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Old 12-14-2010, 09:37 PM
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adjusting the carb on 390

I want to set the mixture screws on an edelbrock 750 for my 390. Everything i've read has people saying you turn one at a time till it bottoms out and the engine runs rough/stumbles, then turn it back out till max idle and do the same for the other screw....Well when i do this my engine doesn't stumble, it idles higher, what's the deal here?
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Old 12-15-2010, 12:06 AM
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You probably have a vacuum leak.
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Old 12-15-2010, 07:45 AM
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http://www.edelbrock.com/automotive_...ers_manual.pdf

Page 6.

Screw IN the idle mix screws, it LEANS the mix. That means if you are bottoming them out, and the idle INCREASES, the mix is already way too rich.

Are you sure the choke is all the way open when you're doing this?
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Old 12-15-2010, 07:54 AM
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I'll agree that i used to have a vacuum leak. It was a bad brake booster and actually, when i changed it out, is when this started happening. It used to idle down when i bottomed the screws. Once i changed the booster (old one would idle engine down pretty low whenever i'd brake), this occurred.

Yes the choke is open, i let the engine get up to running temp to do this. The only thing i don't do is set the actual idle itself by tach because i don't have one so i just guessed and it sounds about 800-900.
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Old 12-15-2010, 08:19 AM
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Hmm... depending on your initial timing, you might have the throttle plates open far enough that you're running on the primary circuit and not just the idle circuit.
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Old 12-15-2010, 03:45 PM
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Any ideas as to what i should do? new carb? have someone rebuild this one and tune it themselves?
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Old 12-15-2010, 03:58 PM
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Originally Posted by NW 150 View Post
Any ideas as to what i should do? new carb? have someone rebuild this one and tune it themselves?
99% of the carbs replaced have nothing wrong with them. All they need is a proper adjustment or a rebuilt and a proper adjustment.
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Old 12-15-2010, 04:55 PM
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It's been taken apart and cleaned out a few months ago, but could have been done better. I don't know where to take it to though.
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Old 12-15-2010, 04:55 PM
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