Spark plugs and rain

[ejensen]

Saturday I flew about an hour in heavy rain with occasional wet snow. It was raining from the time a pulled out of the hanger until 5 minutes before landing. Toward the end of the flight the JPI started showing an 80 degree EGT spread rather than the normal 30. #4 cylinder was showing elevated EGT. I checked the mags and #4 was dead on one mag. The last 10 miles was not raining. After landing, a runup show all back to normal. Next morning runup and flight home (VMC) was completely normal.

I'm assuming something got wet and grounded a plug. If so, is this common? Can anything be done to reduce the chances? This is a Lycoming IO-360 with fine wire plugs.

Eric

 

[Henning]

Saturday I flew about an hour in heavy rain with occasional wet snow. It was raining from the time a pulled out of the hanger until 5 minutes before landing. Toward the end of the flight the JPI started showing an 80 degree EGT spread rather than the normal 30. #4 cylinder was showing elevated EGT. I checked the mags and #4 was dead on one mag. The last 10 miles was not raining. After landing, a runup show all back to normal. Next morning runup and flight home (VMC) was completely normal.

I'm assuming something got wet and grounded a plug. If so, is this common? Can anything be done to reduce the chances? This is a Lycoming IO-360 with fine wire plugs.

Eric

The problem isn't going to be the plugs most likely, not if you lost all cyl on one mag, one or two cyl maybe, but even then not likely because of the way its all designed and goes together. More probable is the mag and p-lead. Maybe somehow enogh water got into the mag to ground out the points, or you have some chaffing somewhere on the lead that when it got wet had enough continuity to ground out. Best thing to do is see if you can find any water tracks on the mags, and with a cold engine, pull the cover off the mag and see if there's been any sign of moisture in there, that's where I would begin as my first suspect is the seal on the mag halves, or possibly even as simple as one of the plugs for the timing mark sight hole is missing, on second though, look there first.

 

[Let'sgoflying!]

Henn I can't see how it could be points or p-lead if only one cylinder quit. My guess is a wire.

 

[ejensen]

Nothing abnormal noticed anywhere else. And I WAS watching close.

 

[Henning]

Henn I can't see how it could be points or p-lead if only one cylinder quit. My guess is a wire.

My bad, miss-read, read "dead on one mag" thinking, incorrectly, one mag dead.
If it's fust one plug, then he probably has a wire going bad at the end with the plug nut and the insulator is no longer insulating fully. If this turns out true, it's time for a new harness, cause it's not the only one going bad.

 

[Tom-D]

Saturday I flew about an hour in heavy rain with occasional wet snow. It was raining from the time a pulled out of the hanger until 5 minutes before landing. Toward the end of the flight the JPI started showing an 80 degree EGT spread rather than the normal 30. #4 cylinder was showing elevated EGT. I checked the mags and #4 was dead on one mag. The last 10 miles was not raining. After landing, a runup show all back to normal. Next morning runup and flight home (VMC) was completely normal.

I'm assuming something got wet and grounded a plug. If so, is this common? Can anything be done to reduce the chances? This is a Lycoming IO-360 with fine wire plugs.

Eric

There are two styles of mag harnesses, one has a 7/8s nut that attaches the plug lead to the plug, and the other has a 3/4 nut.

Which do you have?

The small nut harness does not have a gasket in the connection of the plug to the harness, the large nut harness does have a gasket to stop what you are describing. Water gets in the plug and grounds it out, when it drys out it fires again.

If you want to stop the discrepancy you have, try applying Bendix mag grease to the harness to plug threads

 

[ejensen]

There are two styles of mag harnesses, one has a 7/8s nut that attaches the plug lead to the plug, and the other has a 3/4 nut.

Which do you have?

The small nut harness does not have a gasket in the connection of the plug to the harness, the large nut harness does have a gasket to stop what you are describing. Water gets in the plug and grounds it out, when it drys out it fires again.

If you want to stop the discrepancy you have, try applying Bendix mag grease to the harness to plug threads

I have the 3/4 nut with REM38S plugs. So I don't have the all-weather gasket. I go in for annual next week. We'll check close.

Thanks,

Eric

 

[gismo]

I have the 3/4 nut with REM38S plugs. So I don't have the all-weather gasket. I go in for annual next week. We'll check close.

Another possibility is a distributor problem such as a carbon track and/or crack. In heavy precip there's a lot of moisture flying around the engine compartment and some could get into a magneto and cause you to lose one plug. The wire or the plug end of the wire are more likely the source of the problem though.

 

[Tom-D]

Another possibility is a distributor problem such as a carbon track and/or crack. In heavy precip there's a lot of moisture flying around the engine compartment and some could get into a magneto and cause you to lose one plug. The wire or the plug end of the wire are more likely the source of the problem though.

Carbon tracking causes "Cross fires" which cause, popping, and back fires at full power. The carbon track must be to ground if it were to cause a skip, but in most cases the track is to another plug.

 

[gismo]

Carbon tracking causes "Cross fires" which cause, popping, and back fires at full power. The carbon track must be to ground if it were to cause a skip, but in most cases the track is to another plug.

Makes sense Tom, but I have seen tracks in marine distrubutors that went to ground and killed the spark on one cylinder, so it's at least theoretically possibe in an aircraft magneto. I also have a couple of the original magnetos from the A65 on my Porterfield that would almost always start to run rough if they got wet and this was still a problem after they were "rebuilt". I don't know if it was crossfire or misfire, but it seemed more like misfire. My dad replaced them with newer Bendix ones and the problem went away.

 

[Tom-D]

Makes sense Tom, but I have seen tracks in marine distrubutors that went to ground and killed the spark on one cylinder, so it's at least theoretically possibe in an aircraft magneto. I also have a couple of the original magnetos from the A65 on my Porterfield that would almost always start to run rough if they got wet and this was still a problem after they were "rebuilt". I don't know if it was crossfire or misfire, but it seemed more like misfire. My dad replaced them with newer Bendix ones and the problem went away.

""tracks in marine distrubutors that went to ground""

Yes so have I, but the Distributor block in a Mag is isolated completely away from ground, The track would be all the way around the block and to ground on the other side, And once started it would continue after it was dry. unlike the discrepancy in the first post.

""My dad replaced them with newer Bendix ones and the problem went away""

I'll wager he removed the old Eismann mags and harnesses, and replaced them with SL20's and harnesses. The harness on the old Eismann may or may not have been shielded. The Bendix was better, but still not water proof.

 

[gismo]

""My dad replaced them with newer Bendix ones and the problem went away""

I'll wager he removed the old Eismann mags and harnesses, and replaced them with SL20's and harnesses. The harness on the old Eismann may or may not have been shielded. The Bendix was better, but still not water proof.

I think that's right. I have the old mags somewhere and Eismann sounds familiar. I'm pretty sure they were shielded.