Rough Running and strange things with Mags

Hi John,

Most likely too much idling. If you are going to check the plugs run it up to 3500-4000 rpm for a few minutes or go fly to burn off the dry black soot. When you idle too long the soot can accumulate because of uneven air flow and not peak firing from the bottom plugs vs the top. This always goes away when you're at higher rpms like in cruise. Dry soot from idling is no big deal and very common for most everyone. Too wide a plug gap can add to it. NGK's out of the box are gapped at .030. Our gap should be .024 - .028. according to SI-912-027 dated March 2017, but in the older (not new one) Line Maint. manual section 12-20-00 page 64 for the NGK's it specs out to .023 - .027. I tend to use .025. It's in the middle. I don't use .027 because it just gets wider over time. Your choice. In the new Line Maint. manual they only have the new style Rotax plug gaps that are gapped where they want it right out of the box. The new style Rotax plugs are a wider gap and do not need adjustment out of the box.

If plugs were wet slimy black plugs then that would be different.

Thanks Roger,

I did just that.  Warmed her up and ran it at 4000rpm for a few minutes, and then dropped to idle and then instantly shut her down.  Plugs came out grey - no more black soot.  There was also no sign of missing on the run ups.  Both mags performed exactly the same.

I will look to adjust all the plug gaps etc and do the Carb sync.  I think it just needs that done, and it will be fine.   I am thinking that it missfired because of a long taxi and sooted up those plugs so bad, that it could not cope.

I will keep you posted.

Cheers

J

FWIW, I've chased vibration issues for a while now, but I did find something very similar to your apparent ignition issue at 2500.  Mine does the same thing with A being rough, and B being fairly smooth.  It's a night and day difference, and completely repeatable, so I was convinced it was an ignition issue.  Swapping the coil triggers between A to B changed the problem to B, so I was convinced it was something with the coils, wires, or plug.  I spent LOTS of time and money trying to fix that, but nothing I did ever made any difference. Both ignitions run fine at normal cruise power, and idle, and I found other RPMs where B seems to be rough compared to A. 

Eventually I decided that this must be related to how each cylinder, at a given RPM behaves between the upper and lower plug.  The intakes are far from perfect, and I'm guessing at certain RPMs any given cylinder may be more rich or lean than the others.  I'm further guessing that under those circumstances, the upper or lower plug may work noticeably better than the other.  That's a lot of guessing, but it's the only way I've ever been able to try to explain it.  

Rusty

Thanks Rusty.

I have since flown a few times and it's behaving as it always has.  Very smooth in the air.  So not concerned at this point.

Carb sync coming soon!

This thread is timely. I have recently had a very similar experience in my RV-12. I went out and did my run-up one day and got about a 500 RPM drop and very rough running on "A". I went back, did a little research (nice Lockwood video about this). Next time I went out mag check was fine. Flew about 5 hours, then it cropped up again, 500 RPM drop on "A". "A" was also rough at other power settings.  I did the deal with the choke, enriching a bit to look for roughness change. I got no appreciable roughness change with various choke settings. I shutdown and swapped the vertical connector pair (the harness from the magneto and ignition switches). Unfortunately when I started again things ran a lot smoother. It seemed like the roughness and maybe 200 RPM drop might have moved to "B" but it wasn't really definitive enough to say for sure.

So now I have an intermittent roughness. While waiting for it to crop up again I'd like to go flying. Does anyone know if I can leave the magneto harness swapped for flight or do I need to swap it back? The heavy maintenance manual warns not to do this for a 914 but it doesn't mention any restriction for my 912ULS.

BTW, I had new floats installed over the summer and had carbs redone by LEAF about  20 hours ago. Everything have been running fine up until now.

Bob