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Old 10-02-2008, 09:48 AM
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Idle fuel mixture screws adjust

I've been dialing in my Holley 650 DP. Initially, it was running rich to the point the plugs were black and sooty. I brought the jets down which made an improvement, however I was still on the rich side with the plugs. I decided to check the float levels (which I should of done first), the front was fine, the secondary needed to be adjusted down as the float level was a bit high and fuel was coming out of the sight plug. Since adjusting the float, I took the car for a cruise on the highway, pulled the plug and noticed they are a grey color with a slight tan on the electrode. I have the idle mixture screws at 1 turn from being bottomed out which is running a lean condition. My question is, if I adjust the screws say 1.5 turns, which makes it running better, will that richen the engine to the point that my plugs will be running a little richer than now? I guess what I'm getting at, is will richening the mixture with the screws, create a slightly richer reading at the spark plugs? Thanks guys.

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Old 10-02-2008, 09:51 AM
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always use a vacum gage to adjust idle mixture.
adjust for highest vacum reading connected to an unported vacum source.
then go from there.
if plugs are black and sooty it might mean a jet change is in order or a simple hotter plug can help.
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Old 10-02-2008, 09:53 AM
How fast is fast enough?
 
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yep, set the mixture screws for vacuum... and every so often one screw may need a hair more adjustment than the other.
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Old 10-02-2008, 09:56 AM
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Quote:
Originally Posted by dawg
always use a vacum gage to adjust idle mixture.
adjust for highest vacum reading connected to an unported vacum source.
then go from there.
if plugs are black and sooty it might mean a jet change is in order or a simple hotter plug can help.
Dawg, I did that, and I use the vacuum gauge. I went about a a quarter in more on the idle mixture screws trying to run a little leaner. I'm wondering if the adjustment makes a big difference on the plugs, or maybe I should go up one number on the jetting. Trying to avoid pulling the carb apart, because I'm so close. LOL... I dropped two numbers on the jetting. Was running .67 primary and .73 secondary.
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Old 10-02-2008, 09:58 AM
How fast is fast enough?
 
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I would try steping down a jet size to lean it out and then reset your idle. What is your cam size, compression, and timing?
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Old 10-02-2008, 10:03 AM
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Quote:
Originally Posted by ap72
I would try steping down a jet size to lean it out and then reset your idle. What is your cam size, compression, and timing?
I'm already running a lean condition. Plugs are a light grey color, am thinking I need to richen a bit. Cam is Millings .480 lift with .282 duration (not a big cam) compression is 10:1 and timing is at 14 degrees. I'm running a 5.0 with 351 W heads.
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Old 10-02-2008, 10:07 AM
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The idle mixture screws have no effect off idle. So it would be jetting changes that would be further tuning it. if the plugs are looking tan at regular cruise than it`s just about right.
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