Tips On Holley Carburetors
By Ron Ceridono
As we've said, the Holley and Demon carburetors are quite close in design, so the overhaul procedure is virtually identical. However, if rebuilding a Holley carburetor is something that you'll be doing in the future, we suggest you order a Holley Tuning & Troubleshooting DVD from BoxWrench.net (they also offer a huge poster of a 4150 mechanical secondary carburetor and an excellent DVD on basic engine rebuilding).
Some of the most often asked questions about Holley carburetors have to do with power valve selection, idle adjustment, jet selection, and accelerator pump adjustment. Thanks to the tech team at Holley, here is some advice on each.
Selecting Power Valves
The power enrichment system supplies additional fuel to the main system during heavy load or full power situations. Holley carburetors utilize a vacuum-operated power enrichment system and a selection of power valves is available to "time" this system's operation to the engine's specific needs. Most of the popular Holley "Street Legal" and "Street Performance" carburetors incorporate a power valve blowout protection system in the form of special check valve in the throttle body. This check valve is designed to normally be open, but will quickly seat to close off the internal vacuum passage when a backfire occurs.
If you have a carburetor older than 1992 (or you have experienced an extreme backfire) and suspect a blown power valve, use this simple test. At idle, turn your idle mixture screws (found on the side of the metering block) all the way in. If your engine dies the power valve is not blown.
Each Holley power valve is stamped with a number to indicate the vacuum opening point. For example, the number "65" indicates that the power valve will open when the engine vacuum drops to 6.5 inches or below. To determine which power valve to use, an accurate vacuum gauge should be used. A competition or race engine with a long duration, high-overlap camshaft will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take a vacuum reading with the engine thoroughly warmed up and at idle. If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should be half the intake manifold vacuum reading taken. As an example, a 13-inch Hg vacuum reading divided by 2 indicates a 6.5 power valve. If the reading divided by 2 lands on an even number, you should select the next lowest power valve. Example: An 8-inch Hg vacuum reading divided by 2 equals a No. 4 power valve. Since there is no No. 4 power valve, you should use a 3.5.
Idle Adjustments
The initial adjustment is made by turning the mixture screws (located on each side of the main metering block) in a clockwise direction until they lightly hit bottom. Back them both off 1 1/2 turns. Connect a vacuum gauge to a carburetor vacuum port that has full manifold vacuum at idle. Start the engine and once it has warmed up and the idle stabilized, the choke should be disengaged. Adjust the idle-mixture screws to obtain the highest vacuum reading. In some cases it is advisable to turn the idle screws in just enough to get a 25-rpm drop; this is known as lean-best idle and will help keep the plugs clean.
A passage originating below the throttle plate allows vacuum to act on the spring-loaded diaphragm of the power valve to close it at an established value. | 
Needle-and-seat assemblies are changed during a rebuild. Poor sealing can cause over-rich conditions and flooding. | 
Adjusting the floats on Demon (shown) and Holley carburetors is done externally. Brass floats should be checked for leaks, solid floats on a scale. |