Carb knowledge help needed

[rod7515]

Im wanting to understand some carb theory and tuning knowledge ( holleys) and even though I have worked with many carbs (holleys) over my life I have never been "schooled" on the how it works internally.
The reason I want to understand this a little better is because I plan on going to a 2x4 tunnel ram sometime this year and I want to set up the carbs to be efficient when running on the street. I am thinking about using the one to one linkage setup. I will be using 2 holley 600 CFM carbs on a 408 which should have around 500 HP.
Im sure I will get many different theory's and thats fine because when tuning there are many different ways to get the best setup that will work.
First question is about a carb when idling. Is the fuel drawn down into the intake by the vacuum of the running engine? And if so, how does using 2 carbs effect that vacuum and does an idling running engine draw the same amount of fuel from each carb as it would if it were a single intake carb or does it draw less fuel from each carb then if it were a one carb setup?
When tuning the carbs how many use the engine vacuum gauge in order to tune the carb? Advantages vs draw backs?
When making adjustments using the air screws in the front just under the bowls will it only effect the fuel/air during idle?
In order to change the fuel off idle I need to change jets? Squirters? Or is there another way?
I know I can change ramps on the linkage and I can make changes to the accel pump adjustments but I am more curious about how and what the passages do and how I need to use the circuits for a better setup.
Thanks for any knowledge you can pass on.
Rod

 

[rklein383]

Interesting read here; Zero To Hero! This Guy Reveals Dual-Quad Tuning Secrets! - Hot Rod Network

 

[Mattax]

I am more curious about how and what the passages do and how I need to use the circuits for a better setup.

IMO the best introduction to the principles is the first chapters of Urich and Fisher's Holley Carburetors and Manifolds. Any edition will do, no reason to pay for the latest edition with fuel injection, etc. Also the section on race tuning is useful in a practical way even though doesn't get into the details of the circuitry. In particular the sidebar on proper throttle to transition slot relationship is worth the price alone. (pp. 121-2 in the 1987 edition [Holley Part No. 36-73, HP Books])
The section is called Special Procedures for Wild Camshafts
I've linked it above but the links to copyright material are often temporary.

Chrysler also explained the carburation circuits in several Master technician's reference booklets. Although the general ones are more oriented toward Carters, there's a lot of overlap with Holleys. There's also a couple specifically on the Holleys.
Master Technician Service Conference - Chrysler's Training for Mechanics
The pre-emissions ones from 1966 might be good to start with.

There's a lot of good info about the circuits on Racing Fuel Systems forums. Just that its not all organized neatly.
Dig around in there, lots of good stuff.
here's a couple good ones to start with
AFR Relationship to Throttle Opening (Load)

Importance of Position of Idle Feed Restriction

If you join the forum, ignore any and all requests to also join tapatalk unless you really really want to.

 

[Mattax]

Im sure I will get many different theory's and thats fine because when tuning there are many different ways to get the best setup that will work.

There's many different designs that all work, but many theories are nothing more than thought experiments. When you find references that refer to NACA and SAE papers, books by guys like Obert and Taylor - those are the real deal.

A little preview. The books will show it better:

First question is about a carb when idling. Is the fuel drawn down into the intake by the vacuum of the running engine?

You can look at that way. Most of us do. Its the pressure difference between the fuel bowl and the outlet under the throttle blade that causes the fuel to move. Since the pressure in the bowl in a Holley is atmospheric pressure, we can just look at it as vacuum.

When tuning the carbs how many use the engine vacuum gauge in order to tune the carb? Advantages vs draw backs?

It's always handy.

When making adjustments using the air screws in the front just under the bowls will it only effect the fuel/air during idle?

Not many carbs have idle air bleed adjustment screws. An aftermarket Holley will have idle mix screws that limit the fuel going to the idle port. The fuel through the transfer port at idle is limited by a fixed restriction and the position of the throttle blade.

In order to change the fuel off idle I need to change jets? Squirters? Or is there another way?

On a properly calibrated carb, the jets should have no noticible effect on the 'idle' mix. The 'idle circuit' provides fuel during steady state idle and off-idle and moderate cruising. Its one of the hardest things to learn

The idle circuit is really a misleading name.
Off-idle fueling is tuned by the throttle position relative to the transfer slot, the idle feed restrictions, and the idle air bleeds.

 

[rod7515]

So i found the book on ebay and it should arrive late next week. Should be an interesting bit of information. Thanks for the information and hopefully I can get this setup on and running sometime this summer.
Rod