I have a os 55 ax mounted on its side on a profile aircraft. the fuel tank it mounted on the otherside of the fuselage at the same point as the engine. the engine runs great when right side up and when verticle it can climb for unlimited verticle. Engine stalls when plane flying inverted for 5 seconds or longer. engine will stall while converting a blender to inverted flat spin. keep in mind since the engine is mounted sideways that the engine itslef is not inverted. while draining tank I can get every drop out of tank, trying to figure out why it stalls? Running 2 clicks richer or leaner does not help.

My first guess would be an issue with the clunk or fuel line. If there is a small tear in the clunk line at the bottom it wouldn't cause a problem upright, but it would inverted. What I would do to troubleshoot is get someone to hold the plane for you inverted so that you can watch the line. If you see bubbles, you have a leaky fuel hose. You might also tug on the lines to see if you can find a leaky spot while you're at it.

Turn it upside down and try draining it. The clunk may be stuck on the bottom? How long will it fly right side up?

Insufficient flexability of clunk line if it is clearing the bottom of the tank or otherwise unstuck. The smaller the fuel tank the more this becomes a problem.

-OR-

If you want the most reliable fuel delivery possible with a muffler pressure system then try Tetra or Jett Bubbless tanks. These have no clunk at all with none of the associated problems.

http://www.centralhobbies.com/Fuel/fueltnk2.html

John

Shake the plane while holding it in the various orientations of flight. You should hear the clunk "clunking" around in the tank. If, when inverted, you do not hear the clunk then it's likely that the fuel pickup line is not allowing the clunk to drop into the fuel resulting in a no fuel to the carb. It'll take about 5 seconds to drain the remaining fuel in the fuel line(s).

I popped open the fuel tank today. the clunk was attached. it could get to all 4 corners of the tank without touching the back wall. I inspected the fuel lines inside the tank and could not find any leaks. the tank held pressure from blowing on the two external brass tubes. I have replaced the fuel tank and all fuel and vent lines hoping to find what I might have missed. will test this weekend weather permitting.