Good post. I had problems with my Rotax 912 also. Did not know at first what caused the apparent misfire, could be carburetor of ignition. A pattern began to show: Trouble always began after 40-60 min of flight. Could be vacuum in the fuel system? Could be something heating up and be thoroughly heated ? We dismantled the ignition coils and measured the resistance both primary and secondary circuits. The the whole thing into an oven - 60 deg C for an hour. And there it was! One of the secondary circuits went from approx 15 kOhm to zero i.e. became short-circuited. And this was repeatable: Normal readings came back when cooling the unit, heating again would show a short-circuit. New coils installed - all good 🙂
Thanks for the diagram! It clearly explains that the 912 ignition is not separated into TOP plugs on one switch and BOTTOM on the other, as I’m pretty sure Lycoming and Continental are. I have a ROTAX 912 ULS built in August, 2021. It has 84 hrs tach time. It suddenly dropped from 5500 to 4700 in cruise, and would only rise to 5200 in a VNE dive to 143 MPH. The front 4 spark plugs were fouled. Each carb feeds one front and one rear cylinder, so I thought it had to be an ignition issue. Turns out, the left carb diaphragm had a 1-12” long tear in front, paralleling the perimeter, and the right diaphragm had a 3/8” slit on the 360 radial (pointing toward the prop). New diaphragms have her running better than ever. Of note, as a Yamaha/Kawasaki mechanic in the 80’s-90’s, dozens of customers brought in ignition coils to be dynamically tested with my Kent Moore tester. 99.99999999999999% of them were fine. (The problem was in order, Pulsar (trip) coil, CDI Unit, Source Coil). I suppose kill switch was close to the top, but it would have been eliminated before delving deeper. Thank you again!
I appreciate the feedback and the extra info on the motorcycle ignitions. As I read your comment I was thinking fuel problem also. You seem to have good knowledge and diagnostic skills!
I need to start looking at my channel more often. I did not realize anyone was looking at the videos. Sorry! The modules take the pickup signals and fire the coils.
If I am reading this correctly, the miss is the same on either ignition? Assuming everything is configured correctly, the problem would have to be common to the cylinder(s)affected. Do you have EGT sensors? This would possibly give you an idea as to what cylinder is affected. It would likely be cooler. In my case, I eliminated the possibility of fuel flow issues (which would affect both ignitions) and had tested wires, plugs, and had cleaned up all the grounds before I go to this level of testing. The fact that the engine had to get warm to misfire is what lead me to heating the coils and pickups. Hope this helps.
I basically unplugged the spark plug on the 2 wires for each coil and warmed up the coil with a heat gun. It is a trick I have used on cars in the past. The pug wire ends were in the circuit when I tested them. I simply added the resistance of the coils and plug ends to get my baseline. That works as you may know when you are testing in series. The wires are solid core so no resistance. Hope that answers your question.
Bradley Banitt I couldn’t get the readings through the caps like they wanted,but they were all the same so I figur Ed they were good and took them off and then I got the readings I was suppose to.Thanks
Hi Brad. Can you explain to me where to put the test leads of the meter. I tested the coil by unplugging the sparkplug connector with one test lead in and the other test lead in the other plug connector. But the meter is not showing any resistance. Like an open loop
Good post. I had problems with my Rotax 912 also. Did not know at first what caused the apparent misfire, could be carburetor of ignition. A pattern began to show: Trouble always began after 40-60 min of flight. Could be vacuum in the fuel system? Could be something heating up and be thoroughly heated ? We dismantled the ignition coils and measured the resistance both primary and secondary circuits. The the whole thing into an oven - 60 deg C for an hour. And there it was! One of the secondary circuits went from approx 15 kOhm to zero i.e. became short-circuited. And this was repeatable: Normal readings came back when cooling the unit, heating again would show a short-circuit. New coils installed - all good 🙂
Thanks for the diagram! It clearly explains that the 912 ignition is not separated into TOP plugs on one switch and BOTTOM on the other, as I’m pretty sure Lycoming and Continental are.
I have a ROTAX 912 ULS built in August, 2021. It has 84 hrs tach time. It suddenly dropped from 5500 to 4700 in cruise, and would only rise to 5200 in a VNE dive to 143 MPH.
The front 4 spark plugs were fouled. Each carb feeds one front and one rear cylinder, so I thought it had to be an ignition issue.
Turns out, the left carb diaphragm had a 1-12” long tear in front, paralleling the perimeter, and the right diaphragm had a 3/8” slit on the 360 radial (pointing toward the prop).
New diaphragms have her running better than ever.
Of note, as a Yamaha/Kawasaki mechanic in the 80’s-90’s, dozens of customers brought in ignition coils to be dynamically tested with my Kent Moore tester. 99.99999999999999% of them were fine. (The problem was in order, Pulsar (trip) coil, CDI Unit, Source Coil). I suppose kill switch was close to the top, but it would have been eliminated before delving deeper.
Thank you again!
I appreciate the feedback and the extra info on the motorcycle ignitions. As I read your comment I was thinking fuel problem also. You seem to have good knowledge and diagnostic skills!
Thanks for posting this.
You're welcome!
Super helpful I think this will fix my problem
Glad it helped
Hello Sir, what is the purpose of electric module? The A and B.
I need to start looking at my channel more often. I did not realize anyone was looking at the videos. Sorry! The modules take the pickup signals and fire the coils.
I have a similar problem but the miss occurs even with one ignition off or the other one off, did you try that test?
If I am reading this correctly, the miss is the same on either ignition? Assuming everything is configured correctly, the problem would have to be common to the cylinder(s)affected. Do you have EGT sensors? This would possibly give you an idea as to what cylinder is affected. It would likely be cooler. In my case, I eliminated the possibility of fuel flow issues (which would affect both ignitions) and had tested wires, plugs, and had cleaned up all the grounds before I go to this level of testing. The fact that the engine had to get warm to misfire is what lead me to heating the coils and pickups. Hope this helps.
So you checked resistance between the spark plug wires on the high tension side.
I basically unplugged the spark plug on the 2 wires for each coil and warmed up the coil with a heat gun. It is a trick I have used on cars in the past. The pug wire ends were in the circuit when I tested them. I simply added the resistance of the coils and plug ends to get my baseline. That works as you may know when you are testing in series. The wires are solid core so no resistance. Hope that answers your question.
Bradley Banitt I couldn’t get the readings through the caps like they wanted,but they were all the same so I figur Ed they were good and took them off and then I got the readings I was suppose to.Thanks
wreckum56
Good deal!
Hi Brad. Can you explain to me where to put the test leads of the meter. I tested the coil by unplugging the sparkplug connector with one test lead in and the other test lead in the other plug connector. But the meter is not showing any resistance. Like an open loop
@@gmc2016 just follow the drawing On one coil.
What a great finding and explanation. Thanks for sharing
Glad it was helpful!