I've got a '91 305 TBI,all stock. For a good 2-3 minutes after startup I can hear a light backfire in the exhaust around every 6 seconds. When the idle drops to about 600 RPM it gets very rough dropping to about 450 RPM and going back upto 600 until the car warms up fully. The exhaust also smells A LOT. Does this mean it's running rich? If so,how can I go about fixing it?

 

Pull the plugs, check condition for running rich. Simple and cheap diagnostic.

 

Backfire is usually lean. Rich would stumble and stop. When was the last time u changed the fuel filter? Always need to check for DTC codes before buying anything. Short pins A&B and count the CEL long then short flashes.

Well u have spark so most likey fuel issue (unless u have a bad plug or 2). If no codes then u should get it scanned and look at the O2. But if its doing this on start up then its still in open loop and the problem is not the O2 but something with injection or fuel press.

Get it scanned.

BTW u can buy an Actron 9185 brand new for less than 150 bucks.