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Old 12-13-2017, 08:18 AM
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Tom Vaught Tom Vaught is offline
Boost Engineer
Join Date: Dec 2001
Location: The United States of America
Posts: 30,813

Today going to talk about Holley Carb Bases.
Why they are machined the way that they are and ways to identify different bases and their circuits.

Holley Carb Baseplates and the difference between "Old School" Vacuum 3310-2 type Idle Circuitry and Double Pumper 4779-6 Idle Circuitry.

Also going to show how we used to mod the Baseplate for 4 Corner Idle Circuits. There is a lot more to adding "4 Corner Idle" capability on a 3310-2 and many times it is not really needed for a "Warmed Over" 3310 type carb.

Picture #1 (left Pic) shows a aftermarket base for a Chevelle 3310 (780 cfm) vacuum carb style idle circuit and a Corvette 850 CFM vacuum carb. Several things to notice:

1) The Aftermarket Base has the idle feel holes in the Base set up for both carbs. The Idle Feed Holes on a 850 CFM Carb are straight across from each other. The 600 to 800 cfm carbs have staggered Idle Feed Holes in the base. If you look at the Base you will see that the base has the 8 Mounting screw holes, it has the machined grooves in the base that go (Look at the upper part of the picture) Idle feed holes down toward the lower idle feed holes but the slot does not actually connect the Idle feed Holes together. Picture number #2 will show that detail in a bit.

The "staggered" Idle Feed Holes were used on the 780 CFM carb base 3310-0 and used a 1-11/16" diameter Throttle Blade marked with a 172 number on the PRIMARY Blades. The Secondary Blades were marked with a 173 number on the Throttle Blades. That same Throttle Base Blade could be used on a 650 CFM Double Pumper Carb, 3310 Vacuum Carbs, and all the way up to the 800 CFM Double Pumper Carbs.

The 850 CFM Vacuum Carb and 850 CFM Double Pumper Carb got a unique #180 Throttle Blade in the old days. Along with that previously mentioned different Idle Circuit hole position.

So that is why the AFTERMARKET BASE in Picture #1 has both holes machined. So it will replace both series bases and it probably uses the very slightly larger 1/16" bigger 180 throttle blade.

Picture #2 (Middle Pic) shows a Holley Double Pumper Base designed for a 650 cfm #4777 to 800 cfm #4780 carb. This base is set up for 4 corner idle because you can see two solid blugs pressed into drilled holes in the base that separate the machines "slot" mentioned previously.

As you can see the idle holes vs the Transfer Slot holes are "Staggered" as mentioned earlier. You can also see the "PV "Blow-Out" Protection ball and retainer at the top of the picture close to the "A"). The Secondary side does not have that set of parts so that means the Metering Block on the secondary used a Blocked Power Valve Plug or was not drilled for that PV Install capability.

The base also has Throttle Blades with LARGE "Idle Air Bleed Holes" drilled, so this is a older base that was made before the recent Holley IDLE-EZE parts were available.

NEVER INSTALL THE MIDDLE OF THE BASE TWO SCREWS THAT ARE CLOSE TO THE 4 CORNER IDLE CIRCUIT "PLUGS" ON AN OPEN PLENUM MANIFOLD. The screws could vibrate loose and drop into the engine combustion chamber/wedge the valve. I Never install them in any Holley Carb I touch.

So with the idle circuit slot plugged (on both slots), you can use the 4 Corner Idle Capability and turn your Mixture Screws out 3/4 of a turn on the screw vs 1-1/2 turns on the 2-corner idle carbs.

Picture #3 (Right Pic) shows the OEM Idle Feed SLOT in the base. It connects from the Primary Idle feed Drillng and you can see that it goes to left side of the picture but then stops.
You can also see the Transfer Slot machining in the holes above the idle circuit holes.
"Fuel Bowl Purge" on the rear bowl of the carb is handled by a very small drilled BELOW the Transfer Slot in the base. This allows the carb to remove stale gas if the Secondary barrels are not used for any period of time.

So that covers the basics on the different bases, why the fuel circuits in the base are different for a 2 corner idle carb vs a 4 corner idle carb, The Throttle Blade sizes for easy identification at a swap meet, and Fuel Bowl stale Gas purging.

Tom V.
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"Engineers do stuff for reasons" Tom Vaught

Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward.

Last edited by Tom Vaught; 12-13-2017 at 08:23 AM.