My 912 ULS Wont start

by David Bennett in 912 / 914 Technical Questions 9 years ago

:(
Hi All I own a Piper Sport with a 912 ULS with 550 hrs and 5 yrs this Jan
Battery new 08/14 tested, Spark plugs new, CDIs new with Soft start.
Still difficult to start. Cranks and kicks back. No horrible sounds from sprag clutch yet.
Still have not found the right technique, It will eventually fire if I advance the throttle to full and then back quickly runs rough until choke it closed soft start drops off and runs like a clock. Fear full of damaging sprag if this hard starting continues. Its just not right. Something is wrong. Regards Dave. Down Under Oz.

Re: My 912 ULS Wont start

by Rob Seaton » 9 years ago

Rob Seaton

3 1.32k 240

sounds like the sprag is already damaged.
Install the new flywheel at the same time as you replace the sprag. Then the ignition will fire after TDC and you will not have any more problems with the sprag.
Always replace the sprag housing, its way to much work to find out after you have put it all back together it was also damaged. It may look fine but dont take the chance. trust me on that one :(

Thank you said by: David Bennett

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Re: My 912 ULS Wont start

by David Bennett » 9 years ago

David Bennett

1 1 0

Thanks Rob, Might as well do the gear box (600)at the same time as I imagine the engine needs to come out to do the sprag yes? Runs only on Mogas.

If sprag damage is due to the kick back thing What come first? Is there an ignition fault that causes the kicking back that then causes the sprag to go or is it just wear and tear that compounds its self. Dave.

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Re: My 912 ULS Wont start

by Rob Seaton » 9 years ago

Rob Seaton

3 1.32k 240

Hi Dave, the sprag is in the ignition housing so the back-end has to come off. But you are right, the gearbox is a good thing to freshen up at 600hrs if it has had a rough time.
1-The sprag is worn from hard starting.
2- Then the kickback starts as it is unable to carry the piston through the compression stroke when the plug fires (4 degrees BTDC). It will spin all day with the mags off but when you have add spark the combustion stops, and reverses the piston.
The new flywheel causes the start-up timing to fire after the piston has passed top dead center (3 degrees after top dead center). When you combine this with the soft start or Rotax ASM soft-start you have a very smooth, low power start so the sprag has a much easier life.
Rob

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My 912 ULS Wont start

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