My boyfriend purchased a 1991 Cutlass Supreme for $1200.00. It’s was doing fine for first about 45 days since he purchased it, but just in the last week or so it’s been running really rough. You have to keep your foot on the gas and the breaks when you’re stopped so it doesn’t stall out. We took it to a shop and they scoped it but couldn’t find any issues. They then tried installing a new computer and air intake sencer but it still runs the same. When they checked everything out they also found that it’s been running hot and the spark pluggs are all black. Do you have any suggestions on what else it might be and what it will cost to repair?
FYI… IT is actually a 1991 Cutlass Supreme not 1971. I have fat fingers… 
There’s not enough info to make much of a guess but if the spark plugs are all black (and sooty I assume?) this means too much fuel or a weak spark.
I would check the fuel pressure regulator first. A leaking regulator will soot the plugs up, cause rough running, and kill your gas mileage.