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  1. #1

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    OS FS-120 Surpass Pumped (late '80s?) Trouble Running

    I have an old FS120 Surpass Pumped that I picked up from a friend. I am having trouble getting it running correctly. It has a balanced Top Flite 15x6-10 prop on it. I can get it to produce only 7300 rpm at the high end (needle is about 1 turn out) and can hold about 2500 at the low end - any lower and it kills. What should rpms should I be able to achieve with this? It smoked a lot at low to mid range throttle which said rich to me, so I turned in the small screw that is suppose to control the low speed mixture. The idle improved from 3200 to 2500, but I think it is still too rich (with screw all the way in). Based on my other engines, I would expect ~2000 rpm idle and ~9000-10000 rpm top end. Is that correct for this one? I am using 15% Wildcat Premium Xtra fuel and an OS type F glow plug. Any suggestions?

  2. #2
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    For top-end RPM, you may need a different prop. The one you're using is quite a load. Try a plain 15 x 8 propeller. Any idle below 2,500 is considered "good". Use an O.S. Type F glow plug for best results. Finally, you may need to run several tanks' worth of fuel before the engine will settle down. The engines with the backplate pump would generally need that much running after a long period of non-use because the regulator diaphragm would tend to stiffen, and the running would let the fuel loosen it back again.

    For tuning:

    Your O.S. Max engine with pump should be set up in a slightly different manner than a non-pumped engine. Most modelers tend to get them set with the mixture too rich.One sign your engine's not adjusted correctly would be surging at full throttle. This almost always means it's too rich. Also, the idle may be too rich no matter what you do to the idle mixture. If the idle is leaned a lot, but it's still too rich, the high-speed needle's too rich and needs to be properly set.To get a good setting, you need to use a tachometer. Once the engine's had enough running to accept being leaned to peak RPM, use the tachometer to lean the engine to its peak.At full throttle, lean the engine until the RPM stops increasing. Continue to lean the engine to the first sign of RPM drop or detonation. When you get to that point, richen to the point just before you had the RPM drop or detonation and leave the needle there. DO NOT lean to peak and then richen your usual 1/4-1/2 turn.With a good-quality fuel, the pump system will insure that the engine receives enough fuel. The regulator in the system will make sure that the mixture stays correct as you change the throttle positions. Fly the engine to see how the mixture may change as the engine unloads in the air. You may have to lean or richen slightly, depending upon how it acts. Once you've found the proper setting, you should note the change from the absolute maximum lean position. Now you have a repeatable setting for your engine.Newer O.S. engines with pumps have a broader range for the needle setting, but you can use the same method for all of them.
    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  3. #3

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    Thanks for the quick reply. I will try a 15 x 8 prop and let you know my results.


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