running rich

[fun car guy]

My '89 coupe starts and runs well but, upon inspection, the plugs are black with carbon and the dash display just says zero MPG and there's no smoke from the exhaust.  Is this normal and if not, how do I lean out the mixture? Thanks

[2seater]

Are the plugs checked after being driven for a period of time? For fuel/air mixture control the MAF does the gross calculation of the demand, the O2 sensor fine tunes the running system. The only time the exhaust should be readily visible is in cool/cold weather from condensing water vapor. Beyond that, the exhaust should be pretty much invisible.