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Pumped OS FS-91 surges at full throttle

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Old 03-21-2011, 08:19 AM
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Skylane
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Default Pumped OS FS-91 surges at full throttle

I have a pumped OS FS-91 which is running erratically at full throttle - it surges. It will run well for awhile leaving a visible smoke trail but then it sags and the trail disappears. This happens erratically. Sometimes I can get nearly a fuull flight witout a problem, other times not. I replaced the pump with one from another engine which runs well, but the problem remained. I set the needle for peak rpm and did not enrichen it afterwards, but the problem remains. Any advice would be approciated.

Jeff
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Old 03-22-2011, 06:01 AM
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Default RE: Pumped OS FS-91 surges at full throttle

Usually, surging indicates a too-rich setting of the high-speed needle valve. If you are using a pump system, do not pressurize the fuel tank with pressure from the muffler. Just leave its vent open to the air. Your problem may not be the pump, but the carb, where the regulator is located. You could have problems with the regulator.
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Old 03-22-2011, 06:59 AM
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Default RE: Pumped OS FS-91 surges at full throttle

Thanks, Bax. I am not using muffler pressure and the tank installation works well for my other pumped 91 (this one is down awaiting a new bearing - that's why I'm running the second one). Since the problem engine does run well when it isn't surging, I'm thinking that the mechanical potions of the engine are ok ( I did check the valves and the adjustment is ok). Since I changed out the pump with no effect, that leaves the carburetor, regulator, or needle vlave. I guess the next step is to swap out the carb body and regulator with the other engine and see if that isolates the problem. Before I do that, is there a good way to flush the regulator and carb body? Perhaps there is some trash in there.

Thanks for your advice.

Jeff

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Old 04-26-2011, 08:10 AM
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Default RE: Pumped OS FS-91 surges at full throttle

This is a quick follow-up to my previous question. I determined that the low-end setting was very rich, so I started leaning the low-end 1/8 turn at a time and readjusting the top end. After about 1 1/2 turns I started getting better performance. I continued the process until the low end seemed right. The engine has performed well for the last ten flights with no sign of surging. It now idles at 2000 and has a high end of 10,300 with 20% pro pattern on an APC 13x8. Thanks for your help.

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