Running pig rich?

[Matt Suffern]

Hi all,

Got the new fuel tank in my '86 4kq last Saturday and the car runs great.  Starts up every time without hesitation, pulls strong, doesn't cut out in corners, steady cruising is a breeze.

The only problem is the car runs rich.  Really rich.  At any given time there's a fine black mist emanating from the tailpipe, more so when the car warms up.  At idle a nice haze forms behind me from the unburned fuel mist, and sometimes (not always) at 80% throttle to WOT I can see a cloud pouring out the tailpipe as I accelerate.  Also, as the car warms it will progressively idle lower and lower until it will die unless I jab the gas pedal.

I've replaced the plugs recently and also the coolant temp sensor (the one at the bottom of the coolant neck leading from the head to the radiator).  If anything, the coolant temp sensor replacement made the problem worse, as before it would occasionally idle (albeit high, around ~1200 rpm) when warm.

Any other theories?  Could it be the thermotime switch is faulty and my cold start injector is firing all the time?  I reasoned out the O2 sensor, since it still runs really rich at WOT and idle, two conditions in which the computer is in open loop and doesn't receive O2 sensor input.

When I got the car it didn't have an air filter (a dubious "performance enhancement" by the previous owner), so I installed one right away.  Could it be that the air metering plate is sticking open occasionally at idle from gunk buildup and causing the rich condition?  Any BTDTs would be most helpful.

Thanks all,

Matt
'86 4kq
Chapel Hill, NC