air/vapor lock in avid flyer

[Nico 2]

I am Nico van Dijk, living in the south of Spain and owner of a stol avid flyer EC-YFP. Engine Rotax 582

After flying approx 25 hours total in this plane, I had a engine failure at take-off. I immedeately did cut the engine power and could brake in time. The engine remained running. After checking about everything, I could not find any problem. In order to be more sure, took the carburetters apart and cleaned them. Nothing found, no water, everything very nice and clean. Checked for water in the fuel again, nothing. Checked the free flow at the position of the fuelpump, approx 0.7 Litres/minute, ok.

So I mounted a fuel pressure meter and a fuel level alam at the header tank and tested the engine at ground, all OK. I did take-off again, EG temp ok, power ok worked very fine. A week later I flew at 5400 RPM and had en other engine failure in air. Did cut the engine wich remaied running at 2000 RPM and looked for a place to land. While doing that, I saw the fuel pressur rising to normal, opened the engine slowly and could return to the airfield.

There I tested everything again and found the problem, air or vapor lock.

From the wing tank I had a small fuel valve from Aircraft Spruce number 05-0104x mounted in the line going to the header tank. There is an other of such fuel valves for panel mounting in the panel in order to prevent that the carb's flooding when parked for a long time.

During a test run, I saw a vapor or air bubble underneath the fuelvalve between the wing tank and the header tank. This bubble did not pass the fuelvalve and did completely shut-off the fuel flow. After a few minutes, the fuel level in the header tank lowered and my alarm went off. I knocked the airbubble wich bit by bit started passing the fuel valve. As soon as it was gone, the fuel started flowing and the level in the header tank was restored.

I checked the main tank end the fuelvalve for debris, were perfectly clean. So I took the fuelvalve apart and... surprise. The connections are 5 mm interior, but the valve has only a throughput hole of 3.15 mm (half the expected flow!), leading to the vapor lock problem. The bubble and the fuel can't pass each other in this small hole. I mounted a fuelvalve with a througput hole of 6.5 mm and the problem was gone.

But there is an other such valve in my panel. So also vulnerable to Air / vapor lock. In order to prevent that I did mount a Facet pump (low pressure!!) near the output of the header tank so the rest of the fuel system is a little pressurized. So far everything is fine again. Fuel pressure at the Mikuni pump only approx 4 PSI, At the Facet pump only approx 3 PSI, when both on, 5 - 6 PSI

I contacted Aircraft spruce about this. I explained, but no results yet.

I think that the fuelvalve should have throughput hole the same size as you can see at the connections. This situation is dangeroeus and a vapor or air lock can happen any time. When selling fuelvalves, the seller should advertise the throughput hole!!!

Kind regards

Nico