Zenith Aircraft Builders and Flyers
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Hard starting Rotax 912 ULS
So it started in January. I was having an unusually hard time starting my 912. The first time I killed the battery and had to jump start it. But then it ran fine for a flight. I thought it might have been because it sat for 5 weeks(tied down outside). So I figure I need to fly it more often. Next time out a week later, still hard start but I did not kill the battery. Again it ran fine. Went out about a week later, too windy to fly so I decided to just run it(apparently not a good idea). Again it started hard but I was able to run it till all the temps came up. Go out a week later, beautiful day for flying and it simply refused to start. I bring the battery home to charge it and do some research. I discover that a smaller plug gap is recommended for cold weather. I pull all the plugs. They were >.030. The recommended gap was .025. I clean and re-gap the plugs. Plane fires right up. Great. Go out to fly about a week later and no start. Post on Rotax forum and suggestion is to check the floats. They weighed 6.6 grams a pair which apparently is OK. I bring all this to the attention of my A&P who admonishes me for doing all this on my own. OK so I get new plugs, and we install them, he verifies I reassembled the carbs correctly. He verifies there is spark from both ignition modules. Plane fires right up. We warm it up and check the carb sync. All good. Too late in the day to fly but I go out a couple days later and it again fires right up. A bit windy but I do a couple trips in the pattern. All good. Go out about a week and a half later and no go. Again bring battery home to charge. I decided maybe it's a battery issue so I break down and buy a Earth X 900. I can see it's much better as I'm getting much faster rotation when starting it. It basically started but before I could release the choke and add a little throttle, it died and would not start. So now I'm baffled. The carbs are on year 4 of their 5 year rebuild cycle. I'm thinking of sending the CDI modules off to Lockwood to have them tested. I saw on some other sites people having very similar issues which seemed to lead to an ignition module issue. At $1000 a piece, I'd like to really make sure that this is the problem. Any suggestions? If I rule out the CDI modules I may have to breakdown and have the flight school shop have a look. They probably have Rotax certified mechanics as they have Bristells that they use for flight training and I'd prefer not to take a shotgun approach to resolving the issue.
Replies to This Discussion
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Permalink Reply by Charles Clayton on
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And of course I go out to the field with the intent of trying to start the engine and if that failed just pulling the battery to charge it. Of course it starts. Too windy to fly but I run it till it warms up and ran it a bit at 5000 rpm(being it was chocked and tied down). Mag drop was good. Go figure. The question is will it start when I want to actually fly.
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Permalink Reply by Jerry Griego on
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Make sure that you have zero throttle when trying to start. The Choke will not be effective if there is any forward throttle.
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Permalink Reply by Jim and Amelia Isaacs on
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Also ensure when you pull the cold start (“choke”) knob, the cable allows full travel of the cold start circuit lever on the carbs from fully closed to fully opened, stop to stop.
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My mechanic had asked me to verify that and I did, Looking forward to testing again next week when there is a nice day to actually fly.
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How old are the rubber carb mounts?
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Do you mean the intake manifold flanges? As an FYI I bought my plane to take lessons for sport pilot. After we landed from my demo flight the plane ran rough at low RPM. The owner had it checked and apparently one carb had pulled loose from the intake manifold. In any case he had it fixed and I never had any issues for over a year. One day I go to fly. Engine starts no issues and I go over to the pumps to fill up. When I go to restart it runs rough for about 5 seconds, quits and won't restart. I pull it back to my parking spot and have a look. Both carbs had pulled out of the intake manifold. Both clamps were tight. Hmm. Put everything back together and all was good for some flights after that. Of course I do research and discover the 5 year rubber replacement business. I search through my logs and do not see anywhere where this was ever done. Hmm. So I speak with my A&P, order all the necessary parts, remove my carbs and send them off to Lockwood for their 5 year rebuild(the previous owner's mechanic had sent them there according to the logbook). So all this was done 4 years ago.
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Charles did you get this resolved? If so what what was it?
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Well, I was able to get a couple of flights after my mechanic and I worked on it but the last time I went out to fly it, I couldn't even get one revolution out of the prop before it kicked back. I posted on the Rotax forum and the general consensus was it was the Sprague clutch. That's a job I didn't think my mechanic and I were going to do so I went and spoke with the flight school shop. They agreed it sounded like the clutch and they would have a look. In fact they were doing the clutch on one of their Bristells. Unfortunately, it was going to be a couple weeks before they could get to my plane and I'm still waiting for them to get started.
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