How Tos / Tech

Off-Road Avenger: Tuning A Holley Truck Avenger For The Trail

Let me start by confessing that I am not a carburetor tuning expert. There are those skilled old timers that can tune a carb by ear and carry spare jets in their pocket. For the rest of us though, there is no need to be intimidated by a carburetor. We recently added a 770 cfm aluminum Holley Truck Avenger on top of the 460 engine and wanted to share how we chose the carb and the fine tuning we performed to make it work great on the trail. Note that we were not as concerned with maximum power potential as we were with crisp throttle response, linear power, and the ability to idle at any angle.

We’ll start with some carburetor basics and how they apply to the 4×4 world. We will focus on four barrel carburetors that are commonly run on V8 engines—if you have a four cylinder or six cylinder you are likely better off with a smaller two barrel carb, but many of the same principles still apply.

m_Photo01
m_photo2
m_Photo03
m_Photo04
m_Photo05
m_Photo06

The Truck Avenger uses and a vented crossover tube to keep fuel from sloshing off-road and annular boosters that provide a high vacuum signal for our secondaries. Out of the box, the Truck Avenger performed better than the carb we replaced, but there was still room for improvement.

Our Holley Truck Avenger is a square bore, which means that the primary throttle bores are the same size as the secondaries. Spread bores, like the Quadrajet, use smaller primary throttle bores that can increase mileage and improve throttle response if you are running a smaller displacement engine than our 460. The most important factor is that your intake manifold design matches your carburetor, which we took into consideration when choosing the Edelbrock Performer RPM manifold on our big block.

Mechanical secondaries, like the famous Holley Double Pumper, are preferable for drag racing applications due to their instant response; however, secondaries that use a vacuum signal, like our Truck Avenger, are better suited to a street vehicle since they take factors such as load and vacuum into account in addition to the throttle position. The rate at which the secondaries open on our aluminum Truck Avenger are very easy to adjust by changing the stiffness of the vacuum diaphragm spring. We picked up an assortment of springs from Summit Racing and they are easily swapped out with a Phillips head screwdriver.

Float bowls hold fuel at the ready to be injected by the carburetor. Holley offers both side hung or center hung floats, and our Truck Avenger came with side hung floats at each end of the carb. They use a crossover tube between them to keep fuel from sloshing around during steep climbs and descents. We adjusted the floats as low as possible using the included glass sites. The downside to low floats is that there is less fuel available for full throttle romps, but less fuel in the bowl means that it is less susceptible to sloshing around or spilling into the carb at extreme angles off-road, so it is a compromise we were willing to make.

Holley sends the Truck Avenger (and all Holley carbs for that matter) adjusted for sea level, and out of the box our carb was running rich in the thin air at our 4400 foot elevation. We started by going down two jet sizes on the primaries (74 to 71) and two jet sizes on the secondaries (99 to 97) as recommended by Holley. We bought individual jets from Summit, but they are available as a kit with a variety of sizes as well. The jets were easy to change with a flathead screwdriver.

The accelerator pump is responsible for delivering fuel down the throttle bores when the throttle is first opened. Out of the box we had a stumble off idle that we were able to remedy by adjusting the accelerator pump. First we adjusted the linkage to ensure that it is only lightly making contact at idle with the throttle closed. We also changed the accelerator pump cam from the factory orange cam to a blue cam with a kit from Summit Racing. We moved the linkage to the second screw hole (marked on the throttle linkage) to ensure that the accelerator pump is fully open at wide open throttle.

Tags: , ,

2 Comments

  1. In my experiences, a stumble off idle (especially on a new Holley) is caused by the throttle plates being too far open at idle. This causes the transition circuit to be in use at idle. So, as the throttle plates open, the transition circuit doesn’t function, causing a momentary lean mixture and stumble.

  2. This particular carb has gotten a lot of negative feedback amongst off-road enthusiasts and I hope I can dispel a few myths lingering around this carb and explain a few things that are unique about it.

    For starters, any carburetor can be suitable for off-roading. This particular carb has easily adjusted floats (all 4150 and 4160 carbs do for that matter) which is essential for controlling flood outs during high angle maneuvering.

    This carb has vacuum secondaries which is an essential feature in my opinion for any carb expected to operate well on the street and/or at low rpm.

    This carb has a fancy vent crossover tube further enhancing its ability to prevent high angle flooding.

    This carb is a 4160 series carb so it doesn’t quite have as many tunable features of a 4150 series carb, however the extra tune-ability is likely unnecessary for an off-road application.

    This carb has annular boosters in all 4 barrels! This is the big difference. Annular boosters are typically only used in incredibly large racing carburetors for various reasons however when used appropriately they function exceptionally well in low rpm applications. The only significant shortcoming of the overall superior annular booster is it doesn’t work well on hot manifolds. Annular boosters atomize fuel much better than the traditional down-leg and strait-leg boosters.

    When an annular booster dumps finely atomized fuel into a hot manifold it boils and vaporizes completely, causing a lean condition and often a lean miss. Sound familiar? Off road vehicles almost always use a “hot manifold.” Hot manifolds can usually be summarized as any low rise manifold. Yes, that includes aftermarket aluminum low rise manifolds. The floor on these manifolds are directly exposed to excessive engine heat and that creeps into the floor of the plenum boiling up all of the annular booster’s finely atomized fuel.

    Drop and strait leg boosters don’t atomize the fuel as finely and will in turn cool the hot manifold and vaporization of fuel is minimized.

    Summation: annular booster carbs do make for all around better performance in any application when installed on a cold intake manifold (think air gap). Drop and strait leg booster carbs don’t atomize fuel as effectively however when combined with a hot manifold (think performer) they produce great results just the same.

Leave a Reply

Your email address will not be published. Required fields are marked *

*

This site uses Akismet to reduce spam. Learn how your comment data is processed.