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OS 120 Surpass III

Old 04-20-2007, 05:06 PM
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Default OS 120 Surpass III

good day.

my engine is still quite new ... it used it now and then for about 2 years and now that i finaly got a model to permenantly install it is started to give me troubles. The outline of the problem is this. The engine is constantly running rich. I can run it with good performance with the need open 1 turn from the fully closed position. Tis is with a 13x11 apc prop turning 9000 RPM yron fuels with 20% synthetic to castor blend and am curently running the engine with out a muffler (no noise restrictions back here) Tht needle seems pretty usless at this point, the difference in RPM between 3 turns open and 1 turn open is only around 300 RPM and i can really hear the motor peak.

However although the engine has been performing ok at one setting or another whether it be rich or lean when i pull out the plug, the part without threads is completely black and the exhaust valve is extensively covered in carbon deposits. all this occurs after just one day, about 6 flights, having started the day with a new plug and clean valve

Todays knowing that the above is iniveitable and having cleaned the valve i tuned the engine on 20 20 fuel and was getting good performance fut after several flights the motor began reving at top RPM at all settings.

this is my cry for help, this is an exellent motor but something is wrong what is it ????

thank you
Old 05-01-2007, 01:15 PM
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Default RE: OS 120 Surpass III

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.

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