ImHooked

My Feedback: (1)

I'm getting air bubbles in my fuel supply line while the motor is running at full throttle and obviously it chokes the motor out. It is a K&B .40 2 stroke. I have changed all of the fuel line inside and outside of the tank and made sure the rubber grommet at the tank is sealed good. What else is there that I'm missing?

Thanks for your help.
ImHooked
[sm=confused.gif]

Delta3

Maybe the tank is not sealing properly or maybe you have a small split or hole in one of the lines. If you have another tank that will fit try that and see what happens. Some of the rubber seals on the tanks don't seal properly no matter what you try. Could just be a tank that is faulty.

Try another tank and see if the problem persists.

Charlie P.

With really sever vibration the fuel will foam and cause air-bubbles. Did you balance the prop and cushion the tank from the fuselage with foam rubber or other methods?

My first suspect is a pin-hole in a fuel line. Did you change the one to the klunk inside the tank?

OzMo

My Feedback: (3)

What Charlie said,
Any hard portion of the plane that touches the tank can transfer vibration. A soft foam is more effective than a rigid foam.
Check your lines carefully ESPECIALLY right at the ends of the brass tubes. File the tube end edges smooth and be sure the lines don't tug at all. Also lines that are TOO loose and floppy can be an issue. The tube inside the tank can have the same issues as the outside tank lines and can really drive you crazy, I use the soldered on grips inside the tank to avoid problems.

opjose

As already suggested, balance your prop and put foam around your tank.

I'd also suggest getting one of these:

[link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXD741&P=7]Click Me![/link]

Or

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

They work great.

ImHooked

My Feedback: (1)

Thanks for all the replies. I did replace ALL of the fuel lines inside and out. Everything appears to be tight and snug. The foam around the tank is a little old, I may replace all of that. No, I never have balanced the prop. That prop has been on there a long time, I may just go ahead and change it out and see if that makes a difference. Thanks again for the help.

ImHooked

jetmech05

You aren't saying where you are seeing the bubbles...are the bubbles before the needle valve? or after?....if after the needle valve there is your problem
good luck

ImHooked

My Feedback: (1)

I'm sorry, the bubbles are in the fuel line prior to entering the valves and carb. I have gone thru and replaced all of the fuel lines and now I have changed out all of the old foam for new to try and eliminate any vibration. I guess if that doesn't do it, my next step is a new fuel tank and clunk.

jaka

Hi!
There should never be air bubbles in the fuel line.
That means a hole some where.
Be sure to use a Uni-flow system when you make the new tank installation (Two clunks side by side,one a little shorter) This makes for more even engine runs.

Warnberg

My Feedback: (1)

Balancing the prop greatly reduces vibration as well, you will be supprised at how much a differance a balanced prop can make.....

opjose

Not necessarily, ( but it's a good thing to look for! ).

If the fuel is below 1/2 of a tank and certainly by 1/3 of a tank or less, the clunk WILL become uncovered on downlines with as little as a 20-25 degree angle.

This causes bubbles in the line even when there are no vibrations, airleaks or other problems.

ImHooked

My Feedback: (1)

The wierd thing about this is, this is not a new build. This is a plane and a setup that I have flown for a long time. I changed the tank out about a year ago because the one that was in it was getting pretty dingy looking. This has just kinda popped up and it is baffling to me.

opjose

So what is different between the old tank and the new one.

Do you still have the old tank around so you can compare?

jaka

Hi!
To be certain you must change tank each year...if you don't use a Tettra "Bubbleless" that is.

ImHooked

My Feedback: (1)

They are the same. I have run this 'new' tank for over a year trouble free. WIERD!!

opjose

Remember that the difference can be VERY subtle.

In one case I had something similiar as what you are experiencing occur.

It seemed to occur the most in uplines and when accelerating, but I could not explain why for a good while...

It turned out that the fuel tubing I was using on my replacement tank was stretching due to the G forces involved. I had a standard clunk in the system at the time...

It was bottoming out when the plane accelerated, leaning out the engine to the point of cutoff...

I found this almost by accident...


You must have changed the tubing, fittings and lines, so things are not exactly the same.

Since you state that bubbles tend to form the most at full throttle, it may be that with the higher pressure, air is working it's way around the fittings. Consider putting wire ties, etc. around the tubing where it contacts the fittings to prevent this.

Check to see if bubbles form at full throttle with the tank completely full. If they do, you definitely have an air leak... probably at a nipple.

ImHooked

My Feedback: (1)

Will do, thank you.

beau0090_99

Have you changed anything in the muffler? If you have changed any baffles, it may be producing less pressure for the tank and then the fuel inlet is cavitating. Maybe it's not drawing air through a hole in the line, but either the stopper is loose and the tank is not pressurized, or the muffler pressure is too low and the inlet draw is separating the air in the fuel itself (pulling a vacuum on the fuel).
Something to think about. I also had a similar problem when a tree reached out and grabbed my nexstar. My good friend realized that the clunk slid forward got stuck with the silicone tubing kinked. This was drawing bubbles at high speed. I checked it by shaking tha plane and listening for the klunk, it was not there. Took the tank out and rapped it on the bottom and the clunk unstuck and now it is fine.

Good luck,
Curtis

TedMo

Well I have read all the replies and they covered just about everything. The only thing I can think of that wasn't mentioned is if this is an old tank with the old plumbing that could be it. Now to clarify,you said; you changed all the lines by that I will assume the plastic fuel lines. Well bet the old brass tubing is still there, right? If so that can get pin holes from the nitro in fuel. Just a thought. Might be worth putting new in. Hope something works.

ImHooked

My Feedback: (1)

No brass tubing here. However, by changing the lines and replacing the old foam cushion with the new must have done the trick. It ran perfectly tonight with no bubbles. I got 5 flights in before the sun went down. The end to a good day![sm=wink_smile.gif]