I have recently purchased an old OS 120 4-stroke with the #C-13 pumped carb. It runs like a new engine, but has the common fault of loading up rich at idle, and quitting. With the idle mixture at fully leaned it still is not enough. (Yes , the top end is set as lean as I dare) {New F plugs make no difference) I'm wanting to replace the carb o-rings but can't find a parts schematic for this old engine to get the OS part numbers for the sizes. I'm pretty sure the nitro has caused the o-rings to be leaking pump pressure fuel by this time. The pump is working surprisingly well, and the throttle response is perfect. Can any of the old school glow folks help me size the correct material and sizes for these?

I'd love to get a digital page from the manual too, but seems impossible from what is in other threads.

Hi!
How do you know the O-ring on the high speed needle is leaking air?? I have never seen this happen during my 51 year in this hobby and not seen any O-ring in a model glow engine go bad.
Do you run 5-10% nitro fuel containing 20% castor oil or syntetic oil or a blend of these? Propeller size is also important.16x6 or 15x8 or if you have the engine in a CUB type of plane, 17x4-17x5 or even 18x4.
preferable RAM; Graupner G-Sonic or APC. Avoid Master A props with white tips !!!
Are you sure you have an original OS F plug and not a copy??
Tank size and position is also important. A 320-400 cc tank is suitable for a 20cc four stroke. Tank should also be positioned so the center of the tank is inline with the carb orifice as the plane sits horizontal,this is regardless if you have pumped engine or not. The tank should also sit as close as possible to the engine.