I posted before about a cold start rough idle that I thought was maybe the choke. The choke kicks in (snaps shut when opening the throttle) and the fast idle cam is doing it's job. It also opens up as it warms.
Here's the symptoms: I pump the gas enough to get her to fire (about four pumps) it fires up and idles smoothly for about 7 or 8 seconds and then gets rough, stumbling like it wants to stall, if I leave it alone it will stall OR the RPM's will raise and everything smoothes out and idles at the fast idle screw setting (a little over 1500 RPM's currently). Higher fast idle settings (2000 rpm's) seem to make the stumble worse and for little longer (more likely to stall than smooth out).
While it was stumbling the other day I went around and noticed black puffs from the tail pipe... Which cleared to steam when the idle smoothed out. I think the stepper motor is enriching the mix too much for that short period of time after the initial start and backs the mix back to normal (whatever that happens to be) at which point the fast idle setting can get the RPM's to the level I've set them at.
So what sensor might cause the stepper to go too rich? I just tuned it up which didn't fix the problem. I changed the plugs (all carbonized with some light brown/tan spots, but no fouling) cap and rotor, new wires. There's a small amount of blow by (the little square fiber filter inside the air filter housing has some oil and a bit of residue on the cover at the same spot).
88 YJ with 4.2 and Cater POS.