912ul soft start

by Stephen Foreman in 912 / 914 Technical Questions 8 years ago

Hi, I have a new 912ul fitted to my recently completed ch701. The engine came with the ignition units that have the soft start option which I have connected as per the instructions. Although everything works fine starting the engine with the soft start makes the engine run really rough for the first six seconds or so. My question is is this normal because it seems very strange to me after running another 912ul for ten years without the soft start. The starting process seems horrible now compared to what I'm used to. Thanks for any advice.

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Re: 912ul soft start

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 240

that is not normal; in the start-up mode the ignition is firing after TDC (top dead center) so the engine is producing very little power, it should be very smooth.
Sounds like one module is going into retard/ASM mode and the other is not.
The ASM wires are connected to the 12V starter relay spade connector?
The ASM wires are connected to the violet wires in the lower six-pin connectors?
Ohm them out to make sure crimps are good.
Try disconnecting one module (unplug upper six-pin connector). Does it change anything? this way you can isolate one module from the other.

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Re: 912ul soft start

by Roger Lee » 8 years ago

Roger Lee

126 5.14k 4.58k

Hi Rob,
Maybe you misunderstood him? Isn't this correct?

Hi Stephen,

Yes the engine runs rough for 5-6 seconds at start. It starts in 3 degrees ATDC and keeps it there for a few seconds and then switches over to normal run timing of 26 BTDC. Every engine I have ever seen with the soft start system runs rough just for those few seconds. Mine does too. The old modules started up at 4 BTDC which made the actual firing a tad harder against BTDC. Now it fires ATDC. The old modules fired at 4 BTDC and immediately switched to 26 BTDC. It had no delay in timing switch, but the new style does and allows the engine to spin up and get everything firing before it transfer to 26 BTDC.

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

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Re: 912ul soft start

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 240

Yes that is correct.
I guess we need to define how rough it is. Every ASM start I have done is very smooth; in respect to shaking of of the engine.
If it is truly kicking and bucking then something is wrong.

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Re: 912ul soft start

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 240

Good example of rough vs smooth start: see minute 4:40 of video
http://www.rotax-owner.com/en/videos-topmenu/product-reviews/49-ssm22

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Re: 912ul soft start

by Stephen Foreman » 8 years ago

Stephen Foreman

3 14 0

Hi Roger, thanks for your reply. The engine does run rough but only for the few seconds it was just while it's doing this it seems strange to me after running the other 912ul for 10 years without the soft start option, and that engine seemed to start without so much vibration. But the roughness of the new 912 could never be described as severe it does what you describe. I have disconnected the wire from the starter relay and started the engine, but although the engine started OK it was a bit harsh or so it seemed, so I have reconnected the wire. I was advised by a Rotax technician that the soft start on the 912ul is not really required and is more for the 912uls. I would be interested to know what you think of that comment.

Thanks again Stephen

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912ul soft start

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