Holley 780...???

airbrushguy

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Moved from General Tech

I am almost complete with my 500 Cadillac into a 1949 Hudson project.
I used an aluminum manifold and a 750 Holley and was wondering if anyone has used a smaller carb on their 500. I have an electric choke and am not running any of the smog equipment. I kept the TH400 and added a chevy rear with 3.08s.
the Holley is new but if it sits for awhile it lets out black smoke at idle.
Just interested in what others have used.
Thanks

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v8440

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From all I've read, a stock or mildly built 472 or 500 is better off with a stock intake manifold and quadrajet. I would probably put the quadrajet on the aluminum intake manifold and contact Cliff's High Performance for an application-specific rebuild kit if the quadrajet needs one. The carb that came on your 500 would be an 800 cfm model if I recall correctly. I'm a lifelong holley guy, but quadrajets just seem to do better on these engines until you get well past stock.
 

airbrushguy

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Engine runs and performs well, just seems rich based on black smoke while idling
 

FauxPontiac

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Engine runs and performs well, just seems rich based on black smoke while idling
Sounds like a bad power valve. Was this carburetor made back in the day (prior to power valve back fire protection in 1992)? They can be refitted.



I have owned a couple of vehicles with Holleys. Both performed awesome for 40k miles. When they got to needing work I swapped them out for something I was more familiar with; an AFB style. Which is really just a separate set of problems. We have YouTube now, though, so this should be easy to sort out.
 
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airbrushguy

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Yes, they’re about 1 1/4 turns out
 

airbrushguy

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Original question is just, are there any smaller CFM carburetors that anyone has used successfully?
Thanks
 

Darius

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Original question is just, are there any smaller CFM carburetors that anyone has used successfully?
Thanks
Please forgive the upcoming assumptions -but going to a smaller size carburetor wouldn’t necessarily cure the stated issue, that of running rich. In fact, it could make things worse! (Smaller carb would experience a stronger vacuum draw on the fuel circuit, tending to enriched the mixture)

The jetting is the apparent problem and that’s where your attention should go. If you don’t have experience tuning a carb find someone who does and is willing to help. Even if there is a cost associated it will be less expensive than a new, smaller carb.

Congratulations on your project!
bro. d
 

48Austin

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Cads came with 850 Quadra Jets. So size is not a problem. Is this a cold run problem or all the time?
 
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