Fouled Plug - What?

Shepherd

Shepherd

Final Approach
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Shepherd
How long do I live with it before starting to take the engine apart?
It fouls while sitting and waiting (and waiting, and waiting) for my turn to go.
Run it up to 2100 rpm and it clears.
Brand new plugs, same problem.
 
Do you lean to the brink of cutoff while waiting ?
Oily plug when removed ?
just one cylinder ?
 
Type aircraft / engine?

BY’s are great in all they are approved in.
 
Is the fouled plug on the bottom? Because I put the BY plugs just in the bottom holes and it completely eliminated my fouling problems.
 
I went through this.
Now I lean right after startup and lean it to a point where advancing the throttle will cause a studder.
They have been clean ever since.
 
Leaning will not stop oil fouling. Ever.

Quick lesson in small-Continental cylinder geometry.

upload_2022-4-5_18-27-42.png

That's looking into the cylinder toward the head. You see the intake valve on the left and the exhaust on the right. You see the upper and lower sparkplug holes. Note how close to the cylinder wall that lower plug hole is. You can see where the cylinder wall is by its reflection of the head in it.

When idling, a cold Continental before takeoff is, like most aircooled engines, not at all hot. The clearances between the cylinder and piston, and the ring gaps, have not yet closed up. At idle the throttle is closed or nearly so, so on every intake stroke the piston is sucking in, or trying to suck in, air and fuel, and the restriction of the throttle plate means that there's lots of vacuum in that cylinder on the intake stroke. That vacuum sucks oil past those big clearances and the rings shove it up toward that lower sparkplug hole, where it accumulates and eventually fills the sparkplug well, drowning the sparky end so it doesn't spark anymore. Remember that the cylinder is horizontal, making it even easier for oil to lay in there until it causes problems.

The UREM37BY plugs (Tempest) or REM37BY (Champion) have extended electrodes that get the spark up out of the goop. Very shallow well in those plugs, and they just can't fill up enough to short the electrodes.

upload_2022-4-5_18-37-22.png

Typical REM38E or 40E on the left. UREM37BY on the right. They are approved for a lot of engines, and I used to install them at plug changes in any engine that could take them. I think they even fire better, getting the spark farther out into the mix. And they resist lead and fuel fouling as well. They turn the foul-prone O-235 into a much better engine.
 
Research into what cylinder is the culprit would be prudent. On some engines having an Exhaust Rocker installed on the Intake side may introduce excess oil into the jug. An added benefit is Valve Guides may wear due to insufficient oil from the Intake Rocker.
 
Magman said:
Type aircraft / engine?

BY’s are great in all they are approved in.
Click to expand...

C-85-8 in my PA-17.
Pulled the old Champs, put in qty 8, Tempest UREM40E.
#1 bottom was wet when I replaced it, and woefully out of specs.
The new plug fouled while idling, waiting for my turn in the conga line.
Revved it, cleared it, and went on my way.
But it is a concern.

Question: Can I just replace the one plug with the 37 equivalent for my plane or do I have to swap them all?
 
Shepherd said:
Question: Can I just replace the one plug with the 37 equivalent for my plane or do I have to swap them all?
Click to expand...
I don't know of any legal reason. Some put them in the bottom locations to cure fouling.
 
PaulMillner said:
Yes, or put a fine wire in just that location…
Click to expand...
The fine-wire plug still has a deep well and no electrode protrusion to raise the spark out of the oil. The oil will still drown it.

upload_2022-4-6_17-30-0.jpeg
 
Dan Thomas said:
The fine-wire plug still has a deep well and no electrode protrusion to raise the spark out of the oil. The oil will still drown it.
Click to expand...
Hi Dan,

My experience has been that the higher electric field density of the fine wire allows it to fire when massive electrodes won't. It's interesting that that your experience has been otherwise. Perhaps you needed a hotter heat range to keep those plugs clean?

My last plane had an O360 that was pumping oil pretty good on #2 and #3... changing just those bottom plugs to fine wires greatly improved runup success.

Paul
 
PaulMillner said:
Hi Dan,

My experience has been that the higher electric field density of the fine wire allows it to fire when massive electrodes won't. It's interesting that that your experience has been otherwise. Perhaps you needed a hotter heat range to keep those plugs clean?

My last plane had an O360 that was pumping oil pretty good on #2 and #3... changing just those bottom plugs to fine wires greatly improved runup success.

Paul
Click to expand...

I’m not Dan, but think about the plug. No threads should protrude into the combustion chamber (otherwise pre-ignition from sharp thread hotspot). So the plug thread face sits flush. Now fill the plug up with oil. Spark probably doesn’t jump the electrodes when they are under oil. Dan is saying they need to be raised up and out of oil.
 
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