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OS 120 FS Runnung Rich to Lean

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Old 05-06-2008, 08:46 PM
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allans
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Default OS 120 FS Runnung Rich to Lean

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?
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Old 05-07-2008, 09:26 AM
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Default RE: OS 120 FS Runnung Rich to Lean

Isn't the FS-120 a pumped engine? If so, the position of the tank should have no effect on it and you should not be using pressure at all. Something else is going on.
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Old 05-07-2008, 01:03 PM
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Default RE: OS 120 FS Runnung Rich to Lean

You can get the FS with or without the pump, I have one of each but an engine with A pump is easier to set up.
If the fuel tank is too high then it can do what you described. The center line of the tank should be level with the carb if you can get it set that way. If not then maybe A different shape of tank. A three line tank is just an added line with A clunk so you can fill and drain the tank without removing the pressure line of main carb line. Other then that it doesn't really do anything to help the draw.
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Old 05-07-2008, 02:59 PM
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Default RE: OS 120 FS Runnung Rich to Lean

Sounds like the muffler pressure may be low, or a leak in the muffler pressure line. Also a uniflow tank setup may help.
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Old 05-07-2008, 04:16 PM
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Default RE: OS 120 FS Runnung Rich to Lean

Thank you for the response .

Would not the rich running point to high muffler pressure not low - what do you think?

do you know where I can get info on the uniflow tank system you described?

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Old 05-08-2008, 07:27 AM
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Default RE: OS 120 FS Runnung Rich to Lean

Would not the rich running point to high muffler pressure not low - what do you think?
I understand that the rich running only occurs when adjusting the needle for a half full tank. Other wise it goes too lean and quits. This is a sign of low muffler pressure as the muffler preasure of a foot or two of head often makes the two to three inch's of tank height inconsequential. Here is a link to some uniflow tank information.

http://www.fraserker.com/heli/uniflo...flow_works.htm
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Old 05-08-2008, 08:19 AM
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allans
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Default RE: OS 120 FS Runnung Rich to Lean

The rich running occurs only when the tank is full - not half empty. As the tank empties the engine runs leaner and better -until the flight perfect from there on.

Can you explain pls how low muffler pressure leads to rich running , with a full tank.

Thank you,

Allan




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Old 05-08-2008, 12:52 PM
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Default RE: OS 120 FS Runnung Rich to Lean

Can you explain pls how low muffler pressure leads to rich running , with a full tank.
I told you its because it has too much head when adjusted for proper mixture from a half tank. With muffler pressure this amount of head is usually too small to make that much differance.
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Old 05-08-2008, 02:06 PM
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Default RE: OS 120 FS Runnung Rich to Lean

I understand your reply. The engine was never tuned at the half tank level only the full tank level .
Now your comment makes much more sense.

Thank you and sorry for any confusion.
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Old 05-08-2008, 03:56 PM
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Default RE: OS 120 FS Runnung Rich to Lean

The engine was never tuned at the half tank level only the full tank level .
If it runs correct at half tank then it is adjsuted for half tank. No matter how it got there.
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