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Old 11-19-2017, 12:34 PM
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Tom Vaught Tom Vaught is offline
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Today I would like to post some information about the different Holley carburetor systems and how they inter-relate to each other.

A Holley 4 BBL carburetor has two idle circuits (minimum) in each carburetor (one on each side of the primary metering block).
To get fuel (and air for one of those circuits) the following has to happen: Fuel flows from the fuel pump to the Primary Float Bowl thru the Primary Float Bowl Needle and Seat. The Needle and Seat sets the fuel level in the Primary Float Bowl. That fuel level setting is important and should be one of the first things checked if the engine will actually run at idle.

Then the fuel goes thru a Idle Feed Restriction (IFR), with an orifice typically in the .032" to .039" size. This restriction can be enlarged slightly but typically cannot be replaced easily without drilling and installing a new IFR brass bushing. .032" for smaller 585-600 cfm carbs and .038-.039" for 850 cfm carbs.
The IFRs are mounted at the bottom of the main well and idle well channels in most older Holley carb metering blocks.
Fuel goes thru the main jet, into the main well, thru the IFR, and then fills the Idle Well Channel.
When the idle circuit is active, the idle fuel is drawn upward past the idle air bleed and then turns downward as an air and fuel mixture toward the bottom of the Transfer Slot Passage. The Throttle Blade is opened only .020" (if set correctly) so very little idle fuel can pass thru that square opening. Most of the fuel takes a separate route past the Idle Mixture Screw and down the Idle Discharge Passage.
Usually with a 'two idle circuit' carb, the Idle Mixture Screw ends up being about one and 1/2 turns out from being seated.
This idle system seemed to work great for many years.

Some modifiers decided to make the idle fuel distribution more equal at idle and put a Primary Metering Block on the Secondary side of the carb. They duplicated the Idle Discharge Passage on the Primary side of the Carb on the Secondary Side of the carb. This worked well as a modification but Holley was already supplying a small amount if Idle Fuel thru a couple of fixed small holes from the Transfer Slot Circuit to the intake. This original circuit was designed to keep fuel in the secondary bowl from going "Sour" when the driver never used the Secondary barrels of the carb. The tiny hole constantly removed a slight amount of fuel from the secondary fuel bowl and added it to the idle mixture in the intake manifold.
So a 3310-1 carb in reality is a "4 corner idle" carb, except that you can't adjust the idle mixture on the secondary side without drilling out the idle discharge passage holes.

So about 1978, 'young' Holley Engineers decided that they needed to have idle circuits on all 4 corners ADJUSTABLE. So they tried to do the mods like the smart 'Modifiers' and drill the proper circuits. I saw a couple of their carbs with holes drilled right into the main venturi bores of the carb. OOPS! But they figured it out.
So about Carb list number 4779-6 (-6 being the revision number of the carb) the 750 cfm Double Pumper became a 4 Idle Circuit carb (that was ADJUSTABLE. So now the "Turns Out" of the Idle Mixture Screws went from 1-1/2 "Turns Out" to 3/4 "Turns Out" as the idle fuel flow requirement stayed the same but the Idle Mixture instead of being divided by 2 (TWO Idle Mixture Circuits) went to 4 (FOUR Idle Mixture Circuits). Best deal is to TRY to set each idle circuit screw the same number of turns out because if you have one screw at 1.5 turns out then a different screw has to be barely turned out to have the proper idle mixture in the intake.

With a divided (dual plane) intake you can adjust the mixture so that the front right barrel is 1 turn out and the rear barrel is 1/2 turn out but the mixture blends together in the manifold so really you did nothing except play games with the idle screws. THIS IS ON A 4 CIRCUIT IDLE CARB. A normal two circuit idle carb on a dual plane intake has ONE idle screw controlling the mixture for those 4 cylinders and changes in or out of the screw DO make a difference.

With an Open Plenum intake the cylinder that is on the intake stroke grabs air and fuel from anyplace in the plenum it can find it.
Like a clothes washing machine with water dashing in all directions over and over at idle.

So my point is if you have a open plenum intake you really do not need a 4 corner idle circuit carb, a stock 3310 2 corner idle carb will work fine. It will have the low location Idle Feed Restrictions and will supply liquid fuel to the top of the metering block where the idle air bleed will add air in the proper manner. The newer "High Location" Idle Feed Restrictions are another "better idea" that in reality made the idle calibration more difficult to tune. "Shaker455", "Tuner" (from several carb boards), and I always convert carbs back to the low mount idle feed restriction location. And then the carb works like it was designed to work in the old 3310 carb days.

So that is a explanation of the idle circuit, how it works, what a 4 corner idle circuit is, how it works, and why you should have a Idle Feed Restriction and Idle Air Bleed in the locations they are in. By the way a 3310 carb has an Idle Air Bleed in the .070" hole range. a .002" drilling change WILL CHANGE the idle circuit air/fuel ratio.

Tom V.

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Last edited by Tom Vaught; 11-19-2017 at 12:42 PM.