Rough running engine - benefit of engine monitor in troubleshooting

I just upgraded a panel 9n an RV-6A with an AFS5600. I noticed when flying it prior to the upgrade that there was a significant rpm drop when turning off the electronic ignition and there?s was a little roughness. Otherwise it ran beautifully and flew great. Upon testing the new installation I noticed the same thing, except I also saw that 2 egt?s dropped significantly when going mag only. At idle it ran quite rough, but not too bad at runup rpm. The short version is that the plug wires from the mag to cylinders 2 and 4 were backwards, and had obviously been like that for a very long time. Engine has been running on only 6 plugs for years, and possibly a lot of years. It has passed Condition inspections for harness doesn?t have te cylinder number stamped on it like most do, so it was never caught. One moral of the story is to understand the benefit of having 4 cht and 4 EGT probes. This could have been a huge amount of troubleshooting without that indication

Another example of the benefit of indications on all cylinders is that a friend with a 206 recently spent $11,000 in labor and parts while trying to fix an engine that started running rough at full power. Turns out that one of the mufflers started coming apart and clogged the exhaust pipe. A full engine monitor would have shown a similar EGT drop at full power on the cylinders on that side of the engine, which would likely lead directly to the exhaust being on the short list of things to inspect.

Jesse

My signature line below is founded on exactly your post!

Occasional stumble

What would I look for on the EMS to help troubleshoot an occasional (not every flight) short duration stumble. ECI 0-360 (new in 2008), slick mags, 490 hours TT.

Plugs look fine, compression's good, oil analysis good, mag drop good, start, run-up, idle, take-off power all good. Use the big pull leaning method and the last time the stumble happened I enriched the mixture and had no further instances that flight. Cannot seem to make the problem repeatable.

Read a bunch of threads including about the ACS switch contacts and sticky exhaust valve but would like to see if the EMS can offer some clues if I know what to be looking for.
Thanks
Figs

I would download the data and see what egts (and other data points) do when it stumbles. If an EGT drops or spikes measurably, then you at least know what cylinder to look closer at. Sticky exhaust valves can be tricky, but it?s not a huge job to rheem the guide. It could also be a plug or plug wire. First narrow down the cylinder(s) and go from there.

+1,000,000.


We diagnosed a dirty plug the first time it happened because the egt went to zero during mag test at run-up.

Diagnosed morning sickness in a 172 on the first startup with symptoms, just the same way.