Welcome to the Carburetor Forum; Please Read.

[Jester]

Welcome to the Carb Board everybody. Hopefully you'll find it useful for whatever carb related problem or question you might have, stock or aftermarket. Please note that does not include paint/body, brakes, or tranny questions about your car that happens to be carbed. If your question is on swapping to a carb from either TBI or TPI, please read one or more of the following articles first. Swapping from EFI to a Carb in a Thirdgen How to Convert from TPI to Carb Torque Converter Lockup Switch If you need help choosing the right carb for you, or just want a quick rundown of the options, read this. Choosing the Proper Carb Choosing the Proper Performer Carb If you're looking to breathe a little life into your stock Q-jet, these can be very helpful. * Tuning for Performance with your Computer-Controlled Quadrajet QuadraJet Secondary Metering Rods Many of us here run stock-style non computer controlled distributers. Something more or less unique to this area of thirdgen. If you're looking to tune yours, look here: Non-Computer Controlled HEI Setup For Performance Many of us here run Holleys (by a little over double the percentage of the runner up). If you run one as well, please read Holley's excellent info on the various circuits that make up a Holley. Even if you don't have one, most of the principles that make a carb work are universal to any brand. Very Informative. Holley Technical Information For Edelbrock users, check here for good info on that carb type: Edelbrock Tech Videos Interested in E85? Check out E85 Information (E-85 and jetting) When you ask a question, please describe what system you're running with sufficient details to give potential responders a shot at addressing your particular set-up. Thanks for reading, and Happy Posting.

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[Apeiron]

A mechanical secondary carb has the secondary throttle blades opened directly by the throttle linkage. To prevent bogging due to suddenly going lean as the secondaries are opened before air begins to flow through them, a second accelerator pump is used, which leads to mechanical secondary carbs also being known as "double pumpers". If the carb is sized too large for the engine, it can take a substantial secondary pump shot to prevent bogging, so double pumpers have a reputation for low mileage.

Vacuum secondary carbs have the secondary throttle blades opened by venturi vacuum once the demand from the engine starts drawing enough flow through the carb. An over-sized vacuum secondary carb will open the secondaries later, or won't allow the secondaries to fully open. In effect, the VS carb "sizes" itself to the needs of the engine to an extent, and they have a reputation for getting better mileage than double pumpers.

There's another broad family of carbs that don't have as well-known a name, the "air valve" or "demand" secondary carbs, like the Quadrajet, Edelbrock Performer and Thunder series, and the Carter AFB, AVS and Thermoquad. In all these carbs, the secondary throttle blades are opened directly by the throttle linkage as in a mechanical secondary carb, but an air valve above the secondaries is balanced by a counterweight (AFB and Performer) or a spring (Quadrajet, Thunder, AVS and Thermoquad) doesn't open until the demand of the engine requires it, somewhat similarly to a VS carb.

There are also variable venturi carbs, like the Predator and the SU and such, but they're not very common on our sorts of applications.