Have you replaced the coil? They have a working life span of approx 10 years before they start to break down. If it's not giving a strong spark, it will fail to operate efficiently and burn fuel during the ignition cycle. This would explain the 'rich' fuel mixture symptoms. If the coil is the culprit, dial in your ignition tune once you've installed the new one. Don't use anything exotic for a replacement coil - Accel and MSD do not play nicely with Mitsubishi ignition systems. A transformer coil with a ballast resistor will be a good upgrade, look on eBay -

https://www.ebay.com/itm/12514187636...gAAOSw3o1hE5P6 (here's a start)