I was recently stranded out of town for three days because my RV12 engine would not develop enough RPM to fly. Stranded is a good word to use when you find yourself troubleshooting an engine issue with only the basic tools you carry in the plane. This RV12 will be 5 years old in November and has flown close to 600 hours.
I had flown into an airport with an FBO at 10:30am when the temperature was 90F and returned to leave at 3pm when the ambient air was 100F and the tarmac temperature over 106. All was normal on the flight down. The plane started fine on the return and taxied fine. The run-up, ignition check was unusual. Rather than the normal 60 to 70 RPM drop (each), it had increased to almost 200 (each). Rotax allows 300 to stay in spec. Take off was interesting. At approx 4400 RPM, the throttle kept advancing but the RPM did not. EGT temps did not change. The plane would not develop enough power to fly.
Over the next two days, everything was checked that could be reached easily. Since the problem did not effect either of the two systems independently, I ruled out ignition. It had to be something that impacted an entire cylinder (or more). It was giving the impression that it needed more of something...air or fuel. Fuel pressure gauge read fine. Throttle linkage throw was complete. Choke was not on. Prop blades were not loose or off match-pitch. Car gas (who knows the real octane of car gas) was replaced with 100LL. Spark plug wires checked for attachment. Exhaust connections checked for leaks. Gascolator cleared. On and on no luck. Next day, came back with more tools. Checked for leaking valve. Replaced all plugs (cheapest thing on the engine). Looked for sparking in the dark.
After checking virtually everything possible, without removing major engine parts...it was time to pull the carburetors. Fortunately, I was able to borrow two "known" carbs and swap them out. It worked and I flew home where I could work in my own space at my own pace.
Turns out the rubber diaphragm in one of the carbs was the culprit. It was torn. It does not appear to be ethanol...it is a rubber part but it was not slimy like lawnmower parts, nor was it deformed and full of pin holes. It simply appeared to be torn for whatever reason. Maybe its just time. Both carbs now have all new rubber parts. Rotax recommends replacing all rubber on their engines on a five year replacement schedule. That includes the diaphragm, the o-rings and the rubber intake socket.
I am not mentioning all of this because it has never happened to a Rotax engine. I bring it all up simply because this is one of the original RV12's and has considerable hours. Rotax was new to me when I built the plane. Several of us built ours at the same time and we have stayed close for 5 years just talking and learning about what it takes to keep everything working. Now I think maybe the 5 year hose replacement (and carb rubber) is probably a good idea. I will certainly carry a Bing diaphragm in my plane from now on. The plane won't fly without it.
I had flown into an airport with an FBO at 10:30am when the temperature was 90F and returned to leave at 3pm when the ambient air was 100F and the tarmac temperature over 106. All was normal on the flight down. The plane started fine on the return and taxied fine. The run-up, ignition check was unusual. Rather than the normal 60 to 70 RPM drop (each), it had increased to almost 200 (each). Rotax allows 300 to stay in spec. Take off was interesting. At approx 4400 RPM, the throttle kept advancing but the RPM did not. EGT temps did not change. The plane would not develop enough power to fly.
Over the next two days, everything was checked that could be reached easily. Since the problem did not effect either of the two systems independently, I ruled out ignition. It had to be something that impacted an entire cylinder (or more). It was giving the impression that it needed more of something...air or fuel. Fuel pressure gauge read fine. Throttle linkage throw was complete. Choke was not on. Prop blades were not loose or off match-pitch. Car gas (who knows the real octane of car gas) was replaced with 100LL. Spark plug wires checked for attachment. Exhaust connections checked for leaks. Gascolator cleared. On and on no luck. Next day, came back with more tools. Checked for leaking valve. Replaced all plugs (cheapest thing on the engine). Looked for sparking in the dark.
After checking virtually everything possible, without removing major engine parts...it was time to pull the carburetors. Fortunately, I was able to borrow two "known" carbs and swap them out. It worked and I flew home where I could work in my own space at my own pace.
Turns out the rubber diaphragm in one of the carbs was the culprit. It was torn. It does not appear to be ethanol...it is a rubber part but it was not slimy like lawnmower parts, nor was it deformed and full of pin holes. It simply appeared to be torn for whatever reason. Maybe its just time. Both carbs now have all new rubber parts. Rotax recommends replacing all rubber on their engines on a five year replacement schedule. That includes the diaphragm, the o-rings and the rubber intake socket.
I am not mentioning all of this because it has never happened to a Rotax engine. I bring it all up simply because this is one of the original RV12's and has considerable hours. Rotax was new to me when I built the plane. Several of us built ours at the same time and we have stayed close for 5 years just talking and learning about what it takes to keep everything working. Now I think maybe the 5 year hose replacement (and carb rubber) is probably a good idea. I will certainly carry a Bing diaphragm in my plane from now on. The plane won't fly without it.
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