I have an old (18yrs) Hobbico Avistar Trainer with a K&B 40 on it. I flew it when I was younger, and I recently decided to get back into the sport. I went through the plane and replaced the battery, and checked everything out. I also replaced he fuel line from the tank to the engine, and from the muffler to the tank just to be sure there weren't any problems. The first time I went out a couple of the guys in the local club helped me get it tuned properly and began teaching me to fly. Everything would go great for the first 5 or 8 minutes, and then the engine would lose rpms and begin to sputter although it would never die. The instructor would land the plane ,and we check it out and find nothing wrong refuel and fly again. It would do the same thing. Yesterday I went flying again (different instructor) and the first flight went great untill about 8 minutes in and the problem started again. The new instructor noticed a steady stream of bubbles in the fuel line after the landing. We checked the fuel lines and did not see any problems. He recommended I replace the lines inside the tank.
I went home and did just that, as well as replace the lines on the outside of the tank again. Fired it up in the back yard on the picnic table and immediately had the same bubbles. I do not think it is foam as the bubbles were thier immediately after I started the engine.

Anyone have any Ideas as to where the leak could be????

Hi!

It's probably the tank that needs replacement! 18 years is a lot! Usually the black rubber expander in the front of the tank gets hard and leak.
Replace it or replace the tank.
Best tank, used by pylon racers all over the world is the Tettra "Bubbleless" tank. If you choose the Tettra tank you don´t need to worry about any rubber expander leaking because there is none!

It would be a lot easier for your instructor to locate the problem than we could. Since he (In my opinion) correctly diagnosed the problem the first time, he probably knows what he's doing.

Have him take a look at it.

Some times the brass tubes going thru the rubber stopper get cracked. More commonly the silicon tube gets a leak inside the tank right after the brass tubes. But you have replaced the tubing, so it should not be that.

Excessive vibrations will cause bubbles to form... does your fuel tank have padding around it?

Also these help a lot:

[link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXXF25&P=ML]Click me![/link]

Yep, those do help. I use them on all of my planes. I simply replace the stock clunk with one of those.

I would also just replace the entire fuel system. When it comes to glow fuel, any pin-hole will cause problems.

CGr.

Like Minn said, the instructor would be able to chase down the problem for you but as an instructor I would have my student just replace the tank on an 18 year old plane. For 6 or 7 dollars why fool around with something like that.

Those will help if you have a foaming problem, but they won't do squat if you have a pinhole

Yup, you are so right. I didn't mean to imply that this would fix a pinhole problem, but it would do wonders wit a foaming issue, well, at least it fixed the one I had. I seem to recall that it was on my Tiger 60, but I could be wrong. The tank compartment was way to tight for the tank to be wrapped in foam as suggested in the manual, so I put it on the top and bottom, and then put the anti-foaming clunk in and it all worked.

By the way, the OS ones that was pointed out by opjose are a tad bit expensive. The Dubro ones, although are called something different, work just as well and are quite a bit less expensive, like on the order of about 2 bucks each.

http://www3.towerhobbies.com/cgi-bin...?&I=LXD741&P=7

CGr

Thanks for the help, The tank has foam padding on the top and bottom, although I do not think it is doing anything as it fits tightly in a cutout through the ribs of the fuselage. I will look and see if I can find a tank that will fit in this space, and I will replace the rubber stopper and aluminum tubes with new. The instructor lives about 45 mins away, so I will try these repairs first. If it doesn't fix it I will go to him as I know he builds a lot of stuff fullscale as well as RC.

A sure way to determine and find if you have any leakage from the tank or lines from tank to carb is to remove tank from plane. Submerge totally in water including all lines but with them all plugged,except one that you will blow air into tank with. Any leakage has to show up that way. If all looks good then you are getting air into lines inside of tank.

I've never used foam around any of my fuel tanks and have never experienced this so called foaming of the fuel, the only time i might have noticed this so called foaming in my fuel lines was from a unbalanced prop, so was that a fuel foaming problem or an unbalanced prop problem, i read a statement about fuel foaming is just a myth, could this be true as the only reason my fuel was so called foaming was an unbalanced prop?, i now strictly balance all my props and always use spinner nuts and haven't had a problem since.

And yes i do agree a small pinhole will add bubbles in your fuel line, but once again that is not the fuels fault.

Fuel foaming is sort of like flutter - It will only happen if all of the conditions are right.

In your case, the unbalanced prop was enough to cause it - but that alone should show you that it is not a myth.

I have seen it happen on several airplanes, it's no myth.

I guess i'm just trying to say is fuel foaming really a problem?, or is it another problem that causes it?, i just wonder when everyone says fuel foaming is a problem and you should add foam around your fuel tank, in my case the foam would only mask an unbalanced prop. And isn't flutter usually caused by sloppy linkages and sometimes the size of the gap between the control surfaces?, so is flutter the actual problem or was it the loose linkages that caused it?.
So i guess fuel foaming and flutter is the end result of other problems, and unless you cure the root problem the end result will not go away.

The so called fuel foaming is almost a thing of the past but it still happens so yes it is real. Not too many years ago it was a real problem and happened very often. These days the fuel is blended and uses an anti foaming agent in it so we don't really see it as often as most people think. One of the problems we had even when foam padding was used was people tended to pad the tank then tighten it in so much the padding was usless against vibration. I still pad my tanks but nothing like I used to. Most of my tanks are only padded on the bottom with fiber ties or velcro straps holding them in. I haven't had a foaming problem in years but it can still happen.

An unbalanced prop is not the only cause of vibration: bent propshaft, unbalanced piston to counter-weight/flywheel, unbalanced spinner, etc., etc. The foam is cheap insurance - plus it may prevent an internal soaking of compartment and receiver in the event of a dirt bath by preventing the tank from splitting.

Most often bubbles are from a slit or pinhole in a fuel line. At the first sign I replace them all inside the tank and out rather than waste three hours trying to hunt it down in one line at a time. $1.00 worth of tubing is not worth a lot of aggrivation. Not a bad ideas to replace them annually, anyway.
(Though I never seem to get around to doing that).

I've also had a Chinese built engine that sucked air through the needle valve housing. That one had me pulling my hair out until an old timer showed me how to test for that (and temporarily fix) by sliding a piece of fuel hose over the valve stem. Slick.

Or just smear some vasilone on the needle valve and don't forget the idle mix as that is also a source of air leaks. However neither of these potential leaks will cause bubbles in the line.

One thing not to forget, it is almost impossible to find nice limp fuel line for the clunk line. Make sure the brass tubing is just through the bung enough to get a good seal on the pickup line. To much will prevent the clunk from migrating around the tank as it is susposed to. Also make sure it does move around because if the pickup line is to long, it will allow the klunk to stick on the back of the tank and maybe stick high causing it to pickup air.

When replacing the lines, don't overlook the possibility that the brass tube has a crack.

Now back to my tank problems[:@]

Don

As Charlie P mentioned, the needle valve could be the source, but if the bubbles appear in the line between the tank and needle valve then it is not the problem. Since you said it runs fine for 5 to 8 minutes, I would suspect your problem is occuring after the fuel level drops below half full, which would lead me towards a problem with the pickup line and clunk.

I had a similar problem last year with my trainer. It would run fine for about 3-4 minutes, then lean out and quit. After 4 dead-sticks in a row, I took it home and started checking everything. Finally found a crack in the brass tube just inside the tank. As long as the fuel was above the stopper, I was fine.

It's just one of the ways these things like to mess with our heads.