Fuel siphoning out of tank

Fuel siphoning out of tank
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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> RC Fuels >> Fuel siphoning out of tank

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Fuel siphoning out of tank - 3/10/2002 12:29:18 PM

MiL

Posts: 281
Joined: 1/22/2002
From: Bend, OR, USA
Status: offline

I'm not sure this is the right forum for this question, but i think it will do.

I have a .40 sized Big Stik which has been working fine up untill the other day. I was trying to start the motor and it seemed to keep flooding on me, which never happened before. There was fuel dripping all over the bench and everything, i realized that fuel was actually running out of the carb (the engine is mounted sideways). The fuel was siphoning out of the tank.

Well, when i thought about it, it made sense. After all, if the level of fuel in the tank is higher than the carb, then of course it should siphon out. But why had it never happened before, and isn't the center of the tank supposed to be mounted at the same height as the center of the carb? Thinking about it now, it seems like it should have been siphoning out any time the tank was more than half full.

I blew through the main line and the vent line to make sure there was no blockage. I can not visually check them though due to the design of the plane. I can't see the front of the tank without doing some major surgery to the plane... which i'm not particularly interested in doing. Now, i can't fuel up the plane without it just running back out. I would imagine that something must have changed inside the plane or tank to cause this to happen, but i don't know what.

Can someone straighten me out here?

Post #: 1

Rules, Rules, Rules - 3/10/2002 4:03:41 PM

bob_nj

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Joined: 12/3/2001
From: Vineland, NJ, USA
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The general rule of thumb per Clarence Lee is if the engine is mounted upright, the tank centerline should be real close to 3/8" below the venturi, if on it's side, the centerline should be no higher than the venturi. If nothing else has changed, the only thing that comes to my mind is that maybe a change of weather dictated opening the needle a little and you are just noticing the fuel flow more now. If it is in fact that much, your midrange and idle must be way rich. Let me know_Bob

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Post #: 2

Fuel siphoning out of tank - 3/10/2002 8:23:22 PM

downunder-RCU

Posts: 1125
Joined: 12/27/2001
From: Adelaide, South Australia
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Of course with an inverted engine all the fuel just dribbles out on the ground so you won't flood the engine.....
However, syphoning can be completely eliminated if you use a Uniflow tank set up for the breather line.

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Post #: 3

Fuel siphoning out of tank - 3/10/2002 10:02:48 PM

MiL

Posts: 281
Joined: 1/22/2002
From: Bend, OR, USA
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quote:

syphoning can be completely eliminated if you use a Uniflow tank set up for the breather line.

I don't know what that is, please explain.

Also, i'm not sure exactly where the fuel tank is located, i might have to go see if i can measure it accurately. I did not build this model myself, got it used in a trade.

Anyway, just to make sure you understand what i'm talking about, if i disconnected the line from the carb nipple when the siphoning was happening, a steady stream of fuel would flow out of the line. If i lifted the line so that it was above the fuel level it would stop, and the fuel would recede in the line. However, as soon as i brought the line back down, to hook it up to the carb again, the fuel would start running again.

We're not talking about a slightly fatter fuel flow due to some adjustment here, this is like a waterfall...

Thank you both for your responses

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Post #: 4

Uniflow tank - 3/11/2002 8:24:23 AM

downunder-RCU

Posts: 1125
Joined: 12/27/2001
From: Adelaide, South Australia
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Fuel can only run out of the tank if air can get in so with the normal set up where the breather goes to the top of the tank then there's no obstruction to air coming in.
A Uniflow tank puts the end of the breather close to the clunk, either by tying a flexible line near the clunk or using another clunk. When the engine's not running this breather line fills with fuel to whatever level is in the tank stopping any air from geting in so fuel can't syphon out. When the engine's running the carby has to overcome the head of fuel in the breather line (which is identical to the head of fuel in the tank) to allow air in to replace the fuel drawn out. So there's a very slight drop in pressure inside the tank.
In effect this drop in pressure exactly balances the head of fuel from a full tank all the way down to a near empty tank. This eliminates any change in mixture you might get from full to empty due to head change so it has two advantages. The system also works the same even with muffler pressure.

It's a bit of a difficult concept to visualise but try this with your model (seeing you've got the engine mounted sideways). Your breather almost certainly goes to the top of the tank so fill the tank and wait for the fuel to start syphoning out. Now flip the model over inverted. The breather is now exiting at the bottom of the tank at the same level as the clunk. You've now got a simple Uniflow tank and it'll stop syphoning. If it doesn't then you have an air leak somewhere.

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Post #: 5

Fuel siphoning out of tank - 3/11/2002 1:10:58 PM

MiL

Posts: 281
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From: Bend, OR, USA
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so is the uniflow setup always better? Should i setup the tank of the Extra 300 that i'm currently building that way?

