GY6 starts then dies, or will not idle

The problem it solves

An engine that starts and then dies is telling you it can make combustion but cannot sustain it. That narrows the field enormously — and yet it is the fault most likely to end with somebody buying a carburettor they did not need, because the most common cause on these engines is not in the carburettor at all.

The most experienced posters on the Chinese scooter forums converge on an answer that surprises most owners: when a four-stroke of this type starts running badly, check the valve clearance first. The heads are soft alloy, the valves and seats recede into them with use, and the clearance closes up until a valve is held slightly open. An engine in that state starts on the richer cold mixture and dies as soon as it has to idle on its own.

The second cause is an air leak — at the intake manifold, its gasket, the carburettor spigot or a perished vacuum hose. Extra air the carburettor did not meter makes the mixture lean at idle specifically, because at idle there is so little airflow that a small leak is proportionally enormous.

Third is the mixture itself, and fourth is fuel that is not arriving reliably. The order on this sheet is the order of likelihood, and it is deliberately not the order most people work in.

The mistakes this sheet exists to prevent

• Rebuilding or replacing the carburettor before checking valve clearance, which is the most commonly named cause of exactly this symptom.
• Turning the idle speed screw up until it holds an idle, which hides the fault rather than finding it.
• Chasing an air leak by spraying everything at once, so a change in engine note cannot be attributed to any one joint.
• Assuming the fuel is fine because there is fuel in the tank. Stale fuel and water in the bowl both look like fuel.
• Adjusting the mixture screw without first ruling out the air leak, which means adjusting to compensate for a fault.

How to use it

1. 01Work down in order. The order is the value of the sheet.
2. 02Establish first whether it dies hot, cold, or both — that alone splits the field.
3. 03Do the air-leak spray test one joint at a time, watching the engine note after each.
4. 04Record each answer as you go, including the ones that came back normal.