well....really stumped, my '90 f250 5.8 is rich, rich, rich I've replaced or tested all sensors except O2, which shouldn't matter b/c it runs rich all the time, idle, WOT, cruise especially WOT. Mods are no cat, no a.i.r, headers. Truck starts great, idles rough, revs clean/smooth (WOT)

 

you've got to be kidding..... somebody has got to have an idea

 

Suspect the upper plenum gasket. Spray carb cleaner where the plenum bolts to the lower intake right behind distributor on both sides. If engine revs up, or smooths out, you found it. I hope that you've checked the fuel pressure! Should be 30-40psi at idle.

Robbie