Hey guys, I have a Sig King Kobra with a Super Tiger .61. I just brought the plane out of a six year retirement to find that it has the same problem now as it did then. The problem is that I keep getting air bubbles in the fuel line. The plane runs fine long enough to tune the engine and fly for a minute or so, then it begins to "surge" and lose power all togehter, then die. Before I put the plane away six years ago, I replaced the tank and all fuel lines with no improvement. I also tried different amounts of foam rubber around the tank. I have had it sandwiched in very tight, I have tried no foam at all, and I have tried everything in between. No luck. Someone said try a fuel filter. Tried that. Still no luck. Well, I tried it at home yesterday and it seemed fine, I took it to the field this morning, made a few last minute needle valve adjustments, and took off. Same as always, ran like a striped monkey for about a minute, then began to surge and finally died and gave me a very good test of my skills! (Died in a really bad place). I am out of ideas. Is there anyway that this could be anything to do with the motor or does it have to be a fuel system problem? My father did have one suggestion that I will try if none of you guys have any other ideas. I have the brass tubing from the tank coming out of the front of the firewall to make tubing replacement easy and the holes in the firewall are just large enough for the tubing. Could the vibration from the firewall into the brass tubes be causing these bubbles? I guess I will drill the holes out larger so that the tubing doesn't rest on the firewall. The KK is way to good of an airplane to let sit because of this, but it is a very frustrating problem. I remembered today for the minute or so that I got to fly it just how great it flies. Could any of you guys shed some light on this for me? Thanks a lot in advance.

Zach

Sounds like a fuel tank problem. At least from the discription that you give. I agree with your dad. Make the holes thru the firewall a little larger. Is there foam between the tank and firewall inside? Are the fuel line set up as per the instuctions? Sometimes muffler pressure can cause air bubbles if the line into the tank is wrong.
You may also have a vibration problem. What prop and spinner are you using ? Have you checked them for good balance.
I'm sure you will get some other suggestions as well.
Let us know what you find.

Sounds like it runs great till the fuel level gets below the outlet of the tank. Even though you replaced the inner fuel pickup line, it could be a pin hole or crack in that hose. The little vibration of firewall on brass tubing is not causing the bubbles.
Did you store the plane hanging by the tail (a no-no) ?

Zach,

I had the same surging problem as you some years ago on a ST-61 also.

Unscrew and remove the needle valve and inspect the O-ring. It should be inside the Needle's casing and underneath the O-ring should be a spring. If it looks like the O-ring has an off-center compression mark on it, remove the O-ring, massage it, and replace it. Make sure the spring is in first or you'll never get the O-ring out again. You might have to cajole the O-ring out of the casing with a pin or something.

When you get it running good, there's nothing like a ST-61.

Try running a new piece of fuel line directly from the engine to your gallon of fuel. If the problem doesn't surface, the problem is your tank. Take the tank out, run the engine again - this time with the tank outside the plane. If it happens again, you can rule out fuel foaming, so do like rreiss said, recheck all of the tank lines for pinholes.

If it IS fuel foaming, reinsert the tank making sure the NO part of the tank is touching anything but padding. If it still foams, try adding one short squirt of WD-40 to your jug of fuel.

Does this mean it ran out a tank of fuel on the ground with no problems?

If so, you might just be leaning the engine out too much. Try backing off the engine way to the rich side, so that you know it's "too rich". If you are just backing off of peak "a few clicks" it might not be enough.

It does sound like fuel foaming or an air leak though.