by Jon Mikelonis

Introduction
A few short days after FordMuscle announced the acquisition of Project
Redneck, we received numerous emails from experienced off-roaders
offering truck-specific tips and advice to the staff. In more than one instance, FordMuscle was advised to resist running a Holley if we planned to do anything more than dump-duty with the 1978 F250 4x4. You have to respect the insight of the FordMuscle readership because these guys obviously knew that FM's focus on street machines and nothing more than a freshman orientation in the world of off-roading, might cause us to make some mistakes. Unfortunately, those emails came a little too late. In the article First Load, adding a remanufactured 600 CFM Holley double pumper was among our initial upgrades. While we never had a chance to test it off-road, the carburetor proved to be a mediocre solution even on the street. The shear weight of the F250, combined with the underpowered 351M had us starting in first gear around town just to maintain a signal. If you are at all familiar with these trucks, you'll know that first gear is reserved for pulling stumps out of the ground, not pulling out into highway traffic.



Workarounds do exist for standard Holley carburetors that make them better off-road performers. Adding extensions to the vent tubes and incorporating the Holley vent whistle to the metering block(s) are just some of the common tricks for four-wheelers running Holleys. We'll cover those options at the end of this article. Either way, when Holley offered a Truck Avenger to FordMuscle to test on Project Redneck, we gladly accepted the offer and requested the smallest carb of the series, the 470 CFM model.

Flood-Free duty on inclines and nose-down descents is where the Truck Avenger is designed to showoff its most unique benefits. On some beginner backroads in Northern Nevada, we tested the Truck Avenger at a 30 degree descent. As the video below shows, our F250 maintained an idle and showed no spill-over during the test.

See Project Redneck and The Holley Truck Avenger In Action

How Does it Work? Key Features and Benefits of the Truck Avenger
The Truck Avenger uses Holley's most off-road friendly carburetor design as a foundation and adds a few key features that make it off-road full-proof. Vacuum secondaries and side-hung floats are not unique to the Truck Avenger, but both are advantageous for trail use. By nature, side- hung floats are more resistant to bouncing and control sloshing in the bowl better than center-hung floats. Vacuum secondaries delay fuel metering until sufficient engine speeds are reached. Since many off-roaders spend a great deal of time at low engine speeds, vacuum secondaries are preferred. A vacuum actuated secondary helps prevent the bog effect sometimes associated with carburetors equipped with mechanical secondaries. This can be especially important when climbing out of a hole.

So this brings us to what makes the Truck Avenger unique. While the list of features and benefits from the manufacturer is long, the three key components of this carburetor series according to FordMuscle are its...

One-Piece Vent Tube
Metering Block with Integrated Check-Valve in Main Well
Spring Loaded Needle and Seat

After running Project Redneck on a few trails and around town for a month or so with the Truck Avenger installed, we took the time to pull the carburetor off and get it on the bench for a close-up.

The Truck Avenger just looked appropriate on top of the 351M in Project Redneck. Other than the one-piece vent tube, the Truck Avenger appears to be a standard Holley 4160 carburetor at first glance. However, it's not a 4160, the Truck Avenger is a 4150 with vacuum secondaries, electric choke, side-hung float bowls, and metering blocks with replaceable jets on both the primary and secondary sides.		Without question, the "sexiest" part of the Truck Avenger is the one-piece vent tube. With a standard Holley, extreme angles can force fuel up the vent and straight down the venturis. This "crossover" tube prevents that from happening.
To see for ourselves what made the Truck Avenger tick, we cracked open the primary bowl.		Take note of the spring-loaded needle and seat and the baffle. Both are used to provide good resistance to fuel moving in the bowl. The spring-loaded needle and seat is especially important during a high-speed trail run on rough terrain.
While the 470 CFM Truck Avenger is jetted for small motors, it also works well for an underpowered stock 351M. The 470 CFM Truck Avenger comes equipped with number 57 jets in the primaries and a 2.5 power valve. The combination turned out to work quite well for the F250.		Hello? No that's not a new Holley cell phone, it's a metering block with something rattling inside. That rattling is from the Truck Avenger's exclusive check valve that is integrated into the main well. The check valve prevents fuel from flooding out the boosters on an extreme incline or decline. Truck Avenger main well check valve. It's in there, take a listen.
Off-roading with a heavy vehicle requires a carburetor that supports great low to mid-range throttle response. The Truck Avenger uses "High Vacuum Signal" annular boosters in the primaries. By design, annular boosters create a venturi within the main venturi to create more vacuum.		The Truck Avenger comes with site plugs for setting the float level. These are not intended to remain in place and should be replaced with the standard brass plugs once the floats are set properly.

For a complete run-down of features according to Holley themselves, you can see the Holley Off-Road Truck Avenger series at www.holley.com.

Do-It-Yourselfer's Make Their Own "Truck Avenger"
As we mentioned, off-road enthusiasts have been implementing some of the Truck Avenger's features long before Holley decided to market the product. While it would take some serious work to fully convert a standard Holley to equal the Truck Avenger, a few modifications to your Holley may satisfy your personal off-road requirements. Jody York, a technical contributor at www.classicbroncos.com, offered FordMuscle these DIY tips for those willing to take on the task.

Using 1/4" copper tubing, Jody extended the vent tubes on his 4160 model Holley carburetor. The ones shown here extend to within a 1/2" of the top of the air cleaner.		In this picture, Jody modified a vent whistle to fit a Holley with side-hung floats, they are normally used on center-hung floats. In order to make it work, he cut the whistle down to about 1-1/2" and staked it to the bottom of the metering block. Vent whistles are not used on Truck Avenger carburetors because they have check valves to prevent flooding through the boosters. The vent whistle here serves a similar function.
Using 5/16" copper tubing and a small tubing bender, Jody made up this one-piece vent tube. It looks awfully close to the production Truck Avenger piece.

Small 470 CFM Truck Avenger Performs 'Round Town
Summon your peers about the right-sized carburetor for your application
and you'll get a wide range of opinions. In this hobby, with age comes wisdom and the wise can confirm that there's something valid about backing off your tendency to go with big carburetors. It may seem that a 470 CFM Holley is tiny for a 351 cubic inch motor but that's where the standard carb formula lead us when choosing from the three available Truck Avenger carbs. According to Holley, the 470 CFM unit is best suited for 4-cylinders, V6s, and V8s under 334 cubic inches even though it worked well for our larger displacement application.

Computing the standard carb selection formula for Project Redneck we arrived at a figure of 406 CFM. We came to this result using 351 cubic inches, 5000 RPMs, and a volumetric efficiency of 80%. Most stock, production, low performance motors fall between 75% and 85% in volumetric efficiency, so 80% seemed to be a conservative figure for this our "M" motor.

In this case, experience paid off providing us with great drivability around town and adequate throttle response even with the stock 351M. Besides, at $3.50 a gallon, we could use all the help we could get.