OS 120 FS Idle
 

I have an OS 120 FS that will not idle when the fuel tank is full. If I lower the level of the fuel in the tank it runs fine. High liquid level in the tank causes over rich mixture.
The tank location and engine location are exactly as the manufacturer suggested. The center line of the tank 10mm form the fuel nipple on the carb.

I called OS . This what they suggested . Put a 1 inch loop in the fuel feed line and most of the problem will go away.

I am going to try it.

Does anyone know if this will work and if it does - how and why does it work?

Thank you ,

Allan

RE: OS 120 FS Idle
 

Hi!
The tank rule goes like this!
The center of the tank should be in line with the fuel inlet orifice of the carb when the airplane sits horizontal/flying position (tail high)!
Tank size should be 300-400cc.
No loop is necessary if you have a correct tank position!

RE: OS 120 FS Idle
 

Allan,


While the suggestion from OS *may* solve the 'full tank' problem you are facing, it would also add a little over 3" to the length of the fuel-line, between the carburettor and the tank...

Excessive length in that area affects reliability in a negative way, and also; as the fuel-tank empties in flight, the mixture your engine might face is too lean... The cost of solving an over-rich situation with the tank full, using the suggested method, may be an over-lean situation as the fuel level is reduced...


You wrote nothing about what you tried to do on your own... Did you try closing the low-speed needle in the carburettor a bit, to see if it helps?


Many in this hobby; especially those who are less experienced, regard the low-speed needle, as if it is 'set for life'...:eek:
But this needle tailors the fuel-curve of the engine, from idle to ~80% throttle.

It is in the middle of the carburettor barrel, accessed from the throttle-arm side. Please see the photo, to understand where this needle is located.
It works normally (i.e. clock-wise to close) and you need a small flat screw-driver to manipulate it.


I would define NOT bothering to adjust it, a crime...[:@]

RE: OS 120 FS Idle
 

The plane is an Ultra Stick 120 Lite , so neither the tank nor the engine are moveable . The fuel input nipple is about 10 mm above the center line of the tank. This is within the tolerance stated in the OS engine manual. Moving the enegine up or down will take it off the center line of the fuse - so I don't thinkI want to do that .

If I raise the level of the tank I will increse the height of the liquid level above the fuel nipple and make things worse. This will richen the idle mixture more which is what is happening now.

Do you mean lowering the difference between the liquid level in the tank and the nipple?

Thnak you.

RE: OS 120 FS Idle
 

Dar.

I have adjust the low end and from 80% of the tank capacity to 15% capacity the engine idles perfectly . It is from 100% capacity to 80% that I have the problem. No matter what I do the low end needle I cannot make lean enough to run in this range. It will run in this range with the glow driver on the glow plug.

How does the excessive length affect reliability in a negeative way? And how long is should the fuel be? What would you consider excessive? It is now 6.5 inches.

Thank you,

Allan

RE: OS 120 FS Idle
 

Are you using muffler pressure? This will stabilize you needle settings. You will have to lean you los speed setting a little.

RE: OS 120 FS Idle
 

Allan,


Long fuel-lines make it more difficult for the engine to draw fuel...

It is not a matter of sucking the fuel to a greater height; just over a longer length.

The only possibility I can think of that would make the mixture 'too rich to run', or that much of a difference; is if you are using large diameter fuel-line...
The mid-size (3/32" i/d) is sufficient for this engine.


Also, I powered my USL with an MVVS 26 cc (1.60" ) glow engine.
Isn't this OS a bit small?

RE: OS 120 FS Idle
 

Dar ,

Thank you for the response.

I did not realize that there is a pressure drop over the length of the fuel feed line. I need to give this one some thought. It does make sense though.

The OS 120 FS is bit low on power for this plane. I had an OS 120 AS ( Very powerful) on it and I over flew the tail surfaces and was incurring flutter . So I went back to the FS.

The plane is very light and very easily gains speed. I use big prop with a low pitch ( 16X6, 4 or 17 x4) .

RE: OS 120 FS Idle
 

Hi!
The fuel nipple on the carb has nothing to do with tank rule! It's the fuel intake orifice , in the intake that counts, !
And...if the tank is too high ...why not move it down some!? Just take a Dremel with drum sander and remove some of the formers!! Simple task!

Fuel length is very important!
Always keep it as short as possible!

RE: OS 120 FS Idle
 

The way people got there ugly stick engine carbs in the right places is to "clock" the engine. Some would have an upright cylinder others at 45 or 90 degrees.

RE: OS 120 FS Idle
 

Mine were (are) mounted sideways. Never any trouble

RE: OS 120 FS Idle
 

Allan,


My USL has always been flown with a Bolly wood 18x6 prop.

My engine, like Jim's suggestion, is mounted at 90°, i.e. the head is to the right side (of the plane...) and the exhaust stack points straight down.


I never had any flutter problems either.

RE: OS 120 FS Idle
 

Interseting. Are the gaps in the control surfaces very close ( ie very small ) and are the gaps taped ?

Both of these techiques are have stopped potential flutter on aircraft. I use both.

I will try the lengthened fuel tubing today as a fix for the idle problem when the fuel tank is flull.

Let you know the result.

RE: OS 120 FS Idle
 

Hi!
Lengthening the fuel line will not help!!
The only thing that will help is to use :
1. OS F glow plug
2. 16x6 or 15x8 prop preferable APC or RAM (for best performance).
3. 400-500cc tank, placed according to the "Tank rule"! Set up with Uni-flow! or better yet! A Tettra "bubbleless" tank!
4. As short as possible fuel lines, 2,5-3mm inner diameter.
5. Use fuel with 5-15% nitro

And last but not least... Set the idle needle correctly!

RE: OS 120 FS Idle
 

Jaka,

Thank you, I do everything you mentioned already - although I think the tank is bigger than 500cc- I will check.

I am going to try the longer fuel line today.

I will let you know how it works out.

Allan

RE: OS 120 FS Idle
 

Allan,


I suggest that you try medium fuel-line first, if you have a larger i/d type...


My USL's control surfaces are built with no real gap.
They use the CA hinges, pushed int to contact depth, before CA is applied...

The result is a minuscule gap that is not worth discussing.

But also, this plane is flown slowly, rarely at full-throttle.


Flying it at high-speed will probably increase the chance of flutter setting in.

RE: OS 120 FS Idle
 

I have (had) eight of these Ultra Sticks over the years and I mostly used the tank that came with them. Always mounted the engine sideways.

I've never experienced any problem with them and mixture. Used many different engines including gas engines.