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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Glow Engines >> OS 120 FS
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OS 120 FS - 5/7/2008 2:43:37 AM   
allans


 

Posts: 37
Joined: 11/12/2002
From: Yorktown Hgts, NY, USA
Status: offline 		Hi

I have a Showtime 90 with an OS 120 FS in it . It runs and flies beautifully , but I do have one problem , not fatal , that I would like some help with .

The stock fuel take will hold enough fuel for about 15 minutes of running time for this engine. Way more then I need . I usually stay up 8 to 10 minutes.
If I fill the fuel tank to capacity the engine runs so rich I can barely keep it running , But at the end of the flight the mixture has leaned out and is now running perfectly.
If I under fill the tank , say to half its capacity , I have no problem .

So I am guessing that the vertical height of the top of the tank is adding enough static head pressure to keep the engine running rich until the fuel level is lower in the tank.
Does this make sense ?

If this is wrong then does anybody have any idea what is going on ?

But more importantly how do I fix it or what can I do to ease this problem. The tank does not look like it is easily moved up or down - but I suspect this is not a fix.

Someone mentioned a three line pressurized fuel tank supply arrangement - does anyone know what this is , is it real ?and if it is a potential fix ?

If so how is it done?
       Post #: 1

RE: OS 120 FS - 5/7/2008 5:01:25 AM   
w8ye



Posts: 16708
Joined: 12/11/2001
From: Taylortown, OH, USA
Status: offline 		Sounds like a lack of muffler pressure as well

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       Post #: 2

RE: OS 120 FS - 5/7/2008 1:11:48 PM   
allans


 

Posts: 37
Joined: 11/12/2002
From: Yorktown Hgts, NY, USA
Status: offline 		Thanks for the response .

I think I might have missed your point . Can you tell my why low pressure from the muffler wound result in rich running? wouldn't this result in a lean condition?
Can you explain further ,pls?

Allan



(in reply to allans)
       Post #: 3

RE: OS 120 FS - 5/7/2008 2:06:48 PM   
w8ye



Posts: 16708
Joined: 12/11/2001
From: Taylortown, OH, USA
Status: offline 		The lack of muffler pressure makes the fuel metering system more sensitive to the fuel level in the tank.

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Club Saito Member #7
Original AMA #31261

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       Post #: 4

RE: OS 120 FS - 5/7/2008 2:07:32 PM   
FlyingPilgrim



Posts: 1240
Joined: 9/24/2002
From: Canton, MI, USA
Status: online 		If you are not getting suffiecient pressure from your muffler line to the fuel tank to pressurize the tank, it will make your engine more sensative to fuel level in regards to mixture. Tank pressure from the muffler with a closed (not vented) fuel system will keep a relatively consistent fuel metering from full to empty tank.

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RE: OS 120 FS - 5/7/2008 3:20:06 PM   
Bax


 

Posts: 11559
Joined: 4/26/2002
From: Champaign, IL, USA
Status: offline 		Your fuel tank may be too high or too low. If the centerline of the tank is level with the centerline of the carburetor, or is about 3/8" lower, then it's OK. A very-rich condition with a full tank, and a much leaner setting at the end of the tank suggests that the fuel tank is much too high. With the FS-120, tank pressure is usually not a problem or even a need. Muffler pressure will usually reduce the change in mixture due to fuel head, but won't eliminate it.

We flew for years and years without muffler pressure and didn't have too much problem if the tank was positioned properly, and sized properly for the engine.

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(in reply to FlyingPilgrim)
       Post #: 6

RE: OS 120 FS - 5/7/2008 5:14:14 PM   
blw



Posts: 4321
Joined: 3/15/2004
From: Auburn, AL, USA
Status: online 		A simple thing to try is to use a pin to clean out the pressure nipple on the muffler, and replace the vent line to the tank. Fuel lines can get soft on the inside, and collapse under pressure.

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       Post #: 7

RE: OS 120 FS - 5/7/2008 9:21:34 PM   
jaka


 

Posts: 4239
Joined: 12/11/2001
From: Upplands Vasby, SWEDEN
Status: offline 		Hi!
You are perfectly right! How big tank are you using now?

You can get rid of this problem using a smaller tank...using a Uni-flow set-up with two clunks...and muffler pressure.
I would choose all these three options!


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       Post #: 8

RE: OS 120 FS - 5/7/2008 10:12:22 PM   
allans


 

Posts: 37
Joined: 11/12/2002
From: Yorktown Hgts, NY, USA
Status: offline 		Thank you very much - Can you tell where to get info or piping diagram on a Uniflow tank set up .
If this is what I think I heard described to me - it is the solution I am looking for.

Thank you again,

Allan

(in reply to jaka)
       Post #: 9

RE: OS 120 FS - 5/7/2008 10:28:38 PM   
LeeHop



Posts: 146
Joined: 4/5/2005
From: Aurora, OR, USA
Status: online 		Uniflow links:
How uniflow works
RCU uniflow thread

(in reply to allans)
       Post #: 10

RE: OS 120 FS - 5/8/2008 3:04:04 AM   
icando3d2


 

Posts: 51
Joined: 4/21/2005
From: goshen, VA, USA
Status: offline 		i have the same engine and same plane the engine is less than 3 tanks old and i have problems with mine doing the same thing also it lacks power compared to the old os fs 120 pumped that was on it do you have any power problems and what prop are you running thankns

(in reply to LeeHop)
       Post #: 11

RE: OS 120 FS - 5/8/2008 3:35:15 AM   
allans


 

Posts: 37
Joined: 11/12/2002
From: Yorktown Hgts, NY, USA
Status: offline 		I have had no power or reliability problems. The engine is a horse . I run 15/8 APC when the grass is high at my field and a 15/10 or 16/6 when it is low. The shallow prop gives me better acceration but lower top end . The bigger props give me more consistent power at the same RPM for aerobatics. I am more of pattern type ( or trying to be) flyier then 3D.

The Showtime looks much like a pattern plane after all. The recnt posts all seem to be pointing to a Uniflow Fuel system as the answer . I am going to try it. I ahve an Aeroworks Yak - same engine and the same problem. That plane too is going to get the Uniflow Fuel System.

Now I think I see the advantage of the pump on some of the OS Models - consistent fuel flow . Although some of the people at my field same they are more powerful too.
I am not sure this is true , better running yes - but more power ?

I also have a profile Aeroworks Extra with the same engine and no problem at all. Go Figure.

(in reply to icando3d2)
       Post #: 12

RE: OS 120 FS - 5/8/2008 7:45:09 PM   
djr1007


 

Posts: 199
Joined: 3/21/2005
From: Tucson, AZ, USA
Status: offline 		Have the same setup here also, I mounted the engine sideways, stock muf in the pitts config wouldn't fit. I put a slimline pitts (I think SL), but it has two down tubes 1/2" or more round. I couldn't get the engine to richen up on the ground. I used pliers to pinch the tubes a little, and was able to get richer. pinched a little, got a little more rich. I ended with the equivalent of plugging one tube and now it runs great with full range and full tank to empty. So if you have a pitts with large tubes, that may be your problem (just don't do the DUH like me and pinch both tubes, instead of just plugging one).

(in reply to allans)
       Post #: 13

RE: OS 120 FS - 5/11/2008 2:30:11 AM   
icando3d2


 

Posts: 51
Joined: 4/21/2005
From: goshen, VA, USA
Status: offline 		guys i tried the uniflow setup on my plane and a 16-6 prop problems solves i had a blast this afternoon got some good flights in and actually trusted it to get it low and slow yeah wheeew im so stoked
thanks to this thread

(in reply to djr1007)
       Post #: 14

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