Rotax 912UL black intake

by Przemysław Wiśniewski in 912 / 914 Technical Questions 7 years ago

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Re: Rotax 912UL black intake

by Roger Lee » 7 years ago

Roger Lee

125 5.12k 4.53k

Changing the needle clip for idle rpm won't help you. This is the idle circuit. Have you rodded out the center of the idle jet with a piece of wire. Have you used carb cleaner in a spray can with the small red tube to blow through all the orifices in the carb and did you follow that up with some high pressured air? If not you need to go back and clean the idle circuit better.
And you're sure the choke shafts are turned in the right direction for the small center punch mark?

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Rotax 912UL black intake

by Przemysław Wiśniewski » 7 years ago

Przemysław Wiśniewski

65 247 3

Ok. I did some flights today and I found that module B have problem.
On ground I found, that cylinder 2 when powered by only module 2 is not working. Exhaust pipe is cold. Engine run's rough.
It started to be really bad when I started to touch cables.
I swapped trigger coils, module and finally found that problem is somewhere between HV coil, spark plug cap. I changed spark plug, spark plug cap, removed ignition cable, checked resistance and reinstalled again (by cutting 5mm each side). Problem still exists.
I measured resistance between yellow cable and ground, it's around 0,7ohm. Resistance between spark plug cap of cyl 2 bottom and 1 bottom, gives 15kOhm. There is no connection between HV port (spark plug) and ground.

How I can make sure if coil is faulty?

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Re: Rotax 912UL black intake

by Bill Hertzel » 7 years ago

Bill Hertzel

13 1.95k 7.96k

My first suggestion would have been to swap the spark coil with a known good one.
If the problem stays with the #2 Cylinder, the coil was good.

Thinking it through...
The bottom Plug on Cyl#1 and #2 are power by a common Spark Coil.
If the Bottom Plug on Cyl #1 is firing, the Coil Is Good.

There is no connection from the HV Port to Ground because the opposite spark plug is the ground path.

What you haven't eliminated is the actual HV Wire to the spark plug.
And the problem became worse when you moved the wires.
Look at it again VERY Closely. There may be damage to the insulation that is hidden.
Are you sure it is good? Are you sure you're sure???

If the insulation has worn through or even has hardened and cracked, the spark will take the path of least resistance.
The spark will still spark, just not at the spark plug.

If the fault is not obvious, try looking at the wiring in the dark, really dark, with the engine running.
Be extremely careful trying this. It needs to be so dark that you can barely make out the engine.
If you can see any building lights within 500 ft. it isn't dark enough.
Allow 5 minutes for your eyes to adjust to the darkness.
Any bad plug wires will show a Blue Corona glow on them. Think, "St Elmos Fire". Very Pretty Stuff!
It is actually a quite beautiful way to see what is causing all the grief.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Rotax 912UL black intake

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