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Old 06-27-2012, 12:48 PM   #1
Florin
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OS FS120 dead stick


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New for me SIG 1/4 cub with OS FS 120 (very old) engine.

I had a couple of flights with this plane last year in November when I bought it, then I did not fly it until last week.

Last year I flew it in about freezing temperature and the engine worked really good, now it was about 25 Celsius and the first flight was about 8 minutes and it was awesome - plenty of power, flying like a dream at half throttle.

The second flight - after 1 minute, the engine seemed to not pick up RPM when I throttle up. I was afraid of a dead stick, so I throttled down immediately to idle, then I tried to slowly throttle up, but I felt the engine refused, so I prepared for landing. After few seconds the engine quit and the dead stick landing was relatively OK (one tire came off the rim), but now, on the bench, the engine works just fine on idle, but as soon as I try to throttle up, it will stop.

I also noticed that as soon as I remove the igniter, the engine RPM goes down.

Now, few questions:

#1 - RPM goes down when I remove the igniter - does that mean the glow plug is not operating fine?
#2 - the fuel I used was from the last year. The jug was 70% full, but stored in a cold room - can this still be a problem?
#3 - A guy from the club said the engine is too lean, but I guarantee it was rich enough. After reading the user manual I understood that idle mixture might be too lean, but if that is the case, how could it change in a matter of minutes between the first and second flight?
#4 - the user manual is recommending a type "F" OS glow plug. Is this what you guys are using? for OS four stroke engines?
#5 - what is the best fuel for this engine? The user manual says 5-15% nitro with no less than 18% oil. I never saw the oil percentage on the fuel bottles.

Any input is welcome
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Last edited by Florin; 06-27-2012 at 01:47 PM.
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Old 06-27-2012, 02:54 PM   #2
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Re: OS FS120 dead stick

It sounds like the plug is going out. OS "F" plug is the plug to use. One of the guys at our field, when he has engine running problem the first thing he does is change the plug for a new one and it cures the problem. When he tests the old plug it looks fine. Try a new plug and a new jug of fuel. I have a bad jug of fuel in the garage, it's two years old. If you are loosing RPM when you take the clip off the plug, either the plug is too cold or the fuel not enough nitro or old fuel.
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Old 06-27-2012, 03:00 PM   #3
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Re: OS FS120 dead stick

Thanks Kip.

Here comes another question. What should be the idle RPM?
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Old 06-27-2012, 05:22 PM   #4
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Re: OS FS120 dead stick

Be sure and check if the high speed needle is sucking air. Change O-ring or put on a piece of fuel tubing to seal that area.

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Old 06-27-2012, 09:14 PM   #5
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Re: OS FS120 dead stick

I have an O.S. 120 fs in my Ultimate, and it idles down also when I remove the glow igniter.. I run the low speed rich so it smokes a bit at low rpm and high speed 1/2 turn from peek rpm
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Old 06-27-2012, 10:06 PM   #6
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Re: OS FS120 dead stick

If your RPM drops with the igniter removed that usually means it is a bit rich. If the drop is not too significant I would not worry.

Another indication of mixture at LS is to check response when rapidly opening the throttle after it has idled for a bit. If it quits right off it is too lean. If it stumbles then dies, or stumbles then clears out it is too rich.

A new plug may cure your problem, but there may be another cause to its failure. If the motor sat for a long period with no protection the bearings and other ferrous materials in the engine could have started to corrode. That material can find its way into the combustion chamber and then impinge on the glow wire. This can prevent the catalytic action when exposed to fuel. It will glow when powered fine, but will not work that well unheated. This can also cause the RPM to drop when the power is pulled, if not quit altogether.

Yes, all 4-strokes require a 4-stroke plug and the OS "F" type is a good match. There are others available, but that is all I use.

One other thing you may want to look into is if the motor is overheating as the behavior you described is almost the same for that condition. The fact it ran fine in colder weather, then had this problem in a much warmer environment could be pointing that way. You did not say if the motor was hard to turn over or showed signs of overheating once back on the ground. If the motor is cowled you will have to look into ducting to get enough air over the cylinder and head.

One other place to look is the fuel system. Either a split line or any other air leak such as the HS needle issue williame3590 suggested is a good idea. Even check inside the tank. You did not comment if you refueled before that second flight. If the clunk or line is compromised once fuel drops you could start sucking air.

I run 15% nitro throughout my 2 and 4-stroke fleet with no problems. I've read Saitos can go as high as 30%, but that seems a bit extreme and expensive. 18% lube is fine. If possible check the fuel supplier for specs on their oil content. Castor tolerates hot running much better than synthetic and I try to have at least some in the fuel. Synthetic is a lot cleaner and you can have a mix of the two type lubes with no problems.
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Old 06-27-2012, 11:48 PM   #7
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Re: OS FS120 dead stick

Thanks for the input guys.

