Thorp Air Command - T18.net

It’s happened half a dozen times or so now that I had to return to the hangar after a runup to find oil-fouled spark plugs. Always the bottom plug on #2 cylinder, and occasionally bottom #4. I believe rings there never set well after overhaul. Cylinders were actually re-honed/new rings a second time for the same reason. Just couldn’t get break-in with full power at altitude. Our field is already at 6200 ft and I typically fly 8500/9500. Oil consumption is maybe a quart of Aeroshell 15W/50 over 8 hrs. But it was straight mineral oil for first 50. Overhaul on all 4 included inspection of valve guides and a few replacements. Since overhaul I’ve flown about 250 hrs and still get compression of 78/80 on all 4 (measured hot).

So could I expect break-in to continue and improve, or could oil fouling get worse? Can it become severe and cause failure in flight, or is it a slow tell-tale degradation? It’s a big job to pull the cylinder and rework it again, and no guarantee it will break-in any better than previously.

I heard that fine wire iridium plugs are less prone to fouling and may be a simple solution and can be mixed interchangeably with others. Since top plug isn’t oil-fouled maybe this is just a simple common nuisance to live with? If they work, just wouldn’t want iridium plugs to mask something serious.

Whadya think?
Les

What plugs are you using? 37by's and aggressive leaning while taxiing solved that problem for me.

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Bob Highley
N711SH
SN 835
KLAL

Yep...like Bob said . You also may want to carry a little extra power during taxi and stay away from the idle stop .

RB

Hi Les,
Bummer. Oil fouling is definitely a ring issue. I agree that it most likely did not seat well. Your DA is too high for full power break in that is required. In fact, Lycoming does not recommend above 6500’ DA during break in.
Anyway, it will not get worse. It also will not get better. Leaning and higher RPM should help. It should not happen in the air. You are well above the Lycoming minimum hours. It will just be a pain.
Cubes

Thanks guys. I figured leaning reduces carbon buildup from fuel but always do it anyway, for taxi too. I’ll try iridium plugs and just keep going. Getting a lot nicer weather these days. Love it.

Les

I would consider going back to the straight mineral oil for another 50 hrs. Can't hurt.

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Bob Highley
N711SH
SN 835
KLAL

Hi Bob,
Question - how does going back to mineral oil help? Does it help seat the rings, or what?
John

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John Twigg N12055 SN.79 Pittsburgh, PA (KFWQ)

Yes, to reseat the rings. The alternative is to remove the cylinders and break the glaze with a hone.

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Bob Highley
N711SH
SN 835
KLAL

My two cents. At 250 hours, the rings are as seated as they will be for the life of the motor. The only way to reseat is to remove the cylinder, re-ring, and rehone.
Suggestions to prevent oil fouling have already been stated....increase rpm, lean like crazy, try to keep the cylinder temperature and pressure up a bit during taxi.
I have read fine wire plugs will help prevent fouling, but I have no personal experience with them.
Cubes

Yes, Iridium fine wire plugs made the difference. They seem unaffected by oil fouling. We’re doin well now. Good flying weather this week. Gotta go...

Les