I would check out everything. Have clogged cats messed with mileage in the past?


And for what it's worth.
Getting a supercharged monte and babying it is like getting a really hot wife, and never touching her haha.

 

Well if u said the air filter was black then that just tells u about the maintenance history of that car! I agree with zippy fuel filter and spark plugs first, if that doesn't help one bit then no need to go any deeper hendering it's a much bigger problem than good maintenance can do,