What causes bubbles in the fuel line?

What causes bubbles in the fuel line? The engine on my Nobler runs weird the fuel line is a continious stream of bubbles. I tightened the engine bolts tighter. The fuel tank is clamped solid as a rock. The fuel tank pick up is soldered against the side of the tank all the way along the full length of the tank. Fuel pick up is 1/4" from the back end of the tank. The other copper line that supplies air to the tank is soldered to the side of the tank too and is 1/4" of the tip end of the fuel pick up. I can not get the needle to ajust before take off. The engine runs the same all the time I can turn the needle in 2 turns or out 2 turns and it still runs the same. When the engine first starts it seems to run fine for 2 seconds. Once the airplane is in the air the engine runs rich for about 2 minutes then it runs fine for about 3 minutes then it runs lean at the end of the flight. About the last 5 laps the engine revs up high RPM and dies then restarts over and over for 5 laps. Its like driving down the road in your car and turning the key off and on over and over evey 1/2 second. I removed the needle valve and cleaned it out but it did not help. Flushed the needle out and cleaned out the inside by looking inside with a tiny spot light.
Uniflow tank is 1.125" top to bottom, 3.5" front to back, 2.5" from fuselage out to the tip, 45 degree bend on the outside comes to a 90 degree point in the cents V shape, fuel pick up is in the V. When I first built this airplane and flew it everything worked fine for several flights. None of the glue inside the fuselage is broken, none of the wood is broken.

Gary,
you are getting a vibration causing foaming of the fuel. Try a couple drops of Armor All in the fuel. Not much. Can be a resonance setting up, or whatever, but the Armor All will cure the foaming.
Bill <><

Gary, you are not alone on that "goofy" shut-off. Matt has one right now that does that. I think (not positive, only postulating) the problem on this is that the nose is yawing out somewhat when the engine is running strong and at the very end of the run some fuel is flowing to the front. This uncovers the pick up tube, engines shuts off, less yew (less "pull"), nose points in just enough for the fuel to cover the pick-up tube and flow up the line again. The engine is still turning over and still pumping, so when it gets fuel again it starts up. This causes more yaw and the pick up tube uncovers. (Well, that's the theory, anyway.)

On the bubbling. You have gone to a bit of overkill, but eliminated a lot of things. Vibration in the tank (loose mounting which you say is tight) can cause it, or a vibrating nose itself. Also, the fuel. Shake your jug of fuel and tell me what you see. Does it foam up on top and does the foam linger? If so, add defoamer. The quickest, easiest way is to add one or two drops (just drops) of Armor-All, original formula. Then shake again and see the difference. This may help the problem. And if not, there may be some harmonic vibration in the nose of the airplane. Just be aware that most engines will show some bubbles in the fuel line on the ground. You aren't going to eliminate them all.

Maybe the fuel tank is too rigidly mounted. Currell

>Maybe the fuel tank is too rigidly mounted. Currell

If the nose is vibrating and not solid, then that is a possibility. It is interesting how two different and opposite solutions could be proposed for the same problem. With a solid nose section, we have found you need to wedge the tank is as solidly as possible. If the nose tends to vibrate and flex, however (as with some profiles) then placing some foam rubber under or around the tank can help. Couldn't hurt to try.

Gary,

You didn't mention your engine... Some of the newer types are very smooth running, but older engines, notably Fox 35s, do shake a bit. The engine is the source of your problem -- what else could be?<G>

Even a Fox 35 runs smoother with a very carefully balanced prop. There've been some good posts here and in Stunt News about prop balancing. The engine does its hammering to turn the prop, and if the prop don't balance, "....a whole lot of shakin' goin' on..." (J.L. Lewis, 1958)

The STP treatment helps kill excess bubbling, and those bubbles are what's messing up your needle setting. Is the engine set too rich or too lean? We can't tell, and neither can the engine... the bubbles don't let us.

Surging on cutoff, as Len said, is -- in effect -- fuel sloshing forward when the model slows and back as it accelerates. We sense it like a PIO, pilot induced oscillation, where the engine responds out of phase with the disturbance, making it more noticeable. Reducing the bubbles will help a lot. STP and prop balancing are a good way to that.

