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Viperdoc
Joined: 19 Apr 2014
Posts: 484
Location: 08A
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 Posted: Tue Mar 06, 2018 5:41 pm Post subject: Engine stumble in flight |
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The open valve allows the spark from the aberrant discharge to ignite the fuel mixture in the supercharger. That discharge goes to all 9 cylinders at the same time.
If you are on the wing when it happens you see a big black belch of smoke from the carb and the exhaust stacks.
Doc
Sent from my iPad
| Quote: | On Mar 6, 2018, at 5:46 AM, Vic <vicmolnar(at)aol.com> wrote:
In the photo you can see the aluminium foil of the capacitor, the primary windings are not there, just the HT wires around it.
There are a lot of good ideas in this thread, we certainly know effects from HT harness and sparkplug troubles. But I can tell you, nothing of that all is depending on time and temperature like a failed coil cum capacitor. When we had this stumble first time for real we were thinking along old mogas from last season that may have gone duff. So we had avgas for next flight - and that took us some more flights till we realized that time was that matters. It was about half an hour each time till that stumble showed and could be induced spectacularly on the ground after landing, setting revs to min. 80 percent: You cannot fail to notice for sure !! With a cold engine NOTHING showed when doing the mag test in preflight. So it became crystal clear after realizing the critical time lapse for the effect that the real problem was the coil. I could demonstrate that anytime in my oven by watching the spark length with the hot coil ! A cold bad coil produces exactly the same spark li!
ke a good one.
Just why the engine misses for a complete second on all cylinders in flight is not totally logic to me. Burning into the inlet system with an open valve is hard to imagine. You usually get no ignition without compression in a gasoline engine. While that effect is not yet well explained I continue to be worried about a situation when the engine may not pick up again after that stumble !
Vic
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Looigi
Joined: 20 Apr 2015
Posts: 80
Location: Christchurch, New Zealand
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| Posted: Tue Mar 06, 2018 6:26 pm Post subject: Re: Engine stumble in flight |
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Ok, so I replaced the duff coil and all seems to be happy in the world again.
Ground runs were fine, and we did a proving flight close to the airfield with no apparent issues. I am calling it fixed.
With regards to the cause of the engine stumble, I am going to exert my feminine side and change my mind in favour of the other theory put forwards on here. I still cannot explain how a rogue 'spark' would get to a cylinder with it's intake valve open without cross firing inside the ignition harness, but on reflection I agree that it is a more reasonable explanation. The lack of other classic crossfiring symptoms leads me to still suspect the magneto over the harness.
Many thanks for your input guys. I love this forum for the way we put forward contrary views and discuss the issue without getting upset or personal, which is an affliction other forums suffer from.
Cheers
Chris
Oh, and I still haven't got anything from the guy with the Bendix coils in Yak mags. I will keep chasing him though.
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