bad fuel mileage and exaust popping

[95civicEX]

I just got my 89 240 a little over a month and a half ago, and have already replaced the cat back with an apexi n1 unit, before I replaced the exaust, and now that it's been done, if I rev the motor, it will randomly pop, the car also does this on occasion if I'm cruising and let off the gas and just let the car slow down in gear...is this a bad thing and could this be related to my next problem?

My fuel mileage is terrible, I just filled the car up today - 10 gallons at 170 miles, I know I'm ragging on the car a bit, but this is horrible, my civic used to get 300 miles per fillup, and that was only around 10 or 11 gallons...

any ideas, I've changed the oil, and used fuel injector cleaner for the past two tanks, and have noticed no difference at all...

thanks for any help in advance

oh yeah, almost forgot, the car is an 89, original motor with 130,000+ miles, the sr20det will go in eventually but for now I've gotta get this motor running better