Several things here.
First, on most of the yars that carburators were used after '72, internal
idle orifaces were intentionally undersized so that even with the screws
backed as far out as you can get them without them falling out you will not
have an over rich mixture (turning the screws out richens the mixture at
idle).  It's often a chore to try and get them rich enough to get a decent
idle, especially  if there's any kind of vacuum leak.  Idle mixture,
however, is not a major player in fuel economy.  The range of mixture in the
grand scheme of things generally isn't very significant.
   The single biggest factor in fuel economy and performance is ignition
timing.  This is not limited just to the initial timing setting, the one you
fool with when turning the distributer.  Automatic advance through both the
centrifugal and vacuum advance are equally, if not more important.
   Here's the procedure.  Obtain the full timing/distributer specs.
Connect your known good timing light.  Disconnect (important) the vacuum
advance.  With the vacuum hose off and the idle at or below the correct in
drive idle speed set your timing to spec.  Put the trans in park.  Now,
watching the timing marks with the light open the throttle to above the rpm
indicated that mechanical advance should be all "in".  You should see the
timing mark move smoothly ahead as the speed increases.  You may not be able
to determine the exact amount of advance because you will run out of
markings on the damper.  That's where a high quality light with an advance
feature is helpful.  Once advance is all in you turn the knob on the light
to bring the mark back to the original setting and look at the meter on the
light to see how many degrees ahead it's running.  If you don't have the
means to actually measure advance just make sure it moves smoothly and heads
well off the marks.
   Now let it idle back down.  Check to see if you have idle at the dist
vacuum advance hose at an idle.  Some are designed to have manifold vacuum,
others have vacuum managed through the throttle.  If you have full manifold
vacuum at an idle plug the hose in while watching the timing light.  It
should jump ahead when you plug it in.  If your engine does not have
manifold vacuum at idle raise engine rpm until you have vacuum at the hose
and then plug it in again watching the timing light.  You should see it jump
ahead.    It would also be a good idea to use a hand vacuum pump to test the
diaphram for leaks.
   If after following this procedure you have not uncovered your problem,
the outer ring on the damper may in fact have turned.  Or, you may have
slipped a cog on the timing chain, which would also account for the loss of
power and fuel economy.   If you've ensured that it's not the chain and the
damper has slipped, the best way to time it is on the road.  Take your
wrench with you and with the engine fully warmed up keep advancing the
timing until it starts to ping.  If it starts okay there, just leave it.  If
you have hard starting related to timing (firing against itself
characterized by the crank, abrupt stop, crank, abrupt stop then fires off)
then back it off until it just cranks okay. Carefull here.  Having it fire
against itself can crack the nose on the starter.
   Now that you've eliminated the timing issues you need to set the carb.
With the engine cold the choke should close itself fully once the throttle
is cracked open.  Start the engine.  As soon as it starts the choke break,
the small diaphram on the passenger side, should pull the choke open a
pre-set amount.  If it's not enough the engine will load up when cold.  Too
much and it will stall and hesitate when cold.  If this is okay continue to
let the engine warm.  The choke should come up fairly quickly as the choke
heats.  If it's slow coming off, or has often happens, never comes off you
will have very poor economy and performance.  I believe that model uses warm
air piped into the choke housing through small 5/16 tubes.  Make sure you
can feel suction at the fresh air end of it (the end that plugs into the
carb near the top.  If you don't feel any suction check that the choke cover
is tight and the gasket in place (the round circular plastic choke stat).
If your choke is the type in a well on the intake make sure the exhaust
crossover is not plugged and if equipped with one, that the heat riser valve
on the exhaust manifold is working properly.
   On other check is the thermostatic controls for the air intake on the
air cleaner.  Ensure that the flap in the snorkel is closing when the senser
is cold.  Put ice on it with the engine running to make sure it's closing it
it's already hot.  Make sure the stove on the manifold and the pipe coming
up to the snorkel are intact and in place.
   By now the engine is fully warmed up and choke off long ago.  Start with
the mixture screws out 1 1/2 turns.  Adjust the idle mixture screws out 1/4
of a turn at a time alternating from time to time until the you reach
maximum rpm.  In the old days you would back them out until the rpm started
to drop down after reaching it's peak then you would turn them in 1/4 turn.
Now days you won't see it drop after it's peak.  It will increase and
smoother out then you can turn it out till the cows come home with no
difference.  Leave them at the point that turning no longer changes
anything.  You need a good sensitive tach for this.  Or a good ear.
   Then take two aspirins and call in the morning.
Steve