Running rich car idling too low, running too rich. All plugs, air-filter, dist. cap & button replaced. Knocks off when put in gear. Idles at approx. 600RPMs under no load, should be higher. Strong gas odor and smoke at exhaust. WHERE DO I START TO LOOK........please.

Could be fuel filter,air filter,dirty throttle body.

Replace plugs and wires.

Spark Plugs

Air Filter

Fuel Filter

My first guess would be a weak coil or something going on with the carb.
It could have jumped time but you would probably hear noise coming from the timing chain.
Can you tell if it is not firing on one cylinder ? Checked compression ?
Too rich or too lean on the fuel mixture ?

What year and is it the factory engine? If it is carborated the choke may not be opening enough and is starving it for air and running rich. What spark plugs are you running? Have you replaced the vacume hoses and gone over the engine tightening(torquing) intake, carb and other parts?

Have you replaced the in-line fuel filter?

vacum hose may be off some were

I would suggest also replacing all O2 sensors, PCV valve, air filter, fuel filter.. Just might clear it up.. Not expensive and if issue continues - you will have ruled all that out as a cause..

Have a nice day !

This is because of the improper combustion, so you check the air filter, if the air filter is not good enough then the idling will go down and then the back firing will occur, in the same time check the Electronic ignition system for proper ignition cycle.

my buddies 94 350 has been doing the same thing for years i think its in the computer i would say youll probably have to live with it