Accelerator Pumps: Accelerator pumps function to make up for a lag in fuel flow when there is a sudden opening of the throttle. There are three parts to the accelerator pump mechanism on Holley's The pump diaphragm, the pump cam, and the discharge nozzle. All are adjustable to some degree. Most Holley's comes with a 30cc pump. This means that the pump has a capacity of 30cc, per 10 strokes, or 3 cc per stroke. This can be upgraded to a 50 cc pump if needed. The pump cam determines how the shot is spread out in terms of angle of the throttle lever. Pump cams come in five different colors, each cam has three locating holes. Depending on the color and hole selected, a certain amount of the 3cc or 5cc volume will be delivered over a portion of the throttle opening. For example, the stock pink cam set to the first hole (towards front of the carb) will deliver approximately 2cc over 30 degrees of throttle opening. These rates are not easy to figure out, but can be found in most Holley books. Looking at the profile of the cam will also reveal how aggressive the shot duration is. Finally discharge nozzles work in conjunction with the pump cams in that they determine how quickly the shot is discharged. Virtually all new Holley double pumpers come with 28 primary and 32 secondary shooters. (The size corresponds to the orifice size, measured in thousandths of an inch.) The stock shooters are 99% of the time inadequate and you'll experience a bog when you "punch" the throttle. We're not quite sure why Holley selects these shooters, but it's the first thing you should change. Most engines will respond very well to 35 shooters in both the primary and secondaries. If you drag race the car and use slicks, you may find 37's or 40's are needed to overcome the temporary engine "bog" when the slicks "hook". The key to shooters is to use the smallest size possible that eliminates the bog. Too large of a shooter will cause a stumble. A stumble is when the engine responds well to sudden throttle opening, but then hesitates before settling out. In this case less fuel, or a smaller pump shot is necessary. It should be noted that many times you can make a pump cam adjustment and accomplish the same thing as going up a size in shooters. It's simply a matter of trying out different combinations to find what works best for your car.


Power Valves: Power-valves are simply vacuum operated "switches" that open when engine vacuum reaches a specific level. When the valve is open additional fuel flows through to the main metering system. Power-valves are designed to provided enrichment during high-load/low vacuum conditions. A general rule of thumb is to pick a power-valve with a rating half of the engine manifold vacuum at idle. Stock Holley's come with a 6.5 in.Hg valve which is typically good for cars with 14-18 inches of vacuum.

Other Tuning Tools: The key to tuning your Holley for more power is to make changes that you can measure consistently. The track or dyno are very useful but have their limitations. Directly measuring air-fuel ratio is a very powerful tool in tuning engines. Unfortunately the wide band O2 sensors which have the required sensitivity are quite expensive and not practical for most consumers. Many dyno shops have wide band O2 sensor for use at additional costs. Some companies have developed dash-mount gauges using narrow band "lambda" sensors, however we have found them to be ineffective in telling more than simply gross lean or rich conditions. Finally exhaust gas temperature (EGT) sensors are also powerful tools in nailing down optimum air-fuel ratios.

Obviously we have just touched on some tuning fundamentals, and by no means do we attempt provide an exhaustive source on Holley tuning. The more knowledgeable you become about your Holley, through reading the various books and trying your own changes, the more power and economy you will find. F/M

As we said before, the greatest benefit to Holley carbs is that they are so precisely tunable for a specific engine combination. This makes Holley's the dominant carb for all levels of driving, ranging from the daily commuter to Winston Cup cars. However this does not mean tuning a Holley is easy. It is very easy to mis-tune a Holley and end up with poor economy and performance. The best way to go about adjusting and tuning your Holley is to first understand how it works. There are at least a dozen good Holley tuning books which offer excellent advice. We recommend a very simple book, "The Holley Carburetor Handbook" by Mike Ulrich.

Also critical for tuning a Holley is a jet kit. Considering most speed shops will sell you a pair of jets for $7, the $30 for a full assortment of jets is quite a bargain, and proves to be very convenient when tuning. Some of the other tuning parts, such as shooters, pump cams, and power-valves, don't need much tinkering once you've found the right ones.

Setting idle: Setting the idle speed and mixture on a Holley is very straight forward. First you should set the idle speed, using the adjustment screw against the throttle lever. In our case, since we don't run a choke, we set the idle rpm slightly higher to compensate for cold starts. If you do run a choke be sure to also set the fast idle speed (when the choke is closed.) Idle mixture is set by the tiny adjustment screws in the side of the metering blocks. On Holley's, backing the screw out makes the mixture richer (more fuel passes through the orifice.) We find the best adjustment is no more than two full turns out off the seat. If you have to go more than two turns out to get the engine to idle, you probably have another problem, such as a vacuum leak, or perhaps not enough idle speed. In some cases, especially with big cams and low idle vacuum, you may find that you have to crank the idle speed way up, and the idle mixture screws don't seem to make a difference. The problem here is that as you increase idle speed, more of the transfer slot becomes exposed, and the carb begins to meter through the primary circuit, causing a excessively rich idle mixture. In this case it may be necessary to drill a tiny 1/8" hole in each the throttle plates to allow some air to pass through, enabling you to close the throttle plates. Most Holley books outline this procedure.

Jetting: Typically when we go to the track we make as many as 6 to 8 jet changes depending on weather. Holley carbs meter very efficiently, and thus finding the right jet combination to match the temperature and barometric pressure conditions, as well as your particular engine combo can be a challenge. However, jetting Holley carbs to make maximum power is part of the fun of owning a Holley. Stock jetting for a Holley carb is generally a good start; but in most cases this jetting is a bit rich. For a small percentage of engines, the 'out of the box' jetting may prove to be on the lean side. Experimenting with different jetting is the only way to find extra power, and even fuel economy. We highly recommend purchasing a Holley jet assortment.

Obviously determining optimum jetting is best achieved on an engine or chassis dyno, where you can get consistent runs under similar conditions. Since this isn't an option for most of us, the next best place to tune is at the track, or on a stretch of road where you can safely and consistently drive the car. For most double pumper, we recommend a spread of six jet sizes between primary and secondary jets. Most 200-300 HP engines will run well with 64 primary jets and 70 secondary jets, as a start. You can further tune the primaries by driving the car at part throttle on a long stretch of road (such that the secondaries are not open). Jet down in two size increments until you feel the engine surge at part throttle driving conditions. Then jet up two sizes. This is a lean-best setting for the primaries. Usually six sizes up on the secondaries is perfect, however you can go as high as eight sizes to run rich-best at wide open throttle for added protection against detonation. At the track the secondaries are best tuned by jetting up or down, in two size increments, and observing 1/8 or 1/4 mile speeds and ET.