mar345

For anyone out there who may want to convert an old OS 120II pumped engine to non pumped here's a little story about why and how I did it. Way back about twelve years ago I bought this OS 120 II pumped engine not knowing that it was intended for frequent use for F3A type flying. When not run frequently and allowed to remain idle for a few weeks it could take as much as 24 to 32 ounces of fuel run through it befor the diaphraghm in the regulator loosens up and allows the engine to run reliably. It had never been on an airplane because I could never tune it to my satisfaction. It always ran extremely rich with very wet exhaust in the mid range. I modified the plumbing with tees to reduce pump pressure and leaned the low mixture screw to max, which improved the rich mid range condition. Then I inverted it on the bench to test run it in preparation for putting it into a Hanger 9 Super Cub. Well now I couldn't get a reliable idle below 2700 RPM. So, after getting advice and information pertaining to converting to the non pumped OS120E version I ordered the crankshaft, backplate carburetor and intake manifold from Tower. Took only about an hour to do the actual conversion. I did my first test run inverted on the bench with fuel from a half filled bottle from last season. Big mistake! Engine ran well but couldn't set up a reliable idle. So tried again with a fresh jug of S&W 10%. Made a world of difference. Starts easily, transitions well, idles well and mixture adjustments are easily made with a minimum of fiddling around. So after twelve years this engine will finally find a home on the front of my Super Cub.
MY stats:
S&W 10% fuel
MA Classic 16-6
Max RPM 8900 & 8600 with N.V. backed off to 1 3/8 turns open
Minimun idle 1800 RPM
Reliable idle 2000 RPM

Special thanks to BoB Spear and Bill Baxter for their invaluable input and advice.

tonyc

My Feedback: (4)

I have had both the pumped and the non pumped OS 120. The non pumped is sooooooooooo much more fun to use. I used this one in a Cermark Pitts mounted inverted. Worked just great.

As for the pumped versoin, I sold that at auction years ago and glad that its gone. What a pain in the ----.

tonyc