'90 Civic: Extremely sluggish low-end acceleration, poor fuel economy

A few more issues from my poor little car for anyone who can shed some
light...

My '90 5-spd Civic Ex has been bogging down at low RPMs since I bought
it last fall. When I accelerate from a dead stop I have to goose it to
about 2500rpm and then pop the clutch a bit to get it to take off as
quick as I think it should. Especially in an up-hill situation. (no AC)

If I drive it like other small manual 4-bangers I've owned (Corollas,
etc), it will literally take 5-10 secs to get to the point where I
think I should shift to 2nd. Gas pedal down half way or floored makes
no difference. Also, doesn't matter if the car is at temp or just
started. After 1st gear and I'm moving a bit, there's no problem. I
seem to remember reading about a potentiometer (maybe in the throttle?
can't remember the details) that, if the contacts were dirty, would
cause this type of thing. Maybe I'm just crazy and I dreamed this....

I'm also getting only about 15-18mpg in town and 20-23mpg hwy this
summer... seems incredibly low. Was getting the same last winter, but
wrote it off to cold weather. Spark plugs are about a year old, newish
air filter, and OEM O2 sensor's only 6 months old.

ECU's not throwing any codes.

-Back pressure too low/high? (bad catalytic converter?)
-Bad plug wires? (I'm sure they're not Honda OEM wires, but I think
they're only about a year old)
-Clogged EFI?
-Timing out of adjustment? I didn't fool with the dist. when I changed
the timing belt, mostly because I couldn't find a damn marking anywhere
on the crankshaft pully to save my life. Ran like a top after the belt
change, so I'm confident it's not a tooth off...

HELP!

Any advice would be greatly appreciated...

Regards,
-Bruce