Troubleshooting roughness

RV-7A owner here, with new onset of interesting issue. Setup: 2012 RV-7A, 200 hrs, IO360M1, Whirlwind constant speed prop, 2x PMags, auto spark plugs, G430W, 2x GRT AHRS. Recent replacement of failed #2 alternator.

At the tail end of a cross-country trip last night developed significant engine roughness, much more so than that with some past spark plug fouling. Engine tried to quit on descent with any power setting change. RPM indication jumped around erratically with sputtering at normal leaning levels. Landed without incident. This morning I tried clearing the plugs, then had normal run-up and mag check. Taxiing showed a marked left turn tendency and a significant left tilt of the horizon on the PFD.Horizon returned to normal after takeoff. Engine sounded good, so I took off for a loop in the pattern. Very rough with misfires above 2000 ft, so turned around and landed. Not losing any oil.

Thoughts? It's time for an oil change, and I'll change out the plugs.

Thx, Andy

Sticking exhaust valve?

Bevan

Welcome to VAF!

Your second paragraph hints of multiple symptoms you are experiencing from multiple systems on the aircraft. Unfortunately there is very little diagnostic clues offered that we can use to suggest a course of action. We would be randomly shooting at it in the dark and would only add confusion to your diagnostic path.

Is there someone local that can look at the plane and map out the problem areas and diagnose each system involved?

Much better that randomly (& safer than) shooting in the dark!

I agree it is hard to diagnose remotely, but there are basic things you can check. I like to start with the easy things to check, then proceed with the harder or more expensive things. Also some diagnosis when the problem occurs - does it happen on both fuel tanks? On both left and right ignition? What do the CHT and EGT do? (up or down?; all cylinders or just one?). The quote "significant engine roughness" isn't a lot to go on.

Here's where I would start -

Induction leak? Verify everything is sealed properly. Things can change as the engine gets warmer, so preferably check with the engine warm. While you are at it, verify the air box is secure and not blocked.

Loose spark plug wire? Verify every is firmly attached at both ends of every wire.

Fuel lines clean? Gascolator, carburetor finger screen, fuel injector nozzles, etc. all free of debris?

Fuel vent lines clear? A blocked vent line would take a little time to create enough vacuum to starve the engine.

Timing? Verify your pmags are still timed properly.

Bad spark plug? Automotive plugs are relatively cheap. Replacing them is pretty easy.

Stuck valve? I had one of these, so definitely a possibility, but I'd do the easy stuff first!

Good luck! I hope you find it quickly... and safely.

Timing? Verify your pmags are still timed properly.

Click to expand...

This would be the first thing I would check.

Once flew a Piper Arrow that had a bit of roughness on start that then smoothed out, so all three of us pilots on board figured "musta been a bit of water in the line or something, seems fine now"...did a long, full-power static run-up, all good. Took off, all good, until *right* before entering the clouds at about 700' AGL, then all **** broke loose...running rougher than s**t, immediate return to field, PF made an excellent landing and we shut down, changed our underwear, and grounded the plane.

BOTH mags had been timed but not had the hold-down bolts tightened down, allowing them to rotate at will. How it ran at all is a mystery to me to this day.

So, yeah, I'd check for timing issues first, or some other problem with the mags (I know nothing about P-mags, though).

+1

If you have EI, that is where I would start. Especially when higher altitudes or lower MAP starts the roughness. Dirty oil will not cause performance issues

Larry

pmags have a long history of failures... Good place to start looking

Welcome to VAF

Andy, welcome aboard the good ship VAF.

The first thing that came to my mind is ignition timing, second is fuel delivery.

Good luck with finding this gremlin.

Roughness, tried to quit are a little vague but take it serious. As other suggest:

Ignition? Unlikely both would fail but one misfiring badly (not just shutting down) can all but kill power much less make engine run rough. Check you plugs of course.

Fuel? Kind of obvious but as engine gets super lean with fuel issue (flow or contamination), roughness and loss of power is expected.

Valves? Compression leak down test... One bad cylinder can make the engine shake and cause loss of power.

Upload your engine analyzer data to SavvyAviation and ask for an analysis.

If your engine is an angle valve or high HP/CR parallel valve, you should not be using the standard jumper in settings. You'll need to set timing at 2.4 teeth (~5?) after TDC for correct base timing.