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Old 09-26-2004, 08:48 PM
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SBC runs rough at highway speed

I have a mildly rodded SBC 400 in my truck. Edelbrock 600 CFM carb, Performer RPM intake, MSD 6al, HEI Vacuum Advance Dist. (stock coil), .202 heads and a mild cam (sorry I don't kno\w the grind, but it pulls a decent vacuum at idle).

It runs great at around-town speeds, and even better when I stand on it. It shudders and pops (jeez I hope it isn't detonation) at around 55 (3100 RPM). I can mash it and accelerate out of it. I tried openeing the choke a little when this occurs, but it didn't seem to help, so I thought it may be a rich problem. I went 2 stages leaner in the cruise circuit by changing the metering rods, which helped a little. It only shudders a little bit at 55, but starts to pop around 65 (3700 RPM) now. Could I have been wrong and I should be going in the rich direction?

I haven't ruled out a timing issue, but it behaves so well at acceleration and under 50 MPH, I just don't know. It also only has this problem under load. I can rev it progressively from 3000-4500 in neutral witrh no problems. I think the last time I checked it, I had 10 deg initial and 36 total at 2800 RPM. I need to get my advance light bqack from a deadbeat.

Anyone else have a similar problem, where you are a king at getting groceries or at the drags, but you can't do more than 50 on the way to a show? Thanks in advance, Chris.
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Old 09-26-2004, 08:56 PM
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compression?

Sounds like classic part throttle detonation due to vacuum advance...

K
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Old 09-26-2004, 09:04 PM
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Hmm. I don't know for sure, but the heads aren't anything exotic, they aren't original, but they are still stock GM, so I assumed it couldn't be more than 9/1. I am running with the cheap stuff, so I might try thowing in some octane boost to see if that puts me in the clear. Thanks, Chris.
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Old 09-26-2004, 09:51 PM
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If the vacuum advance adds too much timing at part-throttle, it might do that. See if it adds more than 15-16*.
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Old 09-28-2004, 07:44 AM
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I guessing the carb is going lean. A classic sign is backfiring through the carb, You may need to clean out the air bleeds. You carb manual can tell you where they are located.
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Old 04-19-2007, 11:00 AM
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Thanks for the help. I moved and this project sat for awhile, but I got back into it. The advance was the culprit. When I hooked up an advance light, it was advancing to over 50 degrees. A rebuilt distributor took care of it and it is back to being able to drive at highway sppeds. Thanks again.
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