Carb knowledge help needed

rod7515 said
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:

rod7515 said
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.

rod7515 said
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.

rod7515 said
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.

rod7515 said
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.