I have a basically stock '68 SS 396 that for the past six months or so, has been running a holley 750 vacuum secondary carb with no problems what so ever. The car started and cruised wonderfully, running great only when I kept my foot out of "it". On the occasions that I would romp on it, I experienced noticable lag from running rich. Using the cfm formula (cid x rpm/3456) I realized that I was flowing far too many cfm than my car needed. A friend loaned me a holley 600 that I tore down and rebuilt to make sure that it was sound. I currently have it on my car, but since it was installed, I have experienced severe black smoke from the tailpipes, causing me to change my jetting from 69 to 64 and a 6.5 power valve with the smell of raw fuel in the exhaust at idle. (I was running the 750 with 69 jets, no problems at all). Accompanying the smoke has been a studdering and hesitiation at slow speeds, like a cylinder is not fully firing. I checked and cleaned all of my spark plugs, but the poor performance persisted. Any suggestions whatsoever would be greatly appreciated. Thanks.