Farmboy
MEMBER
Middlebury, VT
A friends 90-8 stopped running today as if mags were shut off. No sputter, no cough, no change in power, just off.
He landed safely.
On the ground, we discovered that the impulse couplers were not snapping when you pulled it through, and compression only on alternating cylinders. So two seemingly good and two seemingly bad.
Spins beautifully, no noise, drag, or other symptoms of an issue.
So engine gurus, our bouncing ideas around on the frozen lake came up with only a few items to cause this.
-With mags not firing, we assume they aren’t turning.
-With two opposite cylinders holding compression, we assume the valves are open on the other two. In listening to the exhaust as it’s pulled through, it appears to confirm this.
(If it wasn’t zero out on a wind swept lake we would have pulled the valve covers to check operation, but it would not have changed how we dealt with it, so we didn’t chance dropping parts for curiosity)
- Provided this theory is on track, then if the cam gear “failed”, fell off”, or the crankshaft failed, sheared, etc at the accessory case... then neither cam nor mags would turn, and therefore create the resulting symptoms we have.
Who has seen something along these lines?
Thanks,
Pb
Transmitted from my FlightPhone on fingers...
He landed safely.
On the ground, we discovered that the impulse couplers were not snapping when you pulled it through, and compression only on alternating cylinders. So two seemingly good and two seemingly bad.
Spins beautifully, no noise, drag, or other symptoms of an issue.
So engine gurus, our bouncing ideas around on the frozen lake came up with only a few items to cause this.
-With mags not firing, we assume they aren’t turning.
-With two opposite cylinders holding compression, we assume the valves are open on the other two. In listening to the exhaust as it’s pulled through, it appears to confirm this.
(If it wasn’t zero out on a wind swept lake we would have pulled the valve covers to check operation, but it would not have changed how we dealt with it, so we didn’t chance dropping parts for curiosity)
- Provided this theory is on track, then if the cam gear “failed”, fell off”, or the crankshaft failed, sheared, etc at the accessory case... then neither cam nor mags would turn, and therefore create the resulting symptoms we have.
Who has seen something along these lines?
Thanks,
Pb
Transmitted from my FlightPhone on fingers...
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