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Old 09-29-2018, 04:40 PM   #1
Bobbed72
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Motor goes from lean to rich at idle.

Have a carbed 5.3 with LS1 cam, Brawler carb, edelbrock manifold. Put a wideband AFR on it this weekend and noticed my fuel ratio goes from 14.5 to 11-12 after the motor runs for a bit. Any input?

Thanks
Jay
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Old 09-29-2018, 05:06 PM   #2
geezer#99
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Re: Motor goes from lean to rich at idle.

Cold intake plenum!
What does it do once the intake is hot?
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Old 09-29-2018, 05:33 PM   #3
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Re: Motor goes from lean to rich at idle.

Adjust it at operating temperature (engine warm). The choke makes up the difference when it's cold.
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Old 09-29-2018, 06:27 PM   #4
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Re: Motor goes from lean to rich at idle.

Quote:
Originally Posted by geezer#99 View Post
Cold intake plenum!
What does it do once the intake is hot?
That makes sense intake is probably heated up when it goes rich.

If I lean it out am I at risk of being too lean before the motor warms up? Electric Choke doesn't stay on long enough. Have it set at just closed when outside temp is 70 degrees.

Last edited by Bobbed72; 09-29-2018 at 06:33 PM.
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Old 09-29-2018, 06:35 PM   #5
geezer#99
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Re: Motor goes from lean to rich at idle.

Intake is cold when it goes rich!
The fuel vapour goes from fine mist to heavier fuel droplets (liquidizes).Heavier fuel doesn’t burn well and goes out the exhaust. Shows rich on your analyzer.
When your intake is hot the fuel vapour stays normal.

Try this test.
When it goes rich shut off the motor for 5 minutes. The intake should warm up from heat soak in the motor. Hot air rises and heats the intake plenum. Should stay lean then.
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Old 09-29-2018, 08:09 PM   #6
Bobbed72
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Re: Motor goes from lean to rich at idle.

Quote:
Originally Posted by geezer#99 View Post
Intake is cold when it goes rich!
The fuel vapour goes from fine mist to heavier fuel droplets (liquidizes).Heavier fuel doesn’t burn well and goes out the exhaust. Shows rich on your analyzer.
When your intake is hot the fuel vapour stays normal.

Try this test.
When it goes rich shut off the motor for 5 minutes. The intake should warm up from heat soak in the motor. Hot air rises and heats the intake plenum. Should stay lean then.
Ok. I was thinking of air density. Hot intake less dense air. Would that contribute to a bog when the motor is cold?
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Old 09-29-2018, 08:12 PM   #7
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Re: Motor goes from lean to rich at idle.

Have to revise my problem statement. Took it for a drive after posting and it was 13.5 at start. Motor cold and within a minute was 11 to 12.
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Old 09-29-2018, 08:17 PM   #8
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Re: Motor goes from lean to rich at idle.

Quote:
Originally Posted by Bobbed72 View Post
Ok. I was thinking of air density. Hot intake less dense air. Would that contribute to a bog when the motor is cold?
It can.
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Old 10-01-2018, 04:26 PM   #9
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Re: Motor goes from lean to rich at idle.

Did you set up the idle mixture using a tach or vacuum guage?
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Old 10-01-2018, 04:44 PM   #10
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Re: Motor goes from lean to rich at idle.

Vacuum gauge. Pulling around 20".
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Old 10-01-2018, 05:11 PM   #11
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Re: Motor goes from lean to rich at idle.

I'm wondering how much of your idle transfer slot is exposed? I've had a couple that I've had to crack open the secondaries to get enough air to idle properly without pulling too much fuel from the slots. Will the engine continue to idle with one or both of the mixture screws turned full in?
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Old 10-01-2018, 05:36 PM   #12
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Re: Motor goes from lean to rich at idle.

Before I put on the AFR gauge I pulled the carb and checked the slot. There was too much on primary so I squared it up. Turned all 4 corners to 3/4 out. Idle jumped to 1100 so trimmed down with the secondary adjustment. Checked the mix with the vac gauge and ended up leaving them at just less than 3/4 to get the AFR to 13. Going all the way in will kill the motor.
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