Mileage / Timing

So my 98 1.0L Standard hatch is getting around 32-36mpg for the last few tanks that I've been paying attention to. I've done a compression test, and it shows 180 +/- 1psi on all cylinders. I've got a higher flowing air filter, with no modifications to the exhaust beyond an aftermarker 'stock replacement' muffler.
I tried to set the timing, but I think it might still be off. I set it to the factory specs detailed on the underhood sticker, but I couldn't find the plug they mentioned that I was to short to disable the electronic timing advance, and I'm guessing that might be throwing it off. I've got new copper plugs in (platinum plugs just get eaten in a few thousand km in Canada, due to chemical content of our gas). I'm running the factory wires, though spark seems healthy.
The exhaust pipe is pretty carboned up, leading me to believe that the car's running rich either due to the timing or maybe a bad O2 sensor which I'll try changing next, probably.

Any other tips from the vets?

the plug you short out to check the timing is between the drivers side strut and the firewall, often has a rubber cap over the plug

I found a 4 pin connector at this location, but the sticker under the hood shows a 6 pin plug.