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.

holley truck avenger carburetor installed on a v8 engine
underside view of barrels on a four barrel carburetor
disassembling a mechanical secondary for a carburetor
float bowls on a holley carburetor with side hung floats
idle mix screws on a holley avenger carburetor
accelerator pumps for a carburetor

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.