Here's a breakdown of the process:
1. Pre-Adjustment Checks:
* Warm-up: Let the engine reach its normal operating temperature. A cold engine will not idle correctly.
* Tune-up: Ensure your engine is properly tuned. This includes:
* Clean air filter: A clogged air filter restricts airflow, affecting the idle.
* Spark plugs and wires: Worn or faulty spark plugs and wires can cause erratic idling.
* Proper PCV system function: A malfunctioning PCV system can affect vacuum and idle.
* Fuel pressure: Check fuel pressure to ensure it's within specifications. A fuel pressure regulator issue can significantly impact idle.
* Vacuum leaks: Carefully inspect all vacuum lines for cracks, leaks, or loose connections. Even a small leak can disrupt idle.
* Throttle body: Make sure the throttle body is clean and the throttle plate moves freely.
2. Idle Speed Adjustment:
The idle speed is typically controlled by the Throttle Position Sensor (TPS) and the Idle Air Control (IAC) motor. However, there's usually also a screw on the throttle body itself that provides a *small* amount of idle speed adjustment.
* Locate the idle speed screw: This is usually on the throttle body, often a small screw with a locknut.
* DO NOT significantly adjust the idle speed screw: This screw is for fine-tuning *after* the TPS and IAC are functioning correctly. Major adjustments here usually indicate a problem elsewhere.
* Diagnostics (if needed): If the idle is erratic or too high/low despite the above checks, you might need a scanner to check the TPS voltage and IAC motor operation. Incorrect values here would need attention *before* adjusting the idle screw. A faulty TPS or IAC often requires replacement.
3. Adjusting the Idle Speed Screw (ONLY AFTER DIAGNOSTICS, IF NECESSARY):
* Slight adjustments: Use a small screwdriver to make minute adjustments to the idle speed screw.
* Test and adjust: After each tiny adjustment, let the engine stabilize and check the idle speed with a tachometer. The ideal idle speed is usually around 650-750 RPM (check your owner's manual for the exact specification).
* Tighten the locknut: Once you've achieved the correct idle speed, carefully tighten the locknut on the idle speed screw to secure the adjustment.
Important Considerations:
* Professional help: If you're uncomfortable working on your engine, it's best to take it to a qualified mechanic. Improper adjustments can cause further damage.
* Year and model variations: The exact location and adjustment procedure may vary slightly depending on the specific year and model of your Chevy TBI 350. Consult a repair manual specific to your vehicle.
* IAC cleaning: Often, cleaning the IAC motor with throttle body cleaner can resolve idle issues without any adjustments to the idle screw.
This process is a guideline. A repair manual specific to your vehicle's year and model will provide the most accurate and detailed instructions and specifications. Always prioritize safety when working on your vehicle.
