Intermittent 4-5 second engine roughness at cruise

[Narwhal]

Twice now this happened:

C182B O-470L Engine 460 hrs since overhaul in 2011, bendix/tcm mags overhauled at the same time.

The engine gets real rough for 4 or 5 seconds then goes away.

First time was climbing at 10,500 with carb heat on, mixture leaned ~ 150 F rich of peak.

Second time cruising at 7,500 with, no carb heat this time, mixture leaned same as before. I had been cruising at 2300 RPM and was advancing the prop to 2400 rpm when it happened.

Both times I was about an hour into the flight.

Of course when it happens I usually change the position of the carb heat and mixture but by the time I do those things it's over.

I suspect an intermittent magneto problem, because the engine feels about like it would with one magneto off when it happens.

Is it possible that a magneto could be failing intermittently or am I on the wrong track.

 

[Daleandee]

Were the mags done at overhaul or since that time? If not, it's time for a look at them either way ...

 

[Jdm]

No telling. Absolutely could be mags. I can think of so many things, bad lead or leads vibrating, plugs fouling, lifter sticking, engine mounts, etc. I wouldn’t rule out an intermittent sticking valve. Even with a good compression test you can still have a valve issue. Don’t ask how I know! So many possibilities. Start with the easy stuff first.

 

[Clip4]

Bladders and Water in fuel.

 

[Jdm]

If you suspect water in a tank have your mechanic remove the drain port. Let a few gallons pour out into a clean white bucket. You wouldn’t believe the junk that comes out of some of these old tanks. I’ve started pulling my quick drains yearly during annual.

 

[Magman]

There is an AD ( maybe 84-10-01) that describes the required technique for

checking sumps.

Something like :

Tail low

Shake wing tip up and down 18 inches each way

Check sump


The procedure is supposed to allow water to pass over wrinkles in the bottom

of the bladder and enable a path to the sump.


Magnetos have a Carbon Brush that is somewhat similar to the graphite in a pencil

but about 3x diameter.

It carries high voltage from the Coil to the rotating Distributor Rotor.

Like a pencil it will wear with use.

Issue is the wear particles stay in the mag.

They are excellent conductors and could affect the operation by misfiring.

Just one of the things checked at Mag Servicing Time.

 

[Salty]

You sure it was that long? 4 - 5 seconds of engine running rough at cruise can feel like a very long time.

 

[Tarrow]

Had that on my Arrow. It ended up being the points in the mag.

 

[Dan Thomas]

Had that on my Arrow. It ended up being the points in the mag.

Ignition problems constitute 90% of performance problems. Most people spend hours and dollars fooling with the fuel system, trying to cure a problem caused by weak ignition. Magnetos work far harder than carbs or injection system components do. Their parts wear out exponentially faster.

Water in fuel will cause really serious hiccups, often stopping the engine altogether. Water, due to its surface tension, can stop fuel from passing though the fine screens and metering jets in the system. Inadequate fuel flow caused by a dirty filter or some obstruction should show up on takeoff, where fuel flows are maxed out.

 

[PaulS]

Hmmm, almost 500 hours on those mags. I'd pull them and get them refreshed.

 

[JOhnH]

Hmmm, almost 500 hours on those mags. I'd pull them and get the refreshed.

Agreed. Even if the problem isn't the mags, you are close enough to TBO to go ahead with it anyway. You'd be doing it soon enough anyway, and personally, I think the fewer changes at a time the better.

 

[Let'sgoflying!]

Thanks for coming back with the update; often we don’t hear the outcome.