FAQ
TL;DR: On a 2T Junak 606 that stalled on throttle, 4 g rollers fixed it; "the vario sleeve had slack and the rollers were too heavy." Start with plug reading and carb checks, but inspect the CVT early. [Elektroda, Królik1981, #17503556]
Why it matters: This helps 2T scooter owners fix start-then-stall and low‑RPM under‑load issues without wasting money on the wrong parts.
Quick Facts
- Revving freely on the stand but bogging under load points to CVT; this case needed a new variator and 4 g rollers. [Elektroda, Królik1981, #17503556]
- A black, wet spark plug indicates a rich mixture; fix fueling before chasing ignition faults. [Elektroda, Anonymous, post #17349984]
- Forum-proven order: warm the engine, read the plug, then lean the mixture via the needle clip before testing ignition. [Elektroda, Zygaqra, post #17349908]
- Some CDIs limit top speed to about 45 km/h on restricted machines, causing low RPM under load. [Elektroda, SKM 1964, #17497147]
- A CDI limiter cuts ignition regardless of load; the engine won’t rev out even on the stand. [Elektroda, SKM 1964, #17502926]
Why does my 2T scooter start then stall when I open the throttle?
It’s either over‑rich fueling or the CVT keeping RPM out of the powerband. Read a warm plug to confirm mixture. If fueling is fine, inspect the variator bushing and roller weight. Heavy rollers or worn sleeves hold RPM down under load. In the Junak 606 case, a fresh variator and 4 g rollers restored pull. "It flies like a smear" after the swap. [Elektroda, Królik1981, #17503556]
What does a black, wet spark plug tell me?
Black and wet means a rich mixture and poor combustion. Warm the engine before reading the plug. Correct fueling before replacing ignition parts. "Black and wet candle is too rich a mixture." If it stays sooty after tuning, check float height and auto‑choke operation. [Elektroda, Anonymous, post #17349984]
Should I read the spark plug hot or cold?
Hot. Run the engine, kill it, then remove the plug. A cold plug reading misleads diagnosis. Make mixture changes only after a proper warm‑up ride. If midrange is rich, lower the needle by raising the clip one notch. Re‑test under load. [Elektroda, Zygaqra, post #17349908]
Which carb screws do I adjust, and how?
Most scooter carbs have an idle‑speed screw and a pilot mixture screw. Tune only when fully warm.
- Set idle speed to spec with the throttle‑stop screw.
- Turn the mixture screw to peak idle, then fine‑tune slightly richer.
- Reset idle, test ride, and read the plug.
This restores clean off‑idle response. [Keihin CVK Tuning Guide]
Can late ignition timing cause bogging on a 2T scooter?
Yes. Retarded advance reduces torque and can cause stalling when you open the throttle. With these symptoms, verify ignition advance early in diagnosis. If it’s late, correct the cause, such as CDI or pickup position. Re‑test after confirming mixture. [Elektroda, Anonymous, post #17349984]
Can I adjust ignition timing on a Junak 606 2T?
There’s little user adjustment. A quality ignition module controls timing electronically by RPM and acceleration. Two‑strokes use modest advance variation compared to four‑strokes. Use the correct CDI and pickup for the engine. Avoid random module swaps without a matching map. [Elektroda, SKM 1964, #17355813]
How do I tell if my CDI is speed‑limited?
A restricted CDI cuts ignition at higher RPM, so the engine stops revving out smoothly. You feel cyclic misfire at the top. This behavior is independent of load and appears on the stand too. That confirms a limiter is active. [Elektroda, SKM 1964, #17502926]
My scooter only reaches ~4500 rpm and ~40 km/h under load. What next?
Inspect the CVT first. Low RPM under load points to heavy rollers, a worn variator bushing, or belt slip. In the case discussed, the plug was tan after tuning, yet speed stayed ~40 km/h with ~4500 rpm. Swapping the top end didn’t help, confirming a transmission issue. [Elektroda, Królik1981, #17495963]
Why didn’t replacing the piston and cylinder restore power?
The fault was in the transmission, not the top end. Roller mass and a loose variator sleeve kept the engine from reaching powerband. A sound variator plus 4 g rollers restored acceleration and speed. Fix the CVT before opening the engine. [Elektroda, Królik1981, #17503556]
What roller weight fixed this Junak 606, and what does weight change?
Four‑gram rollers solved the bog by letting RPM rise before upshifting. Lighter rollers raise revs; heavier rollers lower revs. Choose a weight that holds peak torque during acceleration. Tune in small steps to avoid over‑revving and belt slip. [Elektroda, Królik1981, #17503556]
If it revs to max on the stand, can the CDI still be restricted?
No. A limiter cuts ignition regardless of load, preventing full revs even on the stand. If it revs cleanly unloaded, investigate CVT and fueling instead. That observation rules out a speed‑limited CDI. [Elektroda, SKM 1964, #17502926]
What’s a fast, low‑cost diagnostic order for start‑then‑stall complaints?
Warm the engine and read the plug first. If rich, lower the needle by moving the clip up one notch. Test again under load, then consider ignition checks if symptoms persist. This sequence isolates fueling before electronics. [Elektroda, Zygaqra, post #17349908]
