You can't retard the base timing on an M103. Even if you could, you'll fail the part of the smog check in which they check base timing.
Search the archives for how to tweak the idle mixture. I don't know if this will have any effect at 15mph. NOTE: it might be illegal to do this without a proper license!
FWIW, in CA, it's the seller's responsibility to have the car pass smog even in an as is sale. Keep this in mind if you decide you'd rather not keep the car.