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.

First thing would be to remove the sight plugs on the side of the fuel bowls and check float level. That jet size is getting pretty small.

Just a couple of thoughts, I have never run a 396 with anything less than a 750. Smaller is just not enough, and this is with a mild
engine. Also with the 750 on the 396 minimum
jetting would be 72s on the primaries. The hesitation you had was probably from a lean condition, not a rich one. I would get a good 750, put in the 72s (usually 72-74 gets it done with a mildly built engine) get the timing set and try again. I think you will be suprised how much better it will run.

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66/70 chevelle ss396's
69/71 camaros

The formula you used is the correct formula, but it is almost always lower than what most of us run. But did you do the math right?
396Ci X 6200rpm (my guess at your red line) / 3456 = 710.4166667. For my basically stock 350 it says 350ci X 5500rpm / 3456 = 557.0023148. I am running a 600cfm Holley w/ no probs. I think your probelm is you are under carbing you engine. I would put the 750 back on. Why did you take it off in the first place if it was running fine? Oh well, I hope this helps.

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Bryan Shook
Burgettstown, Pa