I just put in a 355 in my 86 Chevy p/u. It has around 1500-2000 miles on it since complete rebuild. It has new plugs and cap, pretty new wires and rotor and a not too long ago rebuilt Quadrajet. Also keep in mind that 100% of the emissions stuff is gone and that it has factory dual 2 1/2"exhaust with no mufflers or cats. Now for the problem: When sitting at idle it puffs a dark smoke out that is very gassy smelling and it also has a slight miss. When you rev it up to around 1750 rpm there is a really noticeble miss, but not a dead miss. It is not a regular dead miss. It seems to have no power loss and runs great down the highway. It starts great and runs great in the cool mornings, but when I get in it after work it will only run on about 6-7 cylinders till it gets pretty warm. I am thinking it is something in the carburetor, but not sure. Your help is appreciated

Your carb is running rich. Might be a choke problem also.:skull:

Check the valve adjustment, and there are the bowl vent tubes in the carb one of them may have fallen out internally.