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Stranded By My 912ULS

Peterk

Well Known Member
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.
 
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Thanks Pete -

for the excellent write-up about the problem. May well help someone in the future if they see 4400 RPM max on take-off and all else seems ok. I have 640 hours, and near 5 year annual. Good info for us all.
 
Pete, Appreciate the pirep on the carb. Will likely save someone some trouble-shooting time. I'm looking forward to what some of the experienced Rotax guys will have to say about this particular issue.

Jim
RV-12 #264 flying since 2010
 
. . . "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." . . .

Prior to this experience with the engine, had the carburetors ever been removed, disassembled and inspected?
 
Yes. At 200 hours and again at 400 hours per the Rotax manual. Both floats were tested for "float". All parts appeared to have just come out of the dishwasher...no evidence of damage or wear. This will be the 600 hour and the 5 year replacement.
 
Yes. At 200 hours and again at 400 hours per the Rotax manual. Both floats were tested for "float". All parts appeared to have just come out of the dishwasher...no evidence of damage or wear. This will be the 600 hour and the 5 year replacement.
Thanks for the heads up on this. . . . :)
At least now, I will know some of the symptoms.
Based upon this experience, what tools would you recommend to be carried on a short cross-country trip?
 
what tools...a short cross-country trip?

Now that would probably be a whole new thread but i'm sure there are many such recommendations already in VAF. I will tell you the basics are a #2 Phillips, wrenches (3/8, 7/16, 1/2) and basic sockets of the same. The unusual on the carbs is the fuel line that attaches with a 13mm bolt. It's probably smart to keep some other issues in mind if you plan to build an emergency pack...like tires, brakes, etc. - things besides the engine.
 
I'm interested

Great write up - appreciated. I think that would be extremely helpful if those of you who are experienced Rotax flyers would chime in here and give us Rotax rookies a list of which parts, tools and equipment we should carry on trips in the 12. Thank you all for your help.
 
Tool Bags Rule!

I had a similar experience as Pete's, but it involved an ignition module. It occurred a 250 hrs. At the time, ignition modules were sold out across the US. I found a certified set in Canada and as luck would have it, Rotax had a special on them. A pair was $1,000.00 (approx.). If you bought one the price was around $700.00. I replaced them both at the same time and carried the left-over functional module in my tool bag, in the airplane. I am a believer in carrying at least enough tools to get the cowling off and to remove a wheel. Tie wraps, safety wire, spark plug wrench, etc. A spare Voltage Regulator would not be a bad idea either. This is all based on experience with the Rotax. A couple years ago my buddy came limping into Butter Valley Golf Port, a small strip with no services, several hours drive from our airport. I had enough tools to remove the bowl on his Marvel-Schebler, clean the almost imperceptible contamination from it and get him going again. Another time, my buddy Skunkworks, had a flat tire immediately upon landing. We whipped out the little Doctors Bag and removed his wheel and with some help at the airport we were at, he was back on his way shortly.
 
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Hey thanks Pete. I always worry about being stranded by a problem like that.
My RV 12 5th Birthday is next month I think I will do the carb kit at annual time.

I bet you where sure happy to be back in your shop to look things over.


Brad Stiefvater
Salem SD.
 
Thanks

Pete much appreciated you sharing your experience! These sharing of experiences is what makes this site such a benefit to the aviation community.

Wasn't the diaphragm replaced at the 200 hour rebuild?

I have a friend who has operated his kitfox with an 80 horse 912 and while traveling last year he had an ignition failure and getting another one was expensive and kept him stranded for a few days. He now travels with a rebuilt spare.

Great ideas on what to carry on those trips out and about in our RV12s
 
Rotax carbs

I have over 800hrs on these engines -- both 80hp and 100hp on a Titan Tornado II. If you have this many hours you should check this since it does happen and the rubber on these should be replaced at some time interval though I never saw one go bad. Overhauling the carbs is easy except for replacing the butterfly valve. Did that once but would not replace it again since it's a little difficult and really shouldn't be needed. I have seen the gaskets go bad on the chokes. Look for discoloring around the joint between choke and engine block and always check to make sure they are returning to off when choke is released. If they stick it'll run rough.
 
I went to the ROTAX service and maintenance classes about 18 months ago. They did a good job on most items, but the carburetors were not discussed. Does anybody know of a course to take before tackling a carb overhaul?
 
I went to the ROTAX service and maintenance classes about 18 months ago. They did a good job on most items, but the carburetors were not discussed. Does anybody know of a course to take before tackling a carb overhaul?
There's a pretty good video on RotaxOwners.com
 
Wasn't the diaphragm replaced at the 200 hour rebuild?

Rotax recommends inspections at the 200 hour intervals(200/400/600) with a replacement of the diaphragm, o-rings and manifold socket at the 5 year mark. Rotax has published a Service Instruction for Inspection of Carburetors

http://www.rotax-aircraft-engines.com/portaldata/5/dokus/d04512.pdf

notice they say, "replace the float chamber gasket". In other words, don't try to use the old one. (it WILL leak)
 
The Rotax 912 engine's have a 200 hr. inspection, this is not a rebuild. 5-6 years or roughly 800 hrs. is a fair interval for a complete rebuild. There is nothing in writing in any Rotax book that gives a carb rebuild time table.
Rotax considers the complete carb rebuild a Heavy Maint. item. I have tried to get them to move this back to a lower service rating, but they won't budge. The rebuild isn't hard, but there are a few items you must be aware of and not mess it up.

For example;

How to remove the butterfly valve and screws (there is an easy way), how to peen the new butterfly screws, the correct position on the butterfly install, the correct position for the choke shaft, how to clean the idle jet which should always be done, proper diaphragm positioning, proper float arm measuring and positioning and how not to tear and or roll the "O" rings when installing them, making sure you flush every little hole on the carb body.
All easy if you talk to someone or have someone show you.

If anyone runs into trouble you can call me for help.
 
Bing carbs

Yep, Same thing happened to me with one of the Bing carbs on my old BMW motorcycle. Was able to tape the tear with some heavy electric tape which worked fine until replacement diaphragm arrived. Would not recommend this for airplane use.
 
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