poor mpg = Timing issue?

[md_83gt]

Hi. I’m new to the forum. I love the old school Toyotas; my brother has an 78 corona that I loved so I bought a 83 Celica.
The Celica is in great condition and has only 85k original but has a random misfire in which it hits around 15 every once every 7 or so light burst instead of 5, not running at its best, and only gets 20.5 mpg.
The cylinder and vacuum pressure are great, trans fluid is clear w/o shards and breaks don’t drag. I recently did a tune-up (problem was there b4 tune up) I adjusted the timing and rpm. Replaced the sparkplugs, wires, distributor, rotor, oil, oil filter, fuel filter, and added fuel injector cleaner, air cleaner was fine.
Things I’ve noted. Rpm starts fine but drops after a seconded when car first started, after a few minutes it seems to behave right. Car rumbles when shifting gears (automatic). Timing seems to advance fine but I only have the idle timing from the repair manual (5 degrees). I was told by some one that I should short out the “diagnostic connector” when adjusting the timing, I don’t know what this looks like, how I short it out, or if this will even help.
Any input on this would be helpful

Thanks
Walter