Help / Brain Cells Request Continental O-300 Problem

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Victor Bravo

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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?
 

fretman_2

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My annual happened this weekend and I have an O-300D in my 172. Plugs were dirty...I also use MMO in the fuel. Two of the plugs had a little oil on them.
My mechanic had a little device that fit in his vice and was basically a mini-blast cabinet. He put the end of the plug in the orifice, turned on the air, and out came a clean plug end.

Cleaning your plugs could be the difference!! They also may need to be re-gapped. If the center is eroded too badly, they'll need to be replaced.

My mechanic also stressed the importance of leaning while idling and taxiing.
 

Aviacs

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Since the engine is open anyway (cowl off, plugs out)....
take the top plugs out of all the cylinders, be sure the mags, gas are off, get your maglite, and observe all the valves as you pull the prop through. (one jug at a time, obviously)

smt <---- time behind small continentals including (previous) GO300 ownership
 

GeeZee

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Do the plugs happen to be older Champions? They are known junk. Champion finally admitted same and redesigned the internal resistor. Check resistance value for all the plugs. I forget what value range is acceptable but I think anything over 5K ohms is no good.
you probably should just pull that engine and use the jugs to make 3 O-100 engines ;)
 

Turd Ferguson

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Not letting the engine idle is more effective at preventing lead buildup than leaning. Although contrary to historic practices, start the engine, warm it up at no less than 1500 rpm and taxi at no less than 1500 rpm. Much less lead fouling but the old timers around the airport are going to jump up and down and scream.
 

Victor Bravo

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I just got off the phone with Spruce and spent over three hundred Mot!(#&*[email protected]*(#$?&? dollars on 12 new Tempest plugs. May have to cancel my membership with Strippers 'R' Us for a couple of months because of this unwanted expense!

The internet forums (Pilots of America, Mooney, one or two others) were strongly supportive of Tempest over Champion.

I may or may not have the old pugs (Champions that are a minimum of 12 years old, on the airplane when I bought it) cleaned and checked.

But I am STILL interested in the collective wisdom here to help figure out why two entire cylinders with four plugs total stopped firing all at once.
 
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Pops

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And you use mostly 100LL, right ?

Should be glad you don't have a 6 cyl Franklin engine. Yours are 1/2 price.
 
Last edited:

Dan Thomas

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An engine like that should use the UREM37BY plugs, and I think they're approved for the O-300. They have extended electrodes that are really resistant to fouling.

1633383927144.png

Champion REM37BY at the top. UREM37BY is Tempest's version, and a better plug. I used those Tempests in all our engines that were approved for them, with great success, especially in the O-235 in the Citabria 7ECA. Those engines are famous for running cool and fouling sparkplugs. No lost flights due to fouling with the UREM37BYs, though.

Besides that, the O-200 and O-300 have cylinder heads that have the bottom plugs really low in the head so that oil in the cylinder runs into the bottom plugs. Idling when cold will let plenty of oil past the rings in those engines, sometimes even when hot. It's ridiculous. Such leaky rings. It's the low manifold pressure that sucks the oil past the rings at idle.

And yes, a stuck valve is a possibility.
 

TFF

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You putting the lead scavenger in? I am one if something fouls, I pull the plugs. No need to waste time except to clean. If it’s here a problem after clean, then time to start more deep thoughts. If the longer reach plugs are 37s I believe are ok for your engine, put a set on the bottom positions. The bottom are easy to foul. You probably barely cleared the bottom and then you fouled the top if not others on the verge. Clean the plugs then call back.
 

Victor Bravo

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Thank you all for the ongoing assist... Dan you were an hour too late I had already ordered a box of 12 UREM 40E Tempests. Next time I change plugs I will try the extended versions in your photo.

All others, how can I do all my startup, "idle", taxi, etc. at 1500 RPM without burning out my brake pads? When I come in to land, I'm at pretty low manifold pressure. I understand and agree about the low MP sucking oil past the rings, but how can I manage that without wearing out the other parts of the airplane?

TFF are you talking about the TCP stuff? I remember using that on a Lycoming engine once on recommendation from someone, but I never used it in the brand C motors.
 

TFF

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Yes on the TCP. A friend who flew a traffic watch plane years ago put it in the old 172 that did the job. We put it in the tanks for a friend’s Rotax that feeds on only 100LL. I also thought too that you might give the wings a good shake and agitate the fuel MMO mix in case it has sat a while and settled some. Usually there is not that much to cause those kinds of problems though.
 

Map

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If those plugs have been in there for more than 12 years (how many hours on them?) they are way past due for replacement. My experience with spark plugs is that they degrade continuously, and in some engines only last 70 hours, in others a few 100, before performance suffers, regardless of cleaning and regapping. Get new ones in there and also check your ignition harness (chafing, resistance etc.)
 
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A spark plug rep at an IA refresher told us, to avoid lead fouling, you need to shut down your engines properly. The procedure is:
run the engine at 1200 rpm​
SLOWLY lean the mixture, as in it should take about 20 to 30 seconds, pulling it back until she quits.​
What you’re doing is super heating the inside of the cylinder, which vaporizes any balls or turds of lead and blows them out the exhaust valve, giving the atmosphere that sweet, burned avgas smell.
In my experience, O-300s are happier running on mogas (with a bit of MMO) instead of 100LL. Unless the old bird will be sitting a long time, like over winter, or you feel the valves need some lube (which MMO should do for you, so that’s not much of an excuse), or you’re flying cross country and the blue fuel is the only thing available, the lower octane mogas is worth the hassle of buying it at one of those few gas stations that carry it, hauling it to the hangar, scrambling up a ladder with the 5 gallon jug, and trying to get more into the wing than all over it.
 
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