Instructions to Adjust the Carburetor in an RX-7

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Mazda's RX-7 has a fundamentally different kind of combustion engine; it's a rotary engine. However, it is still an internal combustion engine and fed with a carburetor. Non-turbo RX-7s used Nikki four-barrel downdraft carburetors from the first model in 1979 through 1984, and in some models through 1985. Adjustment is essentially the same for any four-barrel downdraft carburetor.

Defining the Scope of Your Tasks

  • Adjusting a carburetor should be distinguished from "rebuilding" or modifying a carburetor, which are far more complex and require an in-depth knowledge of carburetor theory. "Rebuilding" a carburetor is really the disassembling, cleaning and reassembly, with a few minor replacement parts. This is important to note, because any carburetor that needs adjustment may well need to be "rebuilt." Modifying the carburetor requires mastery of carburetor theory. The first step in adjusting your carb is to understand a few basics.

Know What You're Adjusting

  • The carburetor's job is to mix or atomize fuel and air and deliver it to the engine. The trick is to deliver precisely the right amount of air/gas mixture at varying throttle positions. The RX-7s carburetor has multiple jets or "fuel circuits;" each corresponds to a specific throttle position. The idle jet controls mixture with the throttle all the way closed. The primary jets and secondary jets in combination control the mixture at open throttle.

Jetting

  • A carburetor jet isn't as interesting as it sounds. It's just a nozzle. Unlike most garden hose nozzles, jets are not adjustable. If you want to change them, you replace them with a slightly different-sized jet. This asks the important question, "Why are you adjusting it?" Factory jetting is correct. If the car stops performing well, there's probably something other than jets causing the problem; changing them will only mask the symptom.

Idle: The Only True "Adjustment"

  • Idle speed is the only real "adjustment" on the RX-7's carburetor. Everything else is a "replacement, rebuild or modification." You'll find a single, slotted brass screw on the side of the carburetor that controls the air/fuel mixture at idle. Idle should rest at 850 rpm. Turn the screw in one direction, slowly, and idle will accelerate; turn it the other direction and idle will slow. Adjust it to 850. Remember, if only the idle changed and you had to adjust the stock idle setting, there's probably something else wrong.

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