It's junk...sell it to me cheap!
Back fire like into induction system or out the exhaust??
IS this a pretty stock car? Factory air cleaner assembly etc?
If known...when did you last do a tune up?
Plugs, cap rotor WIRES! points fuel filter PCV.valve,etc. Time and miles.
Pull the spark plugs, keep the in order, and take pictures and post.
Smoke on decel is likely from the valve guides.... Valve Guide Wear or deteriorated seals.
~Possibly poor crankcase ventilation.
Further diagnosis on smoking:
Does it smoke on heavy acceleration?
Does it smoke from valve cover oil fill hole with PCV valve removed and revving engine?