What was once a custom tuner...
What was once a custom tuner part is now available off the shelf. Holley's tunable metering block (left) fits its PN 080787-1 HP 2-bbl carb (right) and offers full adjustability of the carb's fuel curve previously only available from specialists.
In the not too distant racing past racers had to build some of the heavy-duty parts they needed from scratch, or customize existing ones to make them suitable for racing. Today, racing parts that were once only available from your or your tuners' ingenuity are now mass-produced and ready off the shelf for even the most entry-level racing.
A case in point is this adjustable metering block derived from the stock metering block for Holley's PN 7448-350 two-barrel (2-bbl) carburetor. This unit was originally designed as a 350-cfm Ford replacement carb for street applications, but it is used nationwide in weekly racing series requiring a 350-cfm 2-bbl. Now, a fully tunable metering block that was once only available as an exotic drilled piece from a custom carburetor tuner or engine builder is offered as a retrofit metering block straight from Holley as PN 134-276. It's been designed particularly for its Keith Dorton Signature Series PN 0-80787-1 HP, a race-ready 350-cfm 2-bbl carburetor. In fact, all new Holley PN 080787-1 HP 2-bbls are being shipped with this tunable metering block already installed.
2-bbl Metering Block Origin While carb and engine tuners do work their tuning trade on the main body of a Holley carb for gains, it's the fine adjusting of the metering block that unlocks power and driveability from the carb. Plenty of professional and amateur carb tuners have taken their drill bits to the holes in the metering block in search of performance. For instance, Circle Track Tech Council member Keith Dorton originally worked with Holley to develop and test a tunable metering block with adjustable screw-in orifices (bleeds) for its 390-cfm, 4-bbl; it was sold as a separate part number and used in top touring series racing.
This cutaway shows the positioning...
This cutaway shows the positioning of the five screw-in emulsion air bleeds for the main circuit of the carb. A tuning "bleed kit" (PN 36-322) contains 10 each of these six other bleed sizes: Blank/solid, 0.020-inch, 0.031-inch, 0.040-inch, 0.046-inch, and 0.052-inch. The blanks are for drilling custom hole sizes.
That extensively adjustable metering block didn't go unnoticed by other engine builders. Indeed, expert Late Model engine builder Darrell Poe of Bear Creek, North Carolina, wanted to see how this 4-bbl adjustable metering block could be adapted to a 2-bbl carb-the Holley 7448 he's required to run on the NASCAR Late Model Stock engines he's noted for. Poe has a True Value Mechanic of the Year National Champion award from 2000, and six yearly regional True Value Mechanic awards, including one in 2002, to attest to how highly his peers deem his capabilities. He started to experiment with the 4-bbl adjustable metering block in the fall of 2001, and showed one of these production-modified metering blocks from Holley for the 390 4-bbl to NASCAR Weekly Racing officials to get a preliminary ruling on its legality with a 2-bbl. Plenty of discussion followed.
They rejected it outright for weekly series racing, even though the Holley factory part had been available for about a year for BGN (Busch Grand National)-level engines. In fact, Poe took some heat from fellow competitors, being accused of this extensive metering block modifying, even though it came from the factory that way. This was sort of a backhanded compliment. Holley's racing field tech representative Bobby Writesman clarified to the NASCAR Weekly Racing powers that Poe was in fact offering up a factory-produced part for the Dorton series 390 4-bbl. This was the initial contact between Holley and Poe, which led to his help in the R&D of a 2-bbl version of the tunable metering block and eventual acceptance. The adjustable 2-bbl metering block became eligible for use in NASCAR's Weekly Racing Series for Late Models beginning in 2002.
What's Adjustable? The Holley factory tunable 2-bbl metering block is easily identified by the numbers "12323" stamped into the casting on its throttle lever side. It comes complete with mixture screws, power valve, jets, gaskets, vent whistle, and a complete set of emulsion bleeds, including power valve channel restrictions, and a high-rpm "anti-siphon bleed" or "kill bleed" for the accelerator pump passage on the jet side of the block.
The five screw-in emulsion...
The five screw-in emulsion air bleeds on each side of the metering block are sized from the factory beginning at the bottom bleed to top one: 0.031-inch, 0.031-inch, 0.031-inch, 0.020-inch, and 0.020-inch. They give a tuner full adjustability to the 2-bbl's fuel curve to fit any engine brand and combination.
Emulsion Air Bleeds For starters, the stock 7748 2-bbl metering block doesn't have emulsion tube holes drilled in it. According to Poe, "Carburetor tuners and engine builders trying to alter the fuel curve started playing around with the emulsion tube channels. They drilled different sized holes to introduce and mix air with the fuel, and to get it coming up into the booster to gain performance. You're ultimately trying to get the engine to operate at say 12.5:1 air/fuel ratio throughout its power curve, from top to bottom. Right out of the box, originally, the stock 2-bbl body and metering block do exactly the opposite of what we want them to do in a racing engine: It runs real lean down low (rpm) and real rich up top."
When you introduce air to this fuel channel, you're emulsifying the fuel-you're introducing air in different volume amounts (controlled by the number of bleed holes and their diameter), and at different points in the fuel curve (controlled by their vertical placement in the channel)-to alter the fuel curve and maintain the best air/fuel ratio for your engine combination. The tuning goal is to optimize the quality of the air/fuel mixture coming from the booster throughout the power band. The Holley 2-bbl tunable metering block has five adjustable emulsion screw-in bleed holes, and generally the largest size bleed will be at the bottom of the main well and the smallest at the top.
Unfortunately, a major problem with modifying these fuel channels with a drill on a stock Holley metering block is that once you make a hole that turns out to be too big and hurts performance, it's very difficult to plug the hole and start over with a smaller hole. You can quickly have a large collection of useless "experimental" metering blocks. That's why Holley offers various size screw-in emulsion bleed jets from 0.000-inch (blank)-0.078-inch for all the tunable orifices in this 2-bbl adjustable metering block.