Some of the most popular Corvettes ever built were the chrome-bumper cars from '56 through '72. Within that model lineup, Holley carburetors saw service from '65 through '72. In some of those model years ('67 is a good example), Holleys were used exclusively. And in the aftermarket, Holley has offered-and still offers-hundreds of different carburetors targeted at different vehicle combinations. Plenty have been retrofitted to Corvettes of all model years, too.
A majority of Holley carburetors were engineered as racing or high performance pieces. That meant they were (and still are) equipped with a number of easy-to-operate external tuning adjustments. For the performance freak, that's good news. You can easily fine-tune the carburetor to suit your application, and you can accomplish the basic tuning without going inside the carburetor. So far so good, but there's a bit of a hitch: If someone doesn't know what they're doing, they can just as easily turn a sweet running Corvette into a dud.
So which way do you turn the knobs, and which knobs do you turn? While not all of the adjustments involve actual "knobs," the basic external setup of a Holley isn't difficult. Everything can be accomplished at home with simple hand tools. For a closer look, check out the following. We're using a very common vacuum-secondary 3310 Holley as the example, and everything shown here applies to most factory installed Holleys, too.
|Difficulty Index - 2 Wrenches|
|Anyone’s Project: no tools required||1 Wrench|
|Beginner: basic tools||2 Wrenches|
|Experienced: special tools||3 Wrenches|
|Accomplished: special tools and outside help||4 Wrenches|
|Professionals Only: send this work out||5 Wrenches|
The tools you need: A vacuum...
The tools you need: A vacuum gauge, a couple of flat-blade screwdrivers, a Phillips screwdriver, and a 5/8-inch combination wrench. Not shown is a 0.015-inch feeler gauge.
Float setup is first. There...
Float setup is first. There are a number of theories about the right and wrong way to adjust floats on a Holley carburetor. One of the accepted methods works like this: On a running engine, set the primary floats so that fuel just seeps out of the sight plug. This means that the side of the bowl should be "wet" as you set the float level.
On the secondary side the...
On the secondary side the sight plug is situated lower on the bowl. In this case, set the floats slightly higher. Occasionally, you'll have to rock the car to get fuel to seep out. By the way, an absorbent shop towel placed on the intake manifold below the sight plug is a good idea to catch wayward gasoline.
To set the floats, loosen...
To set the floats, loosen the slotted lock screw at the top of the fuel bowl, and then turn the large adjustment nut (which actually moves the needle-and-seat assembly up and down). Retighten the lock screw once the adjustment has been finalized. Keep an eye on the sight plughole. Remember, the fuel shouldn't pour out. Several companies offer clear sight plugs that allow you to see the float level without actually removing the plug. That can be a bonus, since raw fuel won't pour onto a hot intake manifold.
With the float level set,...
With the float level set, you can fine-tune the accelerator pump discharge nozzle (shooter). All Holley-equipped Corvettes have at least one shooter. Aftermarket mechanical-secondary Holleys (as well as the carbs on L88 and ZL1 Corvettes) were double pumpers, with two accelerator pumps and two shooters. If the Corvette's initial acceleration produces a hesitation and then picks up, the shooter size must be increased. In some cases, the accelerator pump shooter may be so small it creates a lean condition, and the engine will backfire during acceleration. If the shooter is too large, the off-idle acceleration will not be crisp or clean, and there may be a puff of black smoke out of the headers during acceleration. Holley offers many sizes, shapes, and configurations of shooters. The two high performance models include the tube discharge jobs and the straight-end discharge versions. All shooters are numbered from 25 through 52; the numbers indicate the drill size of the shooter orifice in thousandths of an inch
Accelerator pump shooters...
Accelerator pump shooters are attached to the main body of the carburetor in the venturi area (in this location, behind the bowl vent) with a Phillips-head screw.