Subscribing here, i have same problem with my 87. You still runnin speed density system? Mine also backfires around 5k also when about to shift. I too have a off road x pipe, and have to real smelly exhaust and fuel smell. I bought plugs,rotor,cap,wires,k&n filter cleaner. Gonna tune up, sdjust timing, change oil, flush rad clean up my dirty k&n. Im hoping if i put on a short belt and eliminate the smog pump it will help with the exhaust smell.
1987 Mustang GT 5.0 5spd t-roofs, FRPP 4.10 gears, MM subframe connectors, FRPP a arms, Tokico Hp struts/shocks, alum rad, electric fan, dcc fan control unit, 6g alternator