Please Help! Air bubbles are driving me crazy! - RC Groups
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Oct 03, 2004, 07:02 PM
Registered User

Please Help! Air bubbles are driving me crazy!

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.

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Oct 03, 2004, 08:15 PM
Registered User
Assuming that there are no leaks in the fuel pipes, the most likely cause of bubbles in the fuel line is a poorly fitting needle valve. Simplest cure is to put a piece of fuel tubing over the needle valve where it screws into the carb. Many Super Tigre models have this problem.
Oct 03, 2004, 08:49 PM
Registered User
Another cure for the needle valve would be to brush on some copper coat as a gasket material, works great and has solved many air leak problems, also check for any foreign material in the carb. If the Copper coat fixes the problem, run it for a while to verify, then buy a new needle valve. I would start checking for a mild fuel leak while the engine is at WOT and idle teathered to the ground and tilted straight up.
Oct 03, 2004, 09:07 PM
Registered User
Thanks for the input guys. I will try those things.
Oct 04, 2004, 12:32 AM
The reviewer
XJet's Avatar
Another thing to try is to use different oil in your fuel.

Last week we worked to set up a trainer for a club member who had flown plenty of helicopter stuff but no fixed-wing.

He fitted a TT46 to his model and we ran it in (as per instructions) using fuel that had a moderate level of castor in the fuel.

After a minute or so's running on the ground, air bubbles began to appear in the fuel line.

We replaced the external lines and clunk line and tried increasing the vibration isolation of the tank -- all to no effect so then (just playing on a hunch), I told him to switch to some fuel with pure synthetic lube. The problem disappeared completely.

It seems that some oils (particularly those with a high castor component), foam far more easily than some kinds of pure synthetic.
Oct 04, 2004, 06:40 AM
Registered User
turk1's Avatar
Sirs,can the problem be loss of nipple pressure from muffler?I have such a problem with my TT42 GP(inverted mount) for a time, until I discovered the castor oil reservoir clogges the pressure nipple after 2-3 mins of perfect run and then disaster.Watch the nipple pressure system first.
Oct 04, 2004, 05:49 PM
Registered User
Tony Oliver's Avatar
Take the engine out of the model and run it in a test stand. Connect one tube only to the carb from the pickup of a different tank to the one in the model - make sure the tank position is such that the fuel level is the same as the needle. that way there's no syphon or fuel draw problem. See if you can replicate the 'fault'. If you can, it's the engine. If not, it's still the plumbing/tank/pressure.
Oct 04, 2004, 07:07 PM
Oxford Panic
AndyOne's Avatar
I read in a mag a while ago that putting a couple of drops of vinyl cleaner in a tank of fuel stops frothing which is another cause pf bubbles in fuel.
I'm not quite sure which kind of vinyl cleaner but I guess it's the type used for cleaning plastic parts of cars.


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