Honda Acty — Idle Adjustment

Target Idle Speed

Tools Required

• Tachometer (or scan tool with live RPM) — many smartphones have apps that read rpm via microphone; a laser tach pointed at a reflective mark on the crankshaft pulley also works
• Small flathead screwdriver
• Vacuum plug or finger to temporarily block idle ports (optional, for troubleshooting)

Idle Speed Screw Adjustment

The idle speed screw (throttle stop screw) is located on the carburetor body where the throttle linkage attaches. It controls how far the throttle plate stays open at idle. Turning the screw clockwise increases idle speed; counter-clockwise decreases it.

1. Warm the engine fully to normal operating temperature — at least 10 minutes of driving or idling.
2. All accessories off: headlights, AC, blower fan.
3. Connect a tachometer and observe the current idle rpm.
4. Turn the idle speed screw clockwise to raise rpm, or counter-clockwise to lower it. Make small adjustments — one-quarter turn at a time — and allow the rpm to stabilize after each adjustment.
5. Set idle to 700–800 rpm.

Idle Mixture Screw Adjustment

The idle mixture screw controls the air/fuel ratio at idle. It is typically capped from the factory with a tamper-evident plug (a small brass or plastic cap pressed over the screw). These caps were required under Japanese emissions regulations and are present on most unmodified trucks. Remove the cap to access the screw.

1. Start from a baseline: gently turn the mixture screw clockwise until it just seats (do not tighten), then back it out 2.5 turns. This is a typical starting point.
2. With the engine at normal operating temperature, turn the screw out (counter-clockwise) in quarter-turn increments while listening to the idle. The idle should rise and smooth out as you find the optimal mixture.
3. If the idle peaks and then begins to drop again as you continue turning out, the optimum mixture is at the peak rpm point.
4. After finding the peak, readjust the idle speed screw back to 700–800 rpm if it has changed.
5. As a final check: briefly snap the throttle open and release. The idle should return smoothly without hanging or dipping below the target. If it hangs high, the mixture may be slightly lean; if it dips and stumbles, the mixture may be rich.

AC Idle-Up Screw

On Actys equipped with air conditioning, a separate idle-up screw raises the idle speed when the AC compressor engages. Without it, the engine may stall when the AC is turned on, because the compressor load is significant relative to the small engine displacement.

The AC idle-up screw is located near the main idle speed screw, or on a separate solenoid vacuum port on the carburetor. Turning it clockwise raises the rpm compensation when the AC is active. Adjust until the idle remains stable at 800–900 rpm with the AC running.

Common Idle Problems