puppy.
Now, it's entirely possible that your carbs are so badly worn after 21 years
that this won't work. But be advised that merely installing a Weber on your
car in its current state is likely to do nothing more than cost a lot of money
for something that works no better than what you've got now, let alone what
you can expect after a lproper tune-up.
The key to this, BTW, is not to try to hack on the carbs the way they are
now. They're wrong, and it'll take you forever to get them right if you
start from the current condition. Give up making little adjustments to them,
start from scratch, dial them in and try not to go more than 13 or 14 flats
down from the bridge total.
More on air leaks in the morning. My ride is here...
--Scott
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