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The other night after doing some night work in the 360 the engine began running very rough and would stumble at idle. It almost quit on the way back to the hanger. The next day I went out and checked what I could without uncowling. The engine continued to run and stumble. Thinking fouled plugs I did a couple of high speed taxi runs with no success. So back to the hanger to start troubleshooting.
Pulled the cowl and did not see anything amiss. No leaking fuel or loose parts that might cause the stumble. When I first got the plane it had a fuel, actually air leak at one of the fitting. I went over all the fittings and then reran with the same result.
OK maybe the injectors are plugged. Pulled and checked. They looked fine. While I had them out I did a flow check just to make sure. Ran each injector line into a jar and checked volume. Again OK. Did a second check and watched flow. In addition to the fuel I was getting air bubbles once the injector line was below the fuel level. OK there is an airleak somewhere.
To test I ran a clear tube from the discharge of the engine driven pump back to the fuel tank. With the boost pump running the fuel had a stream of very small bubbles mixed in with the fuel. OK time to look for an air leak.
Because the plane does not have a header tank the boost pump and/or the engine driven pump needs to pull a slight vacuum to pull the fuel into the boost pump which is mounted midway up the firewall. Again I went over all the fittings with no change. Did a water check where I sprayed water on a fitting? If there was a leak the water would plug it for a while and the bubbles would disappear. Again no change. Gascolator drain valve would increase the bubbles when opened but when closed and with the drain covered the bubbles were still there. Finally I moved the fuel selector to off. The bubble increased and then the flow stopped. OK now what? Reopened the valve and things returned to the bubbly abnormal. Finally I put a slight amount of pressure on the boost pump overflow line. It had not indicated any problem so I did not think it could be a problem. I WAS WRONG. With a small amount of pressure, just blowing, The bubbles in the fuel increased. OK that is the source of the air. To confirm the leak I put pressure on the inlet to the pump and got a small amount of fuel out the overflow.
So now I have the culprit. Called Lancair and a new pump will be coming Monday, It is a Dukes and was manufactured in 99. It should not be run dry but early in the life of the plane it had problems with vaporized fuel so the pump probably had a hard life. In any case I hope a new one will fix the problem. I will probably send the old one back to be rebuilt to have as a spare.
I will post another note when the new pump is installed.
Ray .
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