Rough right mag..

[ccsmith51]

I have an O-360-A1F6D on my RV-4. Yesterday when I did my 1700 RPM run up it was VERY rough on the right mag (Bendix dual mag), for a number of attempts. Left mag was fine. I ran it up to 2000 RPM for a moment, and then all was fine. I flew for 0.7 hours and tried both mags at different RPM's (2000 - 2700) and altitudes (1500 - 4500) and didn't have a drop of more than 75 RPM. Flight before was six days earlier, with no problems.

It was a bit chilly (for FL), about 55°. The engine took about 8 seconds of cranking to catch (accelerator pump carb, no primer system). When it caught it ran fine. I leaned once it was started.

Last summer on my trip west I was having a right mag issue with what I would call "backfiring" on run up. I was told that it was not backfiring, but in fact a plug that wasn't always firing and and fuel was igniting in the exhaust pipe. We checked the plugs and replaced the right rear lower plug, and that cured that problem. That was about 20 hours ago.

About three hours ago I replaced all eight spark plugs with new RHM40E's. I have had no issues until yesterday. I can't believe that with only three hours on the plugs, and leaning when taxiing, I could have lead fouling, but I don't know...

My last condition inspection was 3/12, and since the engine logs are incomplete, and since it is a single case dual mag, I decided to have it overhauled. The shop said it looked like the mag had very little time on it. So, it was overhauled (new points and bushings, condensers were fine) and yellow tagged 9 months ago.

I'm trying to give as much background as possible, so when I ask the question y'all will know what I know...

What could cause a very rough mag under the circumstances above?

Thanks,

[airtractor8]

Just because the plugs only have 3hrs on them doesn't mean one of them is not bad. Did one get dropped when installing? You may have just got a dud.
It also could be a bad plug lead. Just touching or handling the end of the lead when installing new plugs can transfer salts from sweaty hands and cause intermittent miss fires. I always spray them with electric contact cleaner before pushing them into the plugs. Did the cap and leads get replaced at overhaul?

[ccsmith51]

Originally Posted by airtractor8

Just because the plugs only have 3hrs on them doesn't mean one of them is not bad. Did one get dropped when installing? You may have just got a dud. It also could be a bad plug lead. Did the cap and leads get replaced at overhaul?

Thanks for the input.

Dropped, not by me, been there, done that... I was very careful not to drop any plugs... I also was very careful about not touching any of the parts with my hands, except for the exterior of the plugs and the exterior of the lead cap.

On the other hand, I bought these from a VAFer that had installed them and realized that he heeded REM's, not RHM's. Perhaps he might have dropped one.

The cap leads did not get replaced. That will be a consideration if the issue continues. I hope to fly tomorrow and will report.

[mahlon_r]

New plugs can foul too in the right conditions. Is your oil consumption stable or does it vary flight to flight or series of flights to series of flights? Were any of the old plugs wet with oil when you removed them to be replaced?
Maybe a cylinder that is pushing a bit of oil, fouling that plug at idle and shut down, that needs time to burn off, after restart and sitting, before the plug is clean enough to fire OK will fire OK.
Good Luck and Merry Christmas,
Mahlon

[RV10inOz]

Time to check everything, from magneto, to mag timing, especially that mag to leads, but first of all do you have an engine monitor? And do you lean a bit or a lot on the ground?

If you have a data file, send it to me.

Merry Christmas!

[ccsmith51]

Thanks again for the input.

I heard from the individual from whom I purchased the plugs. He confirmed that they were never dropped, installed but never run, as the lead caps would not attach because of the wrong size.

Six of the eight plugs I removed were dark brownish and clean. Two, I believe from the same cylinder, were black, but not wet with oil. I will check them again this afternoon, they are on the bench at the hangar and are tagged so I know which plug came from where.

I do not have an engine monitor, so have no data to send. I lean aggressively on the ground, only going rich at run up and departure. My airport is basically sea level.

I'll report how it goes today.

[ccsmith51]

It was quite windy today, so all I did was spend 15 minutes in the pattern shooting three T&G's.

Started up fine. Three full pumps on the throttle and it fired on the fifth blade. I leaned until rough, richened a bit, then slow taxied the 4,000' to the end of the runway. I idled at 1,000 RPM and lean until the OT came to 100°. I went full rich and up to 1700 RPM. About 75 RPM drop on each mag, no problems with the right one this time.

When it happened last time it was six days between flights. This time only two. I don't know if that means anything at all...

So, I have no new info, and will just have to take note of conditions should it happen again, and report and ask for more help.

Merry Christmas to all!

[Ant CB]

My plugs foul when I taxi a long time, especially when I taxi down hill at idle. If you can clear the plugs easily with a run up it sounds reasonably normal.

[ccsmith51]

I flew two flights yesterday. Aggressively lean while taxi, no problems at all on run up. So, it appears that whatever it was, it was an anomaly and has cleared. Thanks for all the input and help.

[RV10inOz]

Do not use full rich for your mag check either.

Advance the throttle and then slightly enrich the mixture until you can sustain the 1700 RPM. You may get a slightly higher drop, but the LOP mixture is a far better test.

And get yourself an EMS so you can see what is happening on each cylinder. They are cheap these days.