Car backfiring

Hey guys.....new here. I have a question regarding a backfiring issue. I recently picked up a supercharged 93 lx. The car ran very well before I messed with it. It had a slight oil leak under the distributor. I marked the distributor on the shaft and the block (for reference). I pulled it, replaced the O ring, degreased the entire engine, washed off the degreaser then started it back up. The engine revved up fine but it backfires once you let off the gas. I checked the timing and it is fine. Also the car is not running near as strong as did before I worked on it. Those were the only two things I did to the car. Any information or guidance would be greatly appreciated before I go and spend a lot of money replacing parts. Thanks!

I forgot to mention that I did use a timing light. I initally set the timing to zero, but it seemed that the computer adjusted it back to about 5° retarded. Like I said, the engine will rev up fine, it just back fires out the exhaust when it revs down, especially under a load.

I did unplug it when I set the timing. However I noticed that when I plugged it back in the timing then retarded about 5°.