I am experienced with the Rotax 912/914 engines. I have received maintenance training up to Heavy Maintenance. But, I do have a problem that I cannot resolve. The engine is mounted on a motor glider and during flight we routinely shut down the engine. For reasons unknown the engine will not come to a complete stoppage as it always has done in the past. Normally the propeller stops turning, and then the propeller is feathered. That normal action was routine for 250 hours on a regular bases with this aircraft. Upon shutting off the rotary switch (off, L,R, Both, Start) to off and while throttle is set to idle position, the engine continues to run, but hesitant, some roughness, partially firing and it sounds and feels like it is dieseling or something. It's certainly not smooth and not normal. If you put the ignition back to both, the engine sounds change to instantly smooth running at idle. I don't dare allow the engine to run any more than a few seconds before switching it back on. We changed out the ignition switch with a new one, no improvement. But look at a little history now.
This condition has occurred on two separate occasions about a year apart. The first occasion we investigated the P-leads to the ignition modules to make sure they were making ground when the ignition switch was turned off, and open when they were turned on. I proved the ground contact was made and it should have shut off the ignition modules. The connectors at the modules were also examined and they were found to be okay, at the six pin plugs at the modules themselves. If the ground leads were open then the ignition modules are alive. I must add too, that upon landing and shutting off the engine it would sometimes shut off by turning the ignition switches off, but sometimes it was necessary to turn off the fuel to get it to shut down while on the ground. Shutting down on the ground was inconsistent. We had new ignition modules available so we put on new ignition modules. That did not change any of the characteristics. So we traced the ignition grounding wires from both ignition modules to the switch. There we found embedded wrapped in black tape on the ignition ground wires a diode in each one of the wires grounding the ignition modules. Okay, those were removed and the wires were connected direct. We called everyone we knew to see why there would be diodes in those wires. A diode is a one way valve for electrical circuits. That circuit should be only either open or closed, doesn't need diodes. Open to operate and closed to shut down. Wah-la, that fixed it. The engine would then shut down in flight, feather the prop, shut down upon landing worked just like it was supposed to do all along. It worked fine for nearly a year.
Second occasion, it’s now doing it again. It want shut down completely in flight, it runs like its dieseling not good. I does shut down on the ground by turning off the ignition modules. Sooo, while in flight there is forward motion at nearly stall speed and that seems to be enough to keep wind milling the propeller, and enough turning of the engine to keep it running. Normally at 60 knots the propeller will stop turning when shut off on all other Lambadas and SunDancers that I have flown equipped the same as this one.
So one may conclude that the turning of the propeller (wind milling) causes the vacuum or movement of air through the carburetor sucking fuel into the venture allowing fuel into the cylinders, and the fuel is igniting from pressure, like a diesel, or maybe hot clinkers inside the cylinder heads. The owner does run 91 octane mo-gas nearly 100 percent of the time. The owner did run 100 LL for a while thinking that he could clean up the clinkers if there were any. The spark plugs are clean with maybe 50 hours on them.
I flew this Lambada just last May 2017 and everything was fine. You could stop the engine, feather the propeller normally, and restart normally. Out of the blue it is doing the same thing all over again, except the engine will shut off normally on the ground.
I will appreciate any ideas that anyone may have. You can contact me at my cell phone: 559-960-7873 or my email address at: joekulbeth.airusa@gmail.com
