So your factory carb just won't handle another rebuild? Perhaps your stocker is just fine but lacks the performance you're after. In either case, one of the first thoughts you'll likely have is bolting up a Holley 4bbl, and while you're at it, an aftermarket intake. Don't worry, you're not alone. Several million of your fellow gearheads have gone down the path before you - with outstanding results.

Anyone planning on serious performance tuning needs the Holley Jet Kit. For about $40, you'll save a lot of trips to the speed shop, time, money and effort by having a full line of main jets on hand. Power Valves are not included and available separately.

Unless you've gone bug-nuts on cam and headwork, an out of the box Holley 4150 or 4160 series will bring you some pretty decent performance as long as you stay with in the recommended CFM range for your engine size and RPM requirements. When selecting a carb, especially for a relatively low RPM application like an off-road vehicle, be conservative in your selection, and realistic in the RPM range where you'll do most of your driving.

HOT TIP

Leave the "Double Pumpers" for the race crowd. A well tuned vacuum secondary will return better performance in the majority of situations you'll encounter, and your gas tank won't drain faster than a flushing toilet!

Once you've made the decision to bolt up….say…. a Holley 670 Street Avenger to a…say…stock 400 CID Ford with an Edelbrock Performer (Coincidence? I think not!), you'll be rewarded with performance that leaves you wondering why you ever waited so long to make the swap. But as impressed as you are with your newfound power, there's more to be had. Sometimes a lot more.

In reality, the Street Avenger series, like any aftermarket performance carb is jetted to provide good, all around performance for a wide variety of applications. In other words, it's a compromise. When you realize that the 670 will run on engines as varied as a 302 Ford or a 454 Chevy, you'll also realize that the jetting requirements for those motors are nowhere near alike. That's when re-jetting the carb comes in.

For most off-roaders, jetting a carb is a lot like rebuilding an automatic. Most of us know the basics on how a carb functions, but there's a big difference between knowing how a carb works, and making it do magic. Fortunately, since most off-road engines will never see the high side of 5000 RPM, there's little need to get into the heavy wizardry of annular boosters, PVCR or Air Bleed tuning. Most wheelers need only concentrate on the Idle Screws, Main Jets, and Power Valves.

In a perfect world, we'd all jet our carbs with the aid of a Dyno. In a semi-perfect world we'd use an exhaust gas analyzer (such as those used by smog stations, or an on-board model that uses an oxygen sensor). In the real world though, we often resort to trial and error.

Before beginning a re-jet, you'll need certain tools. In addition to a basic hand tool set, your best friend will be a vacuum gauge. Add a spark plug wrench (because you'll be looking at your plugs from time to time) and you've got the basics.

Once the float bowl is off, the fun really begins. Take the time to clean and inspect all the parts on the carb while it's on the bench. You could come across problems you didn't know you had before they really cause a ruckus under your hood.

Baseline

To begin the process, evaluate your current set-up. If you're blowing black smoke, there's soot in the exhaust, or your plugs are wet or carbon fouled, you've likely jetted too rich. On the other hand, if you hear the engine detonate (when it's timed properly and using a suitable grade fuel), the plugs are either spotless or heat blistered around the ceramic tip, or you experience surging or backfiring at speed, you're likely too lean - perhaps dangerously so. While an overly rich mixture can't do much if any damage to your engine, a lean condition can burn valves, crack heads, knock holes in pistons, and generally ruin your day.

Once you've established whether you're running on the rich or lean side, then it's time to do a little jetting. We'll take this opportunity to tell you that this is a greatly condensed version. If you are seriously considering jetting your own carb, be sure to pick up any of the several great books from HP Books or S/A Designs on the Holley. They go into great detail on the how's, what's, where's and why's of the modern Holley Carb.

Down In It

After disconnecting the battery, chocking the wheels, and taking all the usual safety precautions, allow the engine to cool down. You'll find it's much easier (and safer for the novice carb god) to work with a cool engine. Hot motors can lead to flash fires and considerable damage and injury.

Remove the air cleaner assembly and mark the location of all vacuum hoses, accessories and wires. Once you've stripped the top end of your engine and exposed the carb, it's a simple 4-bolt process to get the carb off the intake manifold and onto the bench. While experts can jet on the vehicle, remember, this is an easier method for the beginner.

Take the time to clean the carb externally so that no dirt will get in to the carb body once the float bowls and metering blocks are off.

When you get to the jets themselves you'll see the simplicity of Holley's design. When working on the main metering system, there's few surprises. Tuning is a simple matter of screwing the proper jet into the proper hole. Determining what size that jet is can be the tricky part.

Carefully remove the four bolts holding the primary float bowl. The primary bowl is on the foreword facing end of the carb - the side nearest the choke/air horn. If you're using the "blue" gaskets, there's a good chance they'll be reusable.

At this time you'll see the two main jets for the primary side, and the power valve centered above them. Gently tap the aluminum metering block to break it free and remove it from the carb.

Changing jets can be accomplished with a specialty tool available from Holley, Moroso and others, or carefully use a broad bladed screwdriver.

When making a jetting change, change only one or two sizes at a time. For example, if you are too rich with a 65 main jet in the primaries and 68's in the secondaries, change to a 64/67 or a 63/66 combo rather than making huge jumps in sizes. It's more time consuming, but you'll get into less trouble in the long run.

As for the power valve, most mild engines respond well with a #65, 75 or 85 power valve installed. Here's where the vacuum gauge comes into play. You'll need to have a VG installed, or hook up a temporary diagnostic gauge, running it into the cab.

Take a highway cruise (before pulling the carb of course!) and determine the vac level at cruise. Ideally, you'll want a power valve rated 2 inch of vac below the figure you arrive at.

Installing the PV is a simple matter of unscrewing the stock valve from the metering block and reinstalling the new one. Be sure the PV gasket does not restrict flow to the PVCR channels.

Bolting the carb back up is simply the reverse of disassembly. Align the gaskets, cover with a light coating of grease to prevent sticking, and torque to spec.

Note that you'll only find a PV on the primary side. Unless you did the nasty and bought a race carb, the secondary side will have a plug where the PV should be.

After sizing the jets and selecting the right Power Valve, simply reverse the disassembly process and return the carb to service. Remember that this is a precision metering instrument and treat it accordingly. A light touch will save you from replacing costly aluminum bowls, housings, metering plates and more.

Once you've reassembled the carb and reinstalled all the connections, you may need to reset the idle mixture screws. To set the screws, adjust until maximum rpm is reached (per screw) and you're good to go. While you can hear the RPM increase, you'll be better served by using a tach to get an accurate reading.

On The Road Again

Since this is a trial and error process Carbslayer, get thee thy carb upon thee highway, and go forth unto perform thine road test.

Go out and run the vehicle through it's usual routine on and off the road. Recheck the plugs and tailpipe for any signs of change. Don't "trust" seat of the pants feel as the final word on changes to performance. If you're lucky, you'll get it the first time, but if not, it's "back to the bench for you kid!"

Holley Powersports Tech Service
P.O. Box 10360
Bowling Green, KY, 42101-7360
Telephone 270-782-3225
help@support.holley.com
www.Holley.com