Aside, from what Steve mentioned, I don't think high octane is your problem. I use 97 octane all the time and have no problems. There are many reasons you engine could be running rich. Check your airpump. It may not be your problam, but it was mine. With the system inoperable the engine runs rich, usually until hot. I had the same problem, black smoke if I really hit the gas hard, rough idle, etc. My air pump was shot. On of the hoses that leads from it to the engine block corroded off the block so the extra air it pumps into wasn't getting in. Since the hose was broken off water got into the air pump and ruined it. Long story short new hoses and valves and new air pump equals no more problems. No rough idle, no smoke during hard acceleration. It take two minutes to do a visual inspection. Not much to lose from doing it.