flyingcheesehead
Touchdown! Greaser!
Hi all,
Looking for some ideas here.
We have a Mooney Ovation with an IO-550-G (280hp, 2500 RPM max) that used to run turbine-smooth, but over time has developed a slight roughness that I can't figure out (and isn't bad enough to throw a ton of money at yet). We also have an old-school Insight GEM 602 engine monitor which could be helpful, but isn't particularly high resolution.
It's worst when the engine is cold. Right after starting up at idle is when it's the worst. Anything below 1000 RPM doesn't work well at all. It'll still be slightly rough all the way up to cruise power. By the end of a flight, it's slightly better at the high power settings and much better at idle, but it never smooths out to the way it used to be.
There is no abnormal EGT or CHT indication at any power setting that points to a particular cylinder being the source of the roughness.
Mag checks: If we do it at 1000 RPM when it's cold, the engine will quit. I've done plenty of normal runup mag checks and one lean-of-peak check at cruise though I tried to keep that short because my wife was aboard and didn't like it. On a 2000 RPM runup mag check, slightly below 1000 MSL and full rich, neither mag shows more roughness than the other, and all cylinders show a rise in EGT on a single mag, exactly as should be expected and as it has always occurred. Likewise, on the LOP cruise check, neither mag was rougher than the other. I didn't get enough of a chance to play with it and see what the engine monitor indicated for more than a couple seconds, but I didn't see any indication that it was a problem with a single mag or a single cylinder.
My normal leaning procedure at 6,000 feet is to simply lean the engine out until I see all six cylinders have peaked and come down, so I'm probably around 50ºF lean of peak* on the richest cylinder. Up higher at, say, 9,000 I'll only go until maybe the first three have dropped, and above 12,000 I'll run it in the vicinity of peak, not necessarily having any of them drop but seeing the rise slow down as I lean.
* The Insight GEM 602 does not have a specific degree value per bar, but it's supposed to peak around the asterisk and the lowest indication is supposed to be around 800ºF EGT. There's about 15-16 bars in between, so should be about 50ºF per bar on the EGT side. CHT is 25ºF per bar.
One final bit: If I richen up the mixture in cruise, it does smooth out some. Normally I see right about the fuel flow I should for 65% power LOP (12.2 gph * 14.9 = 181.78/280 = 64.9%), but at that flow I get the roughness, whereas if I richen it up to about 13 gph it mostly goes away. That's about the most solid clue I have.
Now, taking it down to the basics: Fuel, air, and spark.
1) Air. I think this is least likely. The Mooney has a balanced induction, nothing has changed here, it hasn't been touched except for taking it apart at annual (~5 months ago) to get at the spark plugs. In addition, I would expect that any blockage or restriction in the induction would cause more roughness at higher RPM, not less. Right?
2) Fuel: I suppose this could be a partially clogged injector, but again I would assume it'd be worse at higher power because there'd likely be a greater difference between fuel flows to each cylinder there. I would also expect this to be visible on the engine monitor. FWIW, the engine driven fuel pump was overhauled at annual ~5 months ago.
3) Spark: I'm guessing this is the culprit, but it's not very obvious. Mags were overhauled at the last annual (5 months ago). Nothing obviously rougher on one mag than the other at any power setting, and nothing obvious that I can see appearing on the engine monitor either. It's not coming from a completely absent spark, but I'm thinking it may be coming from a weak spark.
Any ideas?
Looking for some ideas here.
We have a Mooney Ovation with an IO-550-G (280hp, 2500 RPM max) that used to run turbine-smooth, but over time has developed a slight roughness that I can't figure out (and isn't bad enough to throw a ton of money at yet). We also have an old-school Insight GEM 602 engine monitor which could be helpful, but isn't particularly high resolution.
It's worst when the engine is cold. Right after starting up at idle is when it's the worst. Anything below 1000 RPM doesn't work well at all. It'll still be slightly rough all the way up to cruise power. By the end of a flight, it's slightly better at the high power settings and much better at idle, but it never smooths out to the way it used to be.
There is no abnormal EGT or CHT indication at any power setting that points to a particular cylinder being the source of the roughness.
Mag checks: If we do it at 1000 RPM when it's cold, the engine will quit. I've done plenty of normal runup mag checks and one lean-of-peak check at cruise though I tried to keep that short because my wife was aboard and didn't like it. On a 2000 RPM runup mag check, slightly below 1000 MSL and full rich, neither mag shows more roughness than the other, and all cylinders show a rise in EGT on a single mag, exactly as should be expected and as it has always occurred. Likewise, on the LOP cruise check, neither mag was rougher than the other. I didn't get enough of a chance to play with it and see what the engine monitor indicated for more than a couple seconds, but I didn't see any indication that it was a problem with a single mag or a single cylinder.
My normal leaning procedure at 6,000 feet is to simply lean the engine out until I see all six cylinders have peaked and come down, so I'm probably around 50ºF lean of peak* on the richest cylinder. Up higher at, say, 9,000 I'll only go until maybe the first three have dropped, and above 12,000 I'll run it in the vicinity of peak, not necessarily having any of them drop but seeing the rise slow down as I lean.
* The Insight GEM 602 does not have a specific degree value per bar, but it's supposed to peak around the asterisk and the lowest indication is supposed to be around 800ºF EGT. There's about 15-16 bars in between, so should be about 50ºF per bar on the EGT side. CHT is 25ºF per bar.
One final bit: If I richen up the mixture in cruise, it does smooth out some. Normally I see right about the fuel flow I should for 65% power LOP (12.2 gph * 14.9 = 181.78/280 = 64.9%), but at that flow I get the roughness, whereas if I richen it up to about 13 gph it mostly goes away. That's about the most solid clue I have.
Now, taking it down to the basics: Fuel, air, and spark.
1) Air. I think this is least likely. The Mooney has a balanced induction, nothing has changed here, it hasn't been touched except for taking it apart at annual (~5 months ago) to get at the spark plugs. In addition, I would expect that any blockage or restriction in the induction would cause more roughness at higher RPM, not less. Right?
2) Fuel: I suppose this could be a partially clogged injector, but again I would assume it'd be worse at higher power because there'd likely be a greater difference between fuel flows to each cylinder there. I would also expect this to be visible on the engine monitor. FWIW, the engine driven fuel pump was overhauled at annual ~5 months ago.
3) Spark: I'm guessing this is the culprit, but it's not very obvious. Mags were overhauled at the last annual (5 months ago). Nothing obviously rougher on one mag than the other at any power setting, and nothing obvious that I can see appearing on the engine monitor either. It's not coming from a completely absent spark, but I'm thinking it may be coming from a weak spark.
Any ideas?
