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Dec 03, 2018, 08:20 AM
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Low Speed Idle needle mystery - OS 46FX


Tuning an old OS 46FX yesterday I had a situation where the engine would idle high and then drop rpms quite a bit to a much lower idle after about 30 secs. It would then usually quit or at least stumble heavily on transition to WOT. I kept leaning the low speed needle (LSN) until this went away and I got a good steady idle and smooth transition. However, it felt like the LSN was bottomed out when this occurred. Could this be true? And if so, then what would be causing it to run so rich on the low end that the LSN would have to be closed?

Didn't seem like an air leak, bc the engine ran well at this setting and my experience is that an air leak gives a very unreliable idle. It was good enough that I went ahead and performed a maiden flight on the aircraft, which went flawlessly. Maybe I should just leave well enough alone?
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Dec 03, 2018, 09:04 AM
The great Brutifier
Most likely, the last time the engine ran, was on a different fuel with a higher viscosity oil?

There could have been a blob of congealed oil creating the false sense of bottoming out.
When the LS needle of an FX really bottoms out, you notice that by that the throttle won't close properly (it acts a bit like the throttle stop screw).
Dec 03, 2018, 09:11 AM
Registered User
Quote:
Originally Posted by Brutus1967
Most likely, the last time the engine ran, was on a different fuel with a higher viscosity oil?

There could have been a blob of congealed oil creating the false sense of bottoming out.
When the LS needle of an FX really bottoms out, you notice that by that the throttle won't close properly (it acts a bit like the throttle stop screw).
And if bottomed out the needle could mushroom the end of the spraybar if forced.
Dec 03, 2018, 09:27 AM
The great Brutifier
Could.... but you need actually quite a bit of force for that... More than I exerted on it, which was substantial... ( dumb action when I didn't realize I bottomed it out, and didn't understand why---that---darned---- throttle --- won't---close, hgrmpfh!)
Dec 03, 2018, 11:24 AM
Scratch builder
With the barrel closed, you screw in the LSN, until it stops, then return 2 turns if engine is installed upward, if the engine is installed upside down just open 1.5 LSN, if you already do that I think you must check very well your tank or better use other tank to check the engine.
My experience with LSN is if you have a defective oring, the LSN will close itself and never have a stable low RPM, when you try to adjust , the first run it will be ok, but after 5 minutes of run the LSN lose its position and advance to close position.
Alberto
Dec 03, 2018, 12:18 PM
AMA 537620
ChillPhatCat's Avatar
I would be concerned if it needs to run at a significantly different setting than factory. My 25 FX was having an awful time with transition and idle, until I looked up the factory LSN setting and reset it... the previous owner had mucked it all up, but as soon as I went to factory it became my favorite engine. This was an inverted installation as well, didn't seem to care a bit even with the factory 2 turns.
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Dec 03, 2018, 03:46 PM
Registered User
Factory settings are only approximate. Settings can vary widely based on altitude, fuel, prop, and glow plug used.
Dec 03, 2018, 04:12 PM
The great Brutifier
I have never bothered with "factory settings": there are very simple ways to find good settings:
Open the LS needle all the way, tune HS needle at full throttle, gradually close the throttle and adjust LS as you go, and you'll have a good setting in no time.
You might go back and forth a few times to fine tune, but basically that's it. You can find any engine's proper settings that way.
Dec 03, 2018, 10:18 PM
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ChillPhatCat's Avatar
With this engine you need a screwdriver to turn the LSN... no way I’m doing that with a running engine. Maybe if it were truly a twin needle carb, I’d tweak it. Maybe if I was less concerned about losing a piece of finger... it runs perfectly as is so there is absolutely no motivation for me to sit there for an hour tweaking... But to each their own.
Latest blog entry: Ready for test flight
Dec 04, 2018, 03:24 AM
The great Brutifier
We call very long and thin screwdrivers "carburettor screwdrivers" for a reason
And it is about a 5~10 minute job at best, including the back and forth...
Dec 04, 2018, 06:03 AM
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ChillPhatCat's Avatar
What is going to be the benefit to my perfectly running engine?
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Dec 04, 2018, 07:27 AM
The great Brutifier
to a perfect engine? Nothing... it allready is perfect, isn't it?

But for people that have lost their manual, or cannot find the info for their particular engine, it is a way of getting things sorted...
Dec 04, 2018, 09:38 AM
Registered User
Cougar429's Avatar
Some information lacking that may help point towards a solution here. Pics would help a lot, as well. Obviously all this is based on a correctly assembled and good condition carb and engine.

How is the engine mounted? Upright, inverted? Stock muffler? Fuel?

What is the HS needle setting?

What is the tank height relationship to carb inlet? Has this engine or any other run fine in this installation?

Does changing plugs alter anything? How about idling with the exhaust vent line removed from the muffler? At idle the amount of boost is minimal so if there is residual pressure in the tank from a faulty vent line you would have to deal with that by closing off both needles. The same can be encountered if the tank is too high from head pressure, especially if the tank is above half.

Most OS carbs I have dealt with, as well as their clones, start off with the HS needle 1 3/4 turns out from lightly bottomed. The LS needle end is also usually flush with the end of the barrel, although in the case of this engine the instructions do state to just close the throttle valve, then thread the LS needle in to bottom and then back out 1 1/2 turns.

I can also confirm all these settings get you in the ball park, but need to be tweaked for local condition in both environment and mechanical variance. That means outside temp, humidity, altitude, fuel, prop selection, engine and glow plug condition all have a role to play. Don't forget, often these settings were for an engine manufactured and often tried half a globe away. EVO were pains since they did this then added restrictor rings to both needles. First things to get removed.

Finally, with any engine like this you really need to focus on fine tuning the HS needle first. You need to have good top end peak, then slightly rich. Only then do you get the LS needle to allow for reliable idle. Even once they move the HS to the rear remote location the procedure remained the same.

NOTE: I have dealt with engines dropped off to me where the HS needle was so far out to compensate for the nearly closed LS needle to allow for any kind of idle. Surprised they would run at all.
Dec 04, 2018, 12:22 PM
Registered User
Further info:
Engine is mounted upright. HSN was tuned to be a few 100 rpm less than peak - it is likely 1-1/2 to 1-3/4 turns out from bottom. This was done first and then the LSN was tweaked. I'm going to pull the engine this weekend and take a closer look at the LSN. It's an old engine and the threads may just be gunked up beyond where it was originally set.

I have tried the following method to get me close when starting an unfamiliar engine: with the carb barrel at throttle setting I blow air through a clean piece of fuel tubing until I can just barely hear air coming through the carb. This works pretty well for starting blind.
Dec 04, 2018, 12:49 PM
Registered User
Cougar429's Avatar
Most often with old engines what I find it compressed, shrunk or leaking O-rings. That normally causes lean runs rather than rich.

Does the engine idle longer with better transition with glow heat applied?

How is compression cold and hot?

Is this engine back where it ran before, or has another engine run fine there? Again, asking since fuel tank height and other factors could be affecting run quality.

Off to work now. Joy.


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