I'm bench running a 50 size engine with rings.

The fuel lines and tank was leak checked before mounting the engine by blowing into the vent line on the tank and pinching off the line at the carburator.

The fuel lines and tank was leak checked again after the bench run by blowing into the vent line on the tank and immersing the tank, fuel lines and remote needle valve assembly in water and pinching off the line at the carburator. This test was repeated this time by blowing into the line at the carburator and pinching off the vent line at the tank while immersing the tank, fuel lines and remote needle valve assembly in water. No leaks were found.

The fuel line between the remote needle valve and carburator exhibits a continuous stream of bubbles. Bubbles are continuous no matter how I hold the tank, above the carburator, level with the carburator or below the carburator. Obviously, vibration is not affecting the fuel in the tank because I'm holding the tank.

The fuel line between the tank and remote needle valve does not have bubbles.

Engine runs only if it is fuel rich and four cycling. If I begin to lean it out the engine runs but at any moment it will suddenly die lean.

The needle valve o-ring was replaced even though the previous o-ring was not damaged and did not leak.

Engine has not been in a crash.

Engine has very little time on it. It was broken in with two tanks of fuel while mounted on an Eagle 2. It was flown 6-8 flights running very rich (four cycling) without a problem. When I begin to lean the engine it dies before it ever reaches peak RPM. It's not possible to hold peak RPM and richen the mixture. Engine dies before the adjustment is completed.

Fuel is CoolPower 15 percent.

I disassembled and inspected the remote needle assembly before bench running the engine. I checked all the seals, fuel nipples, examined sealing surfaces under a magnifying glass and reassembled. No problems were found.

What shall I do to fix it?

Doug