396 stuttering - please read - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

 
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post #1 of 4 (permalink) Old Oct 3rd, 99, 03:59 PM Thread Starter
 
Join Date: Sep 1999
Posts: 108
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.
penguinjohn is offline  
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post #2 of 4 (permalink) Old Oct 3rd, 99, 05:34 PM
Senior Tech
 
Join Date: Nov 1998
Location: ohio
Posts: 252
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.
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post #3 of 4 (permalink) Old Oct 4th, 99, 12:56 PM
Tech Team
 
Join Date: Aug 1999
Posts: 70
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
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post #4 of 4 (permalink) Old Oct 4th, 99, 01:10 PM
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Bryan Shook
 
Join Date: Sep 1998
Location: Enola, Pennsylvania
Posts: 428
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
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