How do I fix a high idle RPM that will not go down on a Mazda RX-7 FB?

Fixing a hanging high idle on an FB RX-7 is usually a matter of tracking down mechanical binding or a vacuum leak. Whatever you do, do not try to adjust the idle speed screw yet—if something is mechanically hung up or leaking, adjusting the carb will just mask the problem and cause headaches later.

Difficulty: Easy to Moderate

Here is the diagnostic procedure to track down the cause, starting with the easiest checks:

1. Check for Mechanical Binding

Start at the accelerator pedal and work your way to the engine. Verify there is no debris or zip-ties near the pedal assembly preventing it from returning fully ².

Next, check the throttle linkage on the carburetor (assuming you have a 12A engine). The linkage is prone to sticking. The hot start assist arm is a known culprit; removing this arm and thoroughly cleaning the teeter-totter and springs with carb cleaner and WD-40 often resolves a hanging idle after acceleration ⁷. You should also check the accelerator cable tension by loosening the bracket nuts on top of the carb to ensure the cable isn't holding the throttle open ¹².

2. Hunt for Vacuum Leaks

If the linkage is free, you likely have a vacuum leak. On a stock 12A Nikki carburetor, the idle circuit only functions below roughly 900 RPM. If a vacuum leak pushes your engine speed above that threshold, the idle adjustment screw becomes completely ineffective ⁶.

Common leak points on the FB include:

• Intake manifold gaskets
• The Air Control Valve (ACV) and Anti-Afterburn Valve (AAB)
• The shutter valve
• The "rat's nest" of emissions vacuum lines
• Carb-to-spacer and manifold-to-spacer seals

You can use carb cleaner with a narrow straw to spray around these areas while the engine is running. If the idle surges or drops, you've found your leak. Safety Warning: Do not remove the engine cooling fan to get better access while doing this, as you risk overheating the rotary ⁶.

3. Rule Out the Cruise Control Module (Series 3)

If you have a Series 3 FB (1984–1985) with cruise control, make sure the system is switched off at the dash. The cruise control module, located under the left-side storage bin, is prone to water damage. When it fails, its default failure mode is often to electronically apply throttle, which will hold the RPMs high regardless of clutch or brake input ².

4. GSL-SE Specifics (13B EFI)

If your FB is a GSL-SE model with the fuel-injected 13B, your idle control is electronic rather than carbureted. A high, fluctuating idle on these models is frequently tied to a bad ground connection. Check and clean the main ground connection located at the driver's side strut tower ¹.

Let me know if you are running the carbureted 12A or the fuel-injected 13B, and we can dig deeper into the specific intake components if the basic linkage and vacuum checks don't solve it.