Some more details:
- I added ARO immediately after the flight last year, so I do think it is corrosion (at least not recent started corrosion), but I will check the glow plug
- The motor was not cowled and I did not check if I can turn it immediately. What are the signs of overheating?
- I refueled between the flights
- The fuel lines are good, I checked them under pressure and they hold pressure fine.

Cougar, are you saying that you are using the same fuel from the same jug in both 2 stroke and 4 stroke?

Thanks again.
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Old 06-28-2012, 07:49 AM   #8
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Re: OS FS120 dead stick

Florin, unlike 2-strokes the only direct connection to the crankcase is usually through the small breather fitting and on newer OS 4-strokes they opted to connect that back to the intake to try and reburn any contaminants, (identical to the PCV system on an auto engine). On the earlier versions the line was external and could be disconnected to run the vent outboard. I've read the newest version is all internal and therefore cannot be altered.

I agree with many this is a BAD idea. I do not want to run what could be metal and corrosion particles back into the engine.

What all that means is it is difficult on 4-strokes, even those with open vent lines, to flush out contaminants from the case and valvetrain and coat all the internals with ARO. If any unburnt fuel remains the corrosion I described can begin immediately, so could have also contaminated the glow wire with microscopic particles slowly during each run.

One relatively eacy check for corrosion is to remove the valve cover(s) and have a look at the visible ferrous parts, likely the rocker arm(s). Unless you're running a YS AC motor this can be a difficult place for oil to reach.

Usually overheating have a few indicators: If difficult to turn, even away from compression something is binding up. Possibly bearings, but more likely con rod, piston to liner fit, etc. DO NOT try to spin it to loosen it up while still hot. The damage caused will grow much worse. If it does not return to normal once cooled your only real option is to open the motor up and find out why. Perhaps you can get away with pulling the rear cover and dosing the motor in oil or even soaking it in ATF, basically anything to try and get oil between the parts that are in direct contact.

Regardless of freeing up when cooled down I would then opt to open it up and give it a good flush out anyway. Don't forget the drag was enough to stop the motor in the first place and there is likely a LOT of wear particles floating around inside. I would then have a good boo to check for damage to the piston, liner and even a stuck ring. Burnt oil can also cause the valves to stick in the guides and serious overheating can even weaken valve springs.

I do run the same fuel for 2- and 4-strokes. There really is no difference in how the nitro and methanol work inside the combustion chamber.

However, there are different requirements for lube and what I purchase at the start of each season is specifically labeled to cover both types.

Contrary to popular myth and likely to start an argument is the fact 4-stroke oil commonly contains LESS oil than 2-stroke. The max RPM's are much lower and frankly I have not found a 4-stroke bushing style engine. Due to that I would give the following warning:

You can run 2-stroke oil in a 4-stroke, but I would seriously advise against the opposite.

p.s. One very important item if running a line from a case vent fitting is to keep it as short as possible since it actually "Breathes" in an out as the piston moves up and down. If too long or restrictive any contamination will not be ejected and reingested in the case. In my cowled installations I have no choice if I plan to have that ejected outside in free air to keep the inside of the cowl relatively clean. In those cases the only option is to increase the diameter of the vent line. Since I use plastic tube it is relatively easy to accomplish.

Here are a few examples. First is the Skybolt and you can barely make out the black vent line. That was an early installation of an older Saito and I recently moved the vent fitting to below the cam in the case, identical to their newer version motor. This is not recommended unless you know what you are doing and could only be accomplished during a complete teardown.

Second is the OS installed in the Harvard. The line already exits below the cam on the Surpass motor and that short vent line exits directly out the cowl.

Last is the Saito in Rare Bear. A rather long reach and due to that the vent line is visibly larger diameter than the stock 1/8 poly tube I almost universally use now for fuel lines.
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Old 06-28-2012, 09:37 AM   #9
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Re: OS FS120 dead stick

"One relatively eacy check for corrosion is to remove the valve cover(s) and have a look at the visible ferrous parts, likely the rocker arm(s)."

- I'll check that ASAP and I will post some pictures

"One very important item if running a line from a case vent fitting is to keep it as short as possible"

- my line was installed last week and it is not on the short side. I'll shorten it. Last year I flew without the line (and without the cowl). I had no clue what that line was for, but I got it now that it is quite important

Thanks again.
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Old 07-03-2012, 10:57 PM   #10
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Re: OS FS120 dead stick

"remove the valve cover(s) and have a look at the visible ferrous parts, likely the rocker arm(s)"

OK, I had the chance today and I removed the valve cover. There was only one bolt (instead of two) holding the cover and it was not very tight. I easily removed it with a an allen key that was one size smaller. I don't think that a missing bolt here can cause my problems - I guess this over protects from spilling oil, but not to hold any pressure and cause the engine to stop if pressure is lost.

There was no visible rust inside, everything looked clean and oiled.

I'll get another bolt tomorrow (couldn't find anything at the exact size) and I'll give it a try with fresh fuel. I bought a glow plug as well...just in case...
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Last edited by Florin; 07-03-2012 at 11:09 PM.
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