Carb setting help

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Aug 28, 2023 | 09:05 PM

keno3434

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2nd Gear

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Carb setting help

Just had my 454 rebuilt and the mechanic put a used Holley 650 double on it but didn’t set it up properly… he re-jetted it saying it’s running to lean and now I see nothing but black smoke when idling. I put a vacuum gauge on it and my highest vacuum is when I close the front two mix screws completely closed…
can I assume that the jets he installed are just to large or do I have something else going on…
please help

Aug 28, 2023 | 09:28 PM

Arg0413

Drifting

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From: Baton Rouge Louisiana

Your idle isn't affected by your jetting. It's determined by your transfer slot and idle mixture screws - which you should have 4 corner idle on a DP. Is this a new carb? Have you read Lars' paper on Holley set up?

Aug 28, 2023 | 09:51 PM

lars

Tech Contributor

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From: At my Bar drinking and wrenching in Lafayette Colorado

Jetting has no effect on idle rich/lean condition. In fact, you can take the jets completely out of the carb and the idle will remain unchanged, as long as the carb is idling on the idle circuit. You have something else going on - carb needs a complete baseline setup... Sounds like your hack "mechanic" is a real gem.

Lars

Aug 28, 2023 | 11:23 PM

keno3434

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2nd Gear

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Quote:

Originally Posted by Arg0413

yes 4 idle screws, one in each corner.
no it’s a used carb.
no I have not read Lars.
I was on holleys web site and watched a bunch of set up videos… it said if you have black smoke to down size 2 jet sizes… it didn’t talk about why when I bottom out the mix set screws I get the highest vacuum… normally the highest vacuum should be somewhere around 1-1/2 turns out not turned all the way in.

Last edited by keno3434; Aug 28, 2023 at 11:57 PM.

Aug 29, 2023 | 12:03 AM

lars

Tech Contributor

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From: At my Bar drinking and wrenching in Lafayette Colorado

Few Holleys will run at 1-1/2 turns. Most are at 5/8 to 7/8.

If the engine is running with the mixture screws all the way in, the carb is supplying fuel to the engine through some source other than the idle circuit. Most likely from fuel dribbling out of the main discharge nozzles. You need to tear that carb down and do a correct setup on it.

Aug 29, 2023 | 09:10 AM

Jebbysan

Dr. Detroit

Joined: Mar 2012

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From: New Braunfels Texas

What intake manifold are you running?
We like pics here.....

Jebby

Aug 29, 2023 | 09:58 AM

HeadsU.P.

Le Mans Master

Joined: Aug 2017

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From: Cool Northern Michigan

A nice feature about those 650 DPs is the float setting inspection hole (sight-plug) on the fuel bowls.
Have you glanced at the float level while idling?

Aug 29, 2023 | 10:13 AM

Jebbysan

Dr. Detroit

Joined: Mar 2012

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From: New Braunfels Texas

Quote:

Originally Posted by lars

Main source of fuel dribbling is the metering block gasket not sealing.......always use a new gasket when doing a new jet change......you can reuse it a few times but once it heat cycles it will stick to the main body slightly.....and form itself to the metering block, or if it sat and got dry and you removed the metering block....they usually leak. You said the jets had been changed.....and I bet your "mechanic" just reused the old shitty gasket.
It is easy to see while running....shine a pen light down the primary venturi and see if it is dribbling gas out of the boosters.....if it is, you are taking it apart.

Jebby