The engine is OS Max 40 LA almost new got about 30 runs on it.

Gary,

Standard Disclaimer:
Caution use at your own risk, results may vary, I am not a professional. I need to rely on smart people for their idea's, I don't have any of my own.

All of the previous suggestions are good possibilities.

Have you removed this engine from the airplane and tried running it in a test stand? If you did, how did it run?
Were you able to control the engine speed with the needle valve?

What kit is the engine mounted on? Profile or full body?

>..... When I first built this airplane
>and flew it everything worked fine for several flights.

>Gary Weaver [email protected]

What the others said..

Also, is this a different (new or nicked) propellor? Try a different prop to make sure it isnt way out of balance causing vibration.

>fuel and tell me what you see. Does it foam up on top and
>does the foam linger? If so, add defoamer. The quickest,
>easiest way is to add one or two drops (just drops) of
>Armor-All, original formula.

Leonard,
One or two drops to the entire gallon?

I too have been fighting bubbles recently, and one thing that seemed to help somewhat was to switch from uniflo muffler pressure to uniflo with an open, forward-facing vent. Possibly some of the air blowing into the tank under pressure just 3/8" in front of the pickup is being mixed with the fuel due to vibration and sucked right back into the engine? Uniflo muffler pressure is a popular combination, so .....?

>
>>fuel and tell me what you see. Does it foam up on top and
>>does the foam linger? If so, add defoamer. The quickest,
>>easiest way is to add one or two drops (just drops) of
>>Armor-All, original formula.
>
>Leonard,
>One or two drops to the entire gallon?
>>Kim Mortimore

That is correct, drops! We have a small one ounce bottle with a flip top, filled with Armor-All that we keep in the fuel kit. Every time we open a new jug, we put one or two drops of the stuff in, replace the lid, shake it, and watch for foam. It doesn't take much. If you shake it and the foam stays, add another drop. If you shake it and the foam goes away almost immediately, you are done. Just like when the Dentist drills your teeth, this isn't something that if a little is good, a little more is better. Too much can damage plugs, not to mention what else it may do to the fuel run. When Red Max mixed my fuel (before they didn't like castor) I used to have them double the amount of anti-foamer they put in. So, instead of adding one drop, they would add two. Never charged me extra for it, either.

The defoamer(Armorall) method may let you get decent flights in. If you want to permanently solve the problem you have to change the resonant frequency of the fuse. It might be as simple as removing and reinstalling the tank to change where the tank is touching the fuse. If you have ever run a plastic tank on a rubbery profile you can see the bubbles forming. They can come from a surface of the tank touching the fuse at one point. Vibration can cause the sides of the tank to vibrate and generate bubbles at a point far from the fuse.

If nothing else works, take a hint from the plane. It worked fine new. Vibration has softened up the fuse and now it vibrates more. You could try glassing the inside of the engine and tank compartments, or simply adding a layer of glass over the whole outside of the nose.

Ron B.
F4Fguy

While any or all the above may be the problem.The only times I've ever had this problem,it was the tubing inside the tank.In one instance,it was a corroded hole at a solder joint(my fault for not cleaning up the flux)but,on two other occaisions,it was a split in the brass pickup line.It takes me a long time to get the picture.I now use only soft copper tubing.Another thing I've had on one setup was a pinhole in the silicone feed line outside the tank.

Hope this helps,

LAST EDITED ON Mar-19-04 AT 05:38 AM (CDT)
 
>About the last 5 laps the engine revs up high
>RPM and dies then restarts over and over for 5 laps. Its
>like driving down the road in your car and turning the key
>off and on over and over evey 1/2 second.

This happened a lot back when we were using Supertigre X.15s in F2D Combat. If you set the needle too lean, it'd do the stop-go thing. Sounded more like F2C team racers with a bad setting, just not as fast. This was on a bladder, so it wasn't the tank. We used huge venturis on these things. I'll bet the engine stops firing because it's starved for fuel, then when enough fuel accumulates in the engine, it starts up again, until it is again starved. Towards the end of the bladder, when normally, the engine would run rich, it'd quit blipping and run steadily until the fuel ran out.

I've seen other engines do it, too, just not nearly as much.