I have an orignal OS-Graupner Wankel my dad purchased in the early 70's. It was only used on one plane before it removed and stored until Jan/Feb of this year. Since it was stored with a lubricant to keep the bearings from seizing, it didn't take much effort to clean it out and get it running. It actually fired up and ran the first time the starter motor turned it over with the glow plug lit.

The problem: After running several minutes the engine would begin to sag, cutting engine RPM almost in half. Reducing throttle to slow the engine to a high idle for 30 seconds or more would allow the engine to cool enough that it could rev back up to the 10K-12K rpm range when the throttle was opened up again for a brief time. If I kept it at full throttle, the engine would over-heat and quit. This all happening with a well balanced Topflite 10X6 PowerPoint prop.

Thinking it was something I was doing wrong, I searched RC Universe for info on Wankels and found several threads that discussed similar over-heating problems caused by fuel that lacked proper oil content and the remedy was to add castor oil to the fuel. I am using Cool Power 10% and, per the threads, have added 10 ounces of SIG Pure AA Castor Oil to it.

Latest engine runs as of this afternoon with an 11X4N Master Airscrew prop on it were pretty good but it still sags after the engine runs a minute or so at full throttle. The needle valve has always been adjusted at full throttle for max RPM then backed off several clicks to richen it up. It runs pretty well if not opened to full throttle (~3/4 throttle) and can rev up to a strong full throttle for 15-30 seconds before it starts to sag.
Is there something else I can do to help the engine run cooler that I'm not thinking of? Would pure castor oil or a synthetic castor oil be the recommended choice to mix for this engine?

Thx, Jim

Sagging may be caused by an engine that was never fully broken-in, or has had buildup over time that is causing friction. You may want to try some extended running at very rich settings to see if it just needs more running time. If it's the original-series engine, are you running it with the shroud that came with it, and is it getting enough cooling airflow?

Hey Bill,

I'm pretty sure the engine was broken in long before it went into storage. The Wankel was on a plane my dad and I flew for almost a year before dad had to give up the plane to another modeler (long story). Dad was a stickler about following mfgr's directions on engines. And yes, the cooling ring has been in place when run. We even flew when it was snowing here in Dallas this past Saturday and it still started sagging after the plane had been up for a minute or so.

Now for the sad news... The Yak-54 this engine was pulling around spun in Easter Sunday afternoon when a rudder clevis popped loose during a low level manuever. The main force of the impact was to the Wankel and ended up breaking off the needle valve and exhaust port from the cooling ring. So now, I guess the question is "do you know where I can find a replacement cooling ring for an original Wankel engine? (I think the plane can be fixed (one more time).

If I do find the parts I need, I will try more running on the test stand to make sure it's really loosened up before I put it back in the air.

Because parts for your original-series O.S. Wankel are no longer made and no longer available from us or O.S. in Japan, you'll have to go through one of the dealers who sell parts to engine collectors. They advertise in small ads in the model magazines or in the classified sections of those magazines. You may also need to inquire through online sources such as the RCU Marketplace or online sales/auction services.