Mag check rough

bflynn

bflynn

Final Approach
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Brian Flynn
Long trip last week with 4 stops. On initial startup and after a bladder stop, the left mags ran rough during the mag check. I leaned and ran full for about 30 seconds give or take, tried again and it performed well.

thinking about this later, I know I leaned after start. The second time I was well leaned the trip before, leaned at start and I did the run up pretty quickly after starting. I don’t think there was time to foul the plugs.

it seems the engine was trying to tell me something, but I don’t know what that might be.
 
How aggressively are you leaning on the ground? Just enough to keep it running? How about when you’re in the air? Also, the notion to go full rich for landing isn’t necessary either, the only purpose that serves is for go around. Could be there was a plug or two right on the verge of fouling and the right circumstances were present to reveal itself during that mag check. Could also be a wire intermittently misbehaving or any number of other things.
 
Maybe the engine is telling you it’s easier to burn off deposits in a HOT engine rather than a cold one?

30 second post flight run-up at 2000 rpm will heat the combustion chamber up to permit scavenging. Ideal time to check mags too.

Lycoming suggests this per SI and it worked for USAF too. You really don’t want to discover a mag drop with a load of pad on board.
 
It's telling you that you should have a multi-point engine analyzer.

If not, start resistance testing spark plugs and test everything, moving back to the mag, until you find something that isn't right. Without an engine analyzer, you may be testing for hours and replacing things that aren't broken.

This is how such devices pay for themselves.
 
Sparkplugs foul up. Carbon builds up in the plug well and eventually it fills the gap between the electrodes and shorts the plug. Burning that off by running the RPM up and leaning will only remove the bit between the electrodes, if it removes anything at all. It sure isn't going to clean the carbon out of the plug well. Only removal, cleaning, gapping and testing is going to fix that. Don't expect to get 200 hours between cleanings. Sometimes you only get 100. In the O-235s I didn't even get 50 hours until I installed UREM37BY plugs in them.

That plug well normally fills up either because the pilot doesn't actively lean the engine most of the time. Or else it's sucking oil past the rings or intake valve seals and the oil is burning and fouling the plugs. That doesn't take long at all. The small Continentals have their lower plugs right at the bottom of the cylinder bore, and oil flows into those plugs during idle if the rings are bad enough. Shorts them out. In that case, the runup and leaning can sometimes clear them.

Many engines are approved to take the Tempest UREM37BY plugs (Champion REM37BY). Their electrodes are extended and even if their shallow well fills up with carbon, it can't reach the spark gap to short it. It would take some doing to foul one of those plugs to failure. Their electrodes are right out in the flame and are kept clean.

upload_2022-9-10_16-28-6.jpeg

Top one is REM37BY. Bottom one is an REM38E or 40E.

Another view, but the one on the right is a long-reach (long thread) plug, not one you'd replace with the 37BY. Same well as a short-reach plug, though. Easy to fill up with carbon or oil.

upload_2022-9-10_16-29-36.jpeg

Tempest and Champion have application charts for various engines.
 
172S, injected. Lycoming IO-360-??

The oil is changed and plugs inspected and gapped every 50 hours. I'm wondering if the engine is leaking oil, which is fouling the lower plug and indicative of a coming valve/cylinder failure. Or maybe an injector problem that's fouling the lower plugs. The left mag would imply it's one of the lower plugs in the left hand cylinders.

The RPMs run lower than other 172s I've flown, ~1850 to maintain altitude at 80kt vs 2000. If I leave it at 1500 on final at 65 kts with flaps, it will float long.

Hopefully will find out - I've let the mechanic know the symptoms and we'll see if anything comes of it.
 
bflynn said:
The RPMs run lower than other 172s I've flown, ~1850 to maintain altitude at 80kt vs 2000. If I leave it at 1500 on final at 65 kts with flaps, it will float long.
Click to expand...
I believe the green arc starts at 2100RPM in a C172S.
 
How long have you owned it? Long enough to know when something is not right? Trust your intuition, you know your plane best.

If you haven’t owned it long enough for more than one plug change or cleaning, as Dan said above, build-up happens over a long period. The more aggressively you lean on the ground, the longer you’ll go without significant buildup. You might get some buildup while airborne in various flight phases, and that just adds to it.

Lean on the ground until it sounds like a 455 Buick with a “window rattler” cam (nick-name for the old school KB 107 cam 1960s tech) :D

 
bflynn said:
172S, injected. Lycoming IO-360-??

The oil is changed and plugs inspected and gapped every 50 hours. I'm wondering if the engine is leaking oil, which is fouling the lower plug and indicative of a coming valve/cylinder failure. Or maybe an injector problem that's fouling the lower plugs. The left mag would imply it's one of the lower plugs in the left hand cylinders.

The RPMs run lower than other 172s I've flown, ~1850 to maintain altitude at 80kt vs 2000. If I leave it at 1500 on final at 65 kts with flaps, it will float long.

Hopefully will find out - I've let the mechanic know the symptoms and we'll see if anything comes of it.
Click to expand...

Are you sure it's an S model?
 
asicer said:
I believe the green arc starts at 2100RPM in a C172S.
Click to expand...
Likely. Can't remember. But running an engine at low RPM for cruise is a good way to foul plugs, too. They don't run hot enough to keep the carbon burned off.
 
dmspilot said:
Are you sure it's an S model?
Click to expand...

not pulling the cowling, but pretty sure. I didn’t say cruise, i said 80kts, which is pattern speed.


upload_2022-9-11_8-2-22.jpeg
 
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bflynn said:
not pulling the cowling, but pretty sure. I didn’t say cruise, i said 80kts, which is pattern speed.


View attachment 110443
Click to expand...

I know what you wrote. I asked because the RPM to airspeed ratio sounds like an R model. Removing the cowling won't help as they have the same engine.
 
Roughness that "burns off" after a lean run up is either carbon (likely from not aggressively leaning in flight or during taxi) or oil fouling (from a cylinder starting to pump oil past the oil control rings). Inspecting the affected plugs will usually tell the tale. If all plugs are squeaky clean, then mag issues might creep into the equation. Sometimes a bad coil can be intermittent.
 
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