car's rich, i'm not

[foxxinabox at wideopenwest.com (Foxx (in a box))]

   this is sort of a two parter. i recently added sea foam to gas tank 
to help clean that injectors. for the first week everything was good. 
now, though, when i start up in the morning or afternoon, the car will 
take a second more to start and blow blackish smoke on hard 
acceleration. strangely enough, it does not seem to do this in the 
evening (yes, with enough light to see). the car will also stumble and 
have poor acceleration until the smoke diminishes. i know i need to turn 
the mixture down, but don't a couple of things have to be adjusted to do so?
   also, when i bought my car i was getting about 33 mpg with great 
power. problem was it was leaking like a sieve. i replaced the seals and 
the timing belt and had to take it in to get timed. once i got the car 
back, the best i achieved was 25 mpg with a noticeable loss of power (my 
non car-savvy gf even noticed the difference). i recently replaced the 
o2 sensor (the old was was more of a burnt off nub, like a bolt) and am 
now getting about 28/29 mpg, still crappy power, however. any ideas on 
how they might have botched it?
   both i and my fickle '85 8v await your suggestions.

-- 
  /\_/\	  Foxx (in a box)
< o o >	  http://foxx.tripod.com
   \ /
    ?