FAQ
TL;DR: If your 4T scooter hangs at 4–4.5k rpm, set valves to 0.05/0.08 mm and "Check valve clearance." Fix intake leaks, enricher, and pilot jet; they cause lean idle hang. [Elektroda, barton21, post #17036184]
Why it matters: This FAQ helps 4-stroke (GY6/CVK) scooter owners quickly diagnose and fix slow-return revs for safer, smoother riding.
Quick Facts
- Typical hot idle for many GY6/4T scooters: approx. 1,700–1,900 rpm. [“Kymco Agility 125 Service Manual”]
- Valve clearances often used: intake 0.05 mm, exhaust 0.08 mm (cold). [Elektroda, barton21, post #17036184]
- Mixture (fuel) screw baseline: about 1.5–2.5 turns out from lightly seated. [“GY6 Carburetor Tuning Guide”]
- Two key intake seals: O-ring under the ebonite spacer and O-ring in the intake manifold—replace both. [Elektroda, barton21, post #17028612]
- Auto‑choke (bystarter) begins leaning within ~60–90 seconds and extends fully in ~2–3 minutes. [“Keihin CVK Bystarter Operation”]
Why does my 4T scooter hold 4–4.5k rpm after I close the throttle?
A hanging idle points to a lean condition from unmetered air. Common culprits are intake leaks at the manifold, spacer O-rings, or cracked vacuum hoses. As one expert put it, “Usually, the problem is with the left-hand air that gets on the carburetor connection.” Fix sealing first, then fine-tune the carb. [Elektroda, 1133, post #17022173]
How do I check for intake leaks on the intake side?
Do a quick leak hunt:
- Inspect the intake manifold, ebonite spacer, and both O-rings for cracks, flat spots, or nicks.
- Wiggle vacuum hoses at idle and listen for RPM change; replace any brittle lines.
- Lightly clamp suspect vacuum lines briefly to isolate circuits.
Replace any damaged seals and re-test idle. [Elektroda, barton21, post #17028612]
The throttle snaps back; can the cable still be the issue?
If the cable and linkage return fully to the stop, the cable is not holding revs. Verify correct routing, adequate free play, and smooth action with the seat off. Lubricate if needed. If the butterfly or slide closes, focus on air leaks, idle mixture, and the auto‑choke. [Elektroda, Staszek49, post #17023662]
What mixture screw baseline should I use on a stock CVK carb?
Start at about 1.5–2.5 turns out from lightly seated. Warm the engine. Set idle speed, then turn the mixture screw for the highest, smoothest idle and reset idle speed. If you need less than one or more than three turns, change the pilot jet size. [“GY6 Carburetor Tuning Guide”]
How do I set idle speed correctly on a GY6/4T scooter?
Set hot idle near 1,800 ±100 rpm using the idle-speed screw, not the throttle cable. Adjust with lights on to simulate electrical load. Use a tach if your dash lacks one. Correct idle speed shortens return-to-idle time and stabilizes fueling. [“Kymco Agility 125 Service Manual”]
Can the auto‑choke (enricher) cause a hanging idle, and how do I test it?
Yes. A stuck or weak enricher alters fueling during warm-up. Test it off-bike: apply 12 V to the leads and measure plunger travel over 2–3 minutes; look for roughly 3–5 mm extension. Refit with a good O-ring and confirm it seats in the carb. [“Keihin CVK Bystarter Operation”]
Could a torn carb diaphragm cause a slow rpm drop?
Yes. A pinholed or stiff CV diaphragm can let the slide hang and delay return to idle. Inspect the rubber for cracks and ensure the lip seats evenly in the cap. Replace the diaphragm or cap assembly if damaged. [Elektroda, RafalGT2, post #17040745]
What valve clearances help avoid hot‑idle issues on these engines?
Set the intake to 0.05 mm and the exhaust to 0.08 mm on a cold engine. Incorrect clearances can alter vacuum and idle behavior. As advised in-thread: “Check valve clearance.” Recheck after any top-end work. [Elektroda, barton21, post #17036184]
My scooter only idles if the idle screw is fully in—what does that indicate?
The pilot circuit is too lean or restricted. Remove the carb, clean the pilot jet and idle passages, and confirm the fuel-screw tip, spring, washer, and O‑ring are present. Reset baseline and fine‑tune. Consider upsizing the pilot jet if adjustment range is exhausted. [“Keihin CVK Service Manual”]
Do variator rollers or the clutch cause high idle hang?
CVT parts set gear ratio under load; they do not set idle speed. With the throttle closed, the engine should drop to the carb’s idle setting regardless of roller weight. Edge case: a dragging clutch can spin the wheel at idle but won’t hold engine rpm high. [“CVT Tuning Basics”]
My plug is coffee‑with‑milk color; is the mixture OK?
Tan color indicates overall mixture is close at cruise or load. It does not confirm the idle circuit. Idle can still be lean and cause a hanging idle. Evaluate after an extended idle-only run, or tune using RPM response and mixture screw behavior. [“NGK Spark Plug Reading Guide”]
After replacing the manifold and O‑rings, what else should I inspect?
Confirm the airbox-to-carb boot and clamps seal tightly. Check all vacuum lines, including petcock and emissions hoses. Verify the carb top cover seal and slide movement. Recheck mixture screw parts and pilot jet size. Ensure the auto‑choke fully extends and seals its seat. [“Keihin CVK Service Manual”]
