• Engine: YS91 with pumped carburetor on RC helicopter.
• Behavior: Idles fine, but flames out during spool-up under rotor load.
• Tuning attempts: Multiple high-speed and mid-speed needle combinations tried.
• Glow plug: Replaced; issue persists.
• Key observation: Engine has a better chance of not flaming out when the glow igniter remains attached.

The pattern—stable idle, flameout during spool-up, and improved behavior with the glow igniter attached—points to a combustion stability problem under load. In practice, this usually means that when the engine is asked to deliver more power (rotor loading), it loses either:

• Consistent fuel delivery (lean spike under load),
• Sufficient combustion heat (plug cooling or marginal ignition), or
• Stable mixture/pressure regulation (pump/regulator not keeping up).

The fact that the engine behaves better with the glow igniter attached strongly suggests that the combustion cycle is marginal under load and needs extra heat to stay lit.

1. Fuel Delivery / Pressure Instability (YS Pump & Regulator)

YS engines use a regulated pump system instead of exhaust pressure. If the pump or regulator is not maintaining proper fuel pressure, the engine can:

• Idle and transition fine at low demand,
• Then go lean suddenly when rotor load increases, causing a flameout.

Common YS-specific issues include:

• Weak or damaged pump diaphragm (stiff, cracked, or aged).
• Regulator O-ring leaks causing unstable pressure and mixture.
• Sticky or faulty check valve in the pump system.
• Blockage or restriction in fuel lines or fittings.

2. Glow Plug / Combustion Heat Under Load

The engine running better with the glow igniter attached indicates that the plug may not be maintaining enough heat under load, or the mixture is lean enough that the plug’s heat range is marginal. Even with a new plug:

• The plug may be the wrong heat range for this fuel/engine setup.
• A lean condition under load can cool the plug and destabilize ignition.

3. Midrange / Low-Speed Mixture Too Lean

Flameouts during spool-up often come from the transition circuit (low-speed and midrange), not just the high-speed needle. If the midrange is too lean:

• The engine will idle fine.
• As throttle and load increase, the mixture leans out and the engine dies.

YS carbs are known to be sensitive in the midrange, so even small errors here can cause the exact behavior described.

4. Air Leaks Causing Hidden Lean Condition

Small air leaks can cause a lean spike only when demand rises. On a YS engine, critical leak points include:

• Carburetor base and insulator.
• Backplate and regulator body.
• Fuel fittings, lines, and pump connections.

Even minor leaks can destabilize the mixture under load and cause flameouts.

• High-speed needle alone: Repeated HSN adjustments with no resolution suggest the root cause is not simply main needle position.
• Glow plug alone: Replacing the plug did not resolve the issue, indicating the underlying problem is mixture or fuel delivery related.
• Overheating: The flameout occurs during spool-up, not after prolonged high-power running, so classic overheating is less likely.

Based on the symptoms and typical YS behavior, the most probable root cause is: a fuel delivery or pressure regulation instability in the YS pump/regulator system, causing a lean condition under load.

This aligns with:

• Idle stability but flameout when rotor load is applied.
• Improved behavior with glow igniter attached (combustion marginal, extra heat helps).
• Multiple needle changes failing to resolve the issue.
• YS engines’ known sensitivity to pump/regulator health and sealing.

1. Pump Diaphragm and Regulator

• Inspect the pump diaphragm for stiffness, cracks, or age-related wear.
• Check regulator O-rings and seals for any signs of leakage or damage.
• Verify that the check valve operates freely and does not stick.

2. Fuel System Integrity

• Inspect all fuel lines for soft spots, kinks, or partial collapse under suction.
• Confirm that all fittings are tight and free of air leaks.
• Ensure the fuel tank clunk moves freely and is not restricted.

3. Midrange / Low-Speed Mixture

• Revisit low-speed and midrange tuning with a focus on avoiding lean transition.
• Perform controlled spool-up tests while making small, systematic adjustments.

4. Glow Plug and Heat Range

• Verify that the plug heat range is appropriate for the fuel and engine.
• Note that needing the glow igniter attached suggests the mixture or plug heat is marginal under load.

The engine’s ability to idle correctly but flame out during spool-up, combined with improved behavior when the glow igniter remains attached, strongly indicates a lean or unstable combustion condition under load. On a YS91 pumped-carb engine, this is most commonly caused by issues in the pump/regulator system, fuel delivery, or midrange mixture, rather than the high-speed needle alone.

Addressing pump/regulator health, verifying fuel system integrity, and carefully re-evaluating midrange tuning are the most effective next steps toward resolving the flameout behavior.