If it is always better, why do people continue to put the vent line at the top of the tank?

Thanks for the help

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Post #: 6

Fuel siphoning out of tank - 3/12/2002 8:26:53 AM

downunder-RCU

Posts: 1125
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From: Adelaide, South Australia
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MiL
I think it's because Uniflow tanks were developed for C/L stunt flying where absolute consistency is needed no matter which way up the model's flying so very few RC flyers have even heard of it let alone know how it works. And of course, because it's from C/L then it can't be any good

Hahahaha...just kidding....

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Post #: 7

uniflow setup..? - 8/26/2003 2:27:58 AM

canardlover

Posts: 256
Joined: 8/16/2002
From: Sodertalje, SWEDEN
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Hey there, found this thread while making a search for "uniflow" since I�m considering to try it on a sidemounted Saito 56 where I have problems with engine going too rich initially and then too lean at the end of the flight. Center of tank is in line with carb and tank cannot be lowered further. Many thanks downunder for your description in #5. Still - my question is: could someone please direct me to more info on the uniflow setup with "wiring diagram" then I would be most grateful. Guess it would work just as well on a 4-stroke engine..? I have a slight memory having read that some people maintain "uniflow" could not and will not work properly, being a "bluff" more or less.
Any further info on this is gratefully received.........Cheers/Harald

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Post #: 8

Fuel siphoning out of tank - 8/26/2003 3:31:56 AM

Pheasant Slayer

Posts: 6
Joined: 2/5/2003
From: Dumont, CO, USA
Status: offline

[URL=http://www.fraserker.com/heli/uniflow/how_uniflow_works.htm]How it works[/URL]
This is a good explanation.

[URL=http://www.runryder.com/helicopter/t50828p1/?highlight=Uniflow]pics[/URL]
Some good pics and comments on this thread.

Good luck

(in reply to MiL)

Post #: 9

uniflow - 8/27/2003 1:36:14 AM

canardlover

Posts: 256
Joined: 8/16/2002
From: Sodertalje, SWEDEN
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Many thanks Pheasant Slayer that was some really good stuff. I will read and try uniflow asap......Cheers/Harald

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Post #: 10

Fuel siphoning out of tank - 8/27/2003 11:18:46 AM

downunder-RCU

Posts: 1125
Joined: 12/27/2001
From: Adelaide, South Australia
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Uniflow is such a simple thing to set up and one other advantage is that it's reversible. If you have a uniflow tank but reverse the blanked and muffler pressure lines then you're back to your normal vented tank for comparison purposes.

I've been using RC clunk tanks set up as uniflow in my CL stunters for more years than I care to remember and the affect is quite dramatic. A lot of this is due to the fact that we set the mixture close to the point of breaking from a 4 stroke to a 2 stroke so any variation shows up quite quickly. With a normally vented tank the mixture slowly goes leaner as the tank empties and the revs rise until it can no longer run in a 4 stroke. Obviously this increases the speed of the model and is highly undesireable. Setting richer at takeoff only lengthens the time taken before it becomes unstable. With the tank set as uniflow the revs remain constant (in level flight) until the fuel level drops far enough to uncover the end of the uniflow line.

One thing that should be noted though is that because uniflow compensates for head pressure by lowering the pressure inside the tank slightly from either ambient or muffler pressure then the needle valve has to be opened slightly compared to a normally vented tank. As I mentioned back in post #5, if you flip a model with a normally vented tank to inverted then you've now got a simple uniflow system. This means the pressure in the tank drops slightly which leans out the mixture. A possible answer to those who wonder why it leans out when flying inverted

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Post #: 11

uniflow setup...yep..! - 8/28/2003 1:46:54 AM

canardlover

Posts: 256
Joined: 8/16/2002
From: Sodertalje, SWEDEN
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Hello downunder and many thanks for your clarifications.

"....Setting richer at takeoff only lengthens the time taken before it becomes unstable...."

Yes indeed, in fact I lost a canard pusher for this particular reason, setting rich at take-off and then raising the tank even further immediately after take-off made me unable to clear the trees at the end of the runway and ...whammm.... she hit a birch trunk......RIP..!
Cannot wait to try uniflow, hope weather this weekend will permit me to try..........Cheers/Harald

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Post #: 12

uniflow.....sure thing...!! - 8/31/2003 1:02:29 AM

canardlover

Posts: 256
Joined: 8/16/2002
From: Sodertalje, SWEDEN
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Many thanks for your support and info on the uniflow setup now installed in my Sig Cougar with Saito 56(in the background). Although weather was lousy today I just could not resist to try the brand new uniflow hookup. Took off in a rather stiff breeze and rain but so what..?...Well, it really DID WORK VERY NICELY....engine kept its setting extremely well from full tank to about 25% left when I landed - did not want to provoke a dead-stick in that lousy weather.
Again.....THANKS...and Cheers/Harald

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