Only getting 22mpg HELP!

Hi guys,

After reading the sticky on gas mileage, I've started to replace a few things that could be the culprit to my terrible gas mileage.

Let me start with the specs of my car.

Its a d15b block, d15b7 head, si headgasket (8.55:1 compression)
Automatic transmission
Oem everything.
Tires @ 40psi
No tuning modifications at all.
New NGK Spark Plugs and Wires

Before I was getting an average of 22mpg. I just changed the fuel filter with an autozone replacement and now only got 190 miles out of the next tank of gas!! I verified there's no leaks after the replacement.

I pulled the o2 bung in the cat (obd2 cat on obd1 car, so bung closing the second O2 sensor hole) and it was BLACK as was the exhaust tip.

This leads me to a bad O2 sensor which I am going to change tonight.
I honestly have no clue how to check timing at the distributor, or valve lash so I'll be looking those up shortly after I finish this post. I highly believe it could be the distributor as I noticed recently it was only being held in with 1 bolt. I've since fixed that, so no more oil leaks at least.

From there if anyone has some ideas as to what could be causing such a major drop in economy I'll appreciate the support. I'll also be checking compression and would love to do a leak down test, but I say that term to all the tool rental places around here and they look at me as if I have 3 heads.

Thanks again in advance.

James

P.S. I have an intermitent sputter while the car is warming up, I would go as far as to say it sound like a misfire. NO cel lights

P.P.S I definitely think I have at least a distributor timing issue after reading a thread on how to set it.