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65 died

Farmboy

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...
 
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Did oil pressure stay up after quit? Might indicate accessory gears still holding hands if so.
Gary
 
If either the cam or crank gears disconnect from their respective mounts things go quiet. Continued oil pressure should indicate the pump is still being turned by the cam gear on the descent.

But I've had an engine quit right now from an iced over main jet in the carb. Carb ice can take a little longer to slow things down.

From experience fuel flow stoppage to the carb can cause the engine to lean burp before quitting as the line and bowl drain.

Valve cover removal and valve behavior will tell more.

Gary
 
Make life easy for yourselves, find yourself a big tent, or big tent fly with a big door and set it up over the engine with the engine sticking through the door to the inside. Put heaters on to heat the area and go to work! some 2x and tarps work, but a nice big tent is quick!

Or, if you can pull the plane with a sled to a hangar...

Engine? I start with fuel and go from there. Any oil blowing out?
 
It’s not a fuel issue. I don’t even know how someone would come up with that idea based on the info provided.

I don’t believe the mags or cam is turning. We may find out soon as they will be loading it on a trailer today. But a missing gear in the accessory case would stop turning both cam and mags.
We just expected it to be noisier.


Sent from my iPhone using SuperCub.Org
 
That reminds me of an incident with a J-3 when the A-65 quit cold. The pilot landed safely in a field with a big ??. He started the engine normally, took off, it quit again so he landed in another field. I don't recall how we determined it, but the crankshaft was broken. This was in warm weather so not applicable to your case.

Another incident: The A-65 lost oil pressure and quit in the winter. The kidney tank will flow oil normally with the oil only warm and flowing around the pickup tube. Then as the oil continued to warm, a slug of congealed oil moved and blocked the pickup tube. Oil circulation stopped.
Both of these incidents happened in the Berkshire Hills just South of you.

Easy things first:
How did you determine this?: "-With mags not firing," Did you pull a plug or plug lead? and look or feel?
Take the covers off the mags to determine if the cams are rotating. The impulse(s) not working will not stop the engine when it's already running. Do both mags have impulses on this engine?

Disconnect the tach cable to see if the tach drive is rotating. This will tell you if the camshaft is rotating.

How did you determine? "-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." Which two cylinders? The two front ones or the two back ones? Did these two cylinders hold compression on the exhaust stroke as well as the compression stroke???? If so...camshaft not turning. How do you know which cylinders made the noise when you listened? A zero compression cylinder will still make exhaust noises when pulled through unless it's the intake valve which is open. An open intake valve will cause an engine to quit without warning. I know of two cases of this happening in 250 Comanches, fortunately an airport was nearby. Both cases, engine was shot.

Considering the cold...the first question would be ice build up in the carb?
 
Good questions Pete. And while most of this is simply to advance my diagnosis skills, I enjoy the exercise of whittling down the options until few remain.

We pulled the prop through and found compression on alternating cylinders. On bad two, one had nothing and one had maybe a little. But, the no compression cylinders do produce a little breath of air release out the exhaust. This lead us to believe that the crankshaft is turning, pistons are moving, and valves are open.
I don’t know if it was 1/3 or 2/4.

(Again, the theory was its bad enough we weren’t going to make it run at that time. Don’t get into it here on the ice)

We don’t believe the mags are turning because no audible impulse snap on pulling the prop through. It’s extremely quiet on the backside.



Sent from my iPhone using SuperCub.Org
 
1. 172 goes quiet. Lands on golf course. Pull prop through. No clicks from mags. Nothing turning in accessory case because the crank bolt came loose. You know, the one you’re supposed to check on a Lycoming after a prop strike. I know you are talking Continental, but something broke inside that motor.

2. Stearman, 220 Continental starts running like crap. Makes it back. crank broke but stayed turning but “adjusted” the timing.

Rich
 
So it’s a C90-8, not sure why I assumed it was a 65.
In the -8 accessory case I assume they are the same - crank gear is 4 bolts into end of shaft?
More likely the shaft broke or the bolts came out/sheared/etc?


Sent from my iPhone using SuperCub.Org
 
Farmboy said:
We may find out soon as they will be loading it on a trailer today.
Click to expand...

Farmboy said:
(Again, the theory was its bad enough we weren’t going to make it run at that time. Don’t get into it here on the ice)
We don’t believe the mags are turning because no audible impulse snap on pulling the prop through. It’s extremely quiet on the backside.
Click to expand...
Best decision considering the conditions. The cold could cover up a weak sounding impulse snap.
 
As I recall from my C-90 those two gears (crank-cam) are held on by 4-1/4" short bolts that are then safetied.

Gary
 
Just watch the tach while someone pulls the engine through. It the tach moves, the gears are still engaged and turning. The tach is driven by the oil pump, which is driven by the cam gear.
 
Tom, did you not see the reasons why we didn’t? Lol. And no, setting it on fire wasn’t a good solution either. Lol. But thanks for the suggestion. :)

The owner did take a quick look upon unloading at the hangar today. The cause of the resulting failure is known. We are speculating on the root cause, but perhaps improperly tightened bolts. (There is no known prop strike history)

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Transmitted from my FlightPhone on fingers...
 

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Yikes ! That explains the two no compression cylinders, the valves weren't moving. That gear has been cracked for a long time. Good thing the lake was frozen over.
 
Look at those elongated bolt holes in the crank gear. Been bad for long time. Metal should have been in screen. Any evidence of that recently?

Gary
 
The guy's going to take it to the shop and remove the accessory case and see where it goes from there. I would not have thought you'd see metal in the screen from the elongated bolt holes unless something else was wearing as well. I don't know the maintenance history either.

When you guys say a long time - what are you thinking for how long it takes to elongate? I would have thought if those bolts loosened up to the point of shearing, it wouldn't take long for it to happen.

Pete, are you thinking the gear was cracked before, or perhaps as a result of the bolts loose and working?
pb
 
From the pictures it appears the gear cracked, failed and then sheared the bolts when it came apart. It broke in two+ pieces. Thus it separated under load which pulled the pieces apart. Unless they were under torqued originally they weren't loose since the safety wire is still attached.

This is the second engine recently with cracked gears in the accessory case. The fellow in South Africa found them on his new to him J-3. I think he determined there had been a sudden stoppage in the engines history. There could have been on this engine as well, many many hours ago? With a previous owner. All prop strikes aren't reported. How many times has a damaged prop just been straightened or replaced with no written evidence?
 
Pretty much what we thought standing on the lake at zero yesterday. Got up to zero this morning and the crew had it apart and secured on the trailer heading home in under 3 hours.

IMG_20210131_124233.jpg

Glenn
 

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I bet that the bolts were finger/hand wrench tight and then it sat for awhile and later it was assumed that they were torqued and it got wired

Glenn
 
There's potentially two bolts available for the C-90 - one's applicable for the -8 P/N 21346 (1/4"-28x7/16); the other's for the -12 P/N 22532 (1/4"-28x1.00"). Might be a reason if they're sized accordingly as the crank gears are different as well. Might check if there's any chance for bottoming out before clamping the gear.

Gary

Pics below. The correct bolt 21346 is fully threaded and shorter. The one remaining on the crankshaft end pic appears to be fully threaded. The longer one has a partially unthreaded shank so it's unlikely they were installed.
 

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I'm thinking Glenn's post #27 is a pretty good insight. The reason why, this is not usually a problem area on a small Continentals.
 
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