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Old 06-15-2010, 07:53 PM
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350 with a vacuum leak, yet leaning carb improves idle

Hey everybody, as you can tell from the title of the post, I have a chevy 350 that has a vacuum leak around the number 1 and 3 intake ports. I have leaned out the idle and it has greatly improved idle quality. When I started, it was at about 14in of vacuum and now after leaning it out I am running at about 16.5 in. I definitely have a vacuum leak because I was almost able to stall the engine by soaking the area around the number 1 and 3 cylinders. So simply put, what can be going on here? This is the exact opposite of what I think should happen when I lean it out further. Just so you guys know, the only reason I came across this is I noticed my mileage has dropped from 15 to about 11 and after changing plug wires (others got cooked on my headers) my idle is horrible. Any help would be greatly appreciated. Thanks in advance guys.

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Old 06-15-2010, 09:11 PM
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Thats because the idle screws(assuming thats what you mean)don't lean per se, they increase or decrease a set mix of air and fuel.

Last edited by 406 bug; 06-15-2010 at 10:00 PM.
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Old 06-15-2010, 10:01 PM
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But doesn't turning the screws clockwise on an edelbrock carb technically increase the air entering the engine? So assuming I am completely confused (which I may be, I prefer fuel injected motors lol) is it maybe just the vacuum leak causing my issue and not being to rich as well?

Last edited by chevy_c10; 06-15-2010 at 10:08 PM.
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Old 06-15-2010, 10:20 PM
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The idle (mixture) screws on holleys do not control the idle circuit air/fuel ratio and I believe this is also true on your carb.
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Old 06-15-2010, 10:30 PM
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So essentially all I am doing is increasing the idle speed via the "idle mixture screws" which is covering up the issue?
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Old 06-15-2010, 11:26 PM
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406 is correct that opening the air mixture screws adds the quantity of emulsified air/fuel to the carb throats. It doesn't add just air or just fuel- it adds more of both. To add more fuel, you'd need to open the idle restriction or decrease the size of the idle air bleed.

So, adding fuel/air via the idle mixture screws does, in effect, tend to mask over a vacuum leak- to some extent.

The problem lies in that while the added mixture will help the vacuum leaking cylinder(s), it will over-richen the others.
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