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Help / Brain Cells Request Continental O-300 Problem

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Victor Bravo

Well-Known Member
Supporting Member
Joined
Jul 30, 2014
Messages
13,447
Location
KWHP, Los Angeles CA, USA
Slightly OT because it's bolted to a Cessna, but I had an issue with my O-300 Saturday morning, and I know we have some highly experienced people here:

I've been running the engine as lean as possible on the ground and leaning ASAP in the air, because I use Marvel Oil in the gas, and the plugs can get loaded up... leading to a mag drop on runup.

Saturday morning I taxied out to the runup area, and did my mag check. Had more of a drop than is desirable (150), so I leaned it way out and rana it up for a minute to burn out the oil or soot. After a minute, did the mag check again and the mag drop was down to 50, so more or less good to go. But the oil hadn't warmed up to the minimum takeoff yet, so I just sat there with it at 1500 for another minute.

While I was sitting there, I lost one cylinder - it got noticeably rough and the RPM went down. So I figured I needed to re-lean it and burn some more crap out. But that did nothing. It sounded like it was running on five jugs, regardless of switching mags, regardless of mixture, regardless of throttle setting.

So I aborted the flight and took it back to the hangar. Removed the cowlings, didn't see anything obvious externally. No broken plug wires, no cracked cylinders, etc.

I made a couple of calls to friends who have screwed with this stuff more than I. One person said that yes in fact he did actually once have TWO fouled spark plugs in the same cylinder, and that would explain why the cylinder wasn't firing on either mag. So he says start the engine for ten seconds, then shut it down, and see which exhaust pipe is not hot, and that will identify which cylinder is the culprit.

So I closed the hangar, drove 2 !)#*$ hours out to the desert to do my BFR flight in an ancient 2-33 glider and get accepted into the local soaring club (hooray!), and later that night I came back to do the engine test. Started the engine, ran it for 30 seconds. It was running rougher than it did in the morning. Oh S**T, probably swallowed a valve, or broke a rocker, or maybe there's a connecting rod broke in half. When I shut it down, I had someone run over and touch the exhaust pipes. He says that the front TWO cylinders on the left side (#4 and #6) are cold. WTF???

So I'm thinking in order for that to happen from spark plug fouling, FOUR spark plugs would have to fail or short out or crack all at once. Or BOTH magnetos would have to have a partial failure of the distributor for those two same cylinders. Not very likely.

So I took the spark plugs out. Most of the lower plugs were lead fouled (the little shiny balls), and some of them had what looked like little tan colored mouse turds in them. Not black, light tan, and not spherical... elongated like little mouse turds. The upper plugs seemed better, no balls or turds, a couple were a little sooty but not horrible.

So the big question is... has anybody had it happen that bad plugs cause two cylinders to stop firing at the same time? The problem I had can easily be explained by plug fouling if it only happened on one mag, and switching to the other mag brought the RPM back up and smoother. But switching mags didn't affect this problem - it ran on five cylinders (the first run-up) on both mags and regardless of mixture or throttle.

Other than bad spark plugs (I'm ordering new ones already), what else could cause these symptoms?
 
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