I have an FS-120S III engine that I am having problems getting to run up to speed. Right now wide open with a 16x6 prop
I can get it to turn 8000 rpm. The engine is on a 90 size Funtana ARF. This engine is barely enough to keep the plane in the
air.

Does anyone have any suggestions or know of any place where they explain the pump system or any troubleshooting steps
I could try. It is tuned on the high end to max rpms.

Thanks

Steve

Well, you can start [link=http://www.osengines.com/manuals/fs-120s-iii-manual.pdf]here[/link], that's the manual. I run a 16x6 and a 15x10 on mine, but they're on war birds. On the Funtana, I'd try a 15x4W. With the 16x6, I get about 8700 RPM on SIG 15% fuel. I tried running some 30% with a #8 glow plug, but I get more RPMs with the 15%. That's enough to get a 13 pound P51 off the ground. Once the plane is in the air, there's alot less resistance on the prop. I can hear it turnung faster than it did on the ground. I even traded props, and the non pumped 120 performed almost identically.

You didn't say what series engine you have the OS faqs page has the correct set up for all of the pumped engines ,and unless the pump has been messed with adjustment wise it should be factory set for your particular engine .
http://www.osengines.com/faq/faq-pumped.html try this and see if what your after is there .