M20G Rough Running Engine?

[3914N]

I've finally decided that the slight engine roughness I thought was in my head is actually real.  Here's my observations from a normal flight:

• Preflight: somewhat dirty oil (40 hours since last change), passes mag check at 1700 every single time
• Climb: I seem to start noticing the roughness around 5-10 minutes after departure.  Leaning helps a bit but doesn't completely get rid of the issue
• Cruise: At my normal altitude (~8,000) and 21"MP.  Engine seems to be happiest somewhere around 50-100 deg ROP, but still seems rough in those ranges.  Going full rich makes the engine run extremely rough.  I don't notice a huge difference between 2400-2500 RPM
• Descent: pulling power back to 15" makes roughness go away (or at least so I can't feel it)

This is roughness, not vibration.  Feels similar to if I leaned excessively and one cylinder isn't firing quite right.  It's been 11 months since the last annual and I haven't done anything to the engine besides an oil change.  I only have a single-probe EGT/CHT so I have no idea what all the cylinders are doing.

So what do you think?  Fuel distribution issue requiring a carb overhaul?  Bad cylinder?  Dirty plugs?  Mag timing?  Something else?

I'm down for annual in a few weeks so I'm sure we'll find it.  Just want to mentally prepare myself for the AMU's and figure out how safe it is to fly.  What do you all think?

Edited July 20, 2016 by 3914N

[Andy95W]

Try doing a magneto check at your normal cruise altitude and normal cruise power.

[N9453V]

What type of mags and how long since the last mag inspection? I had a new slick mag that failed on about 50 hours short of the first 500 hour inspection?

-Andrew

[Piloto]

A multi probe EGT/CHT will help you a lot analyzing the problem.

José

[Raptor05121]

Could it be a prop imbalance?

[Yetti]

Plugs last cleaned and gapped?

[carusoam]

Do you lean aggressively during taxi?  

If not, have you experienced the lead balls that form and fall into the lower plugs?

If you are handy with a plug wrench, you can pull the lower plugs for inspection.  Allowable maintenance for a PP.

How many hours does it take for the oil to get dark?

How long does it take to burn a quart of oil?

PP ideas from an old M20C driver...

Best regards,

-a-

[3914N]

Looks like lots of people suspecting magnetos.  I haven't done a mag check in flight before, but I'll do that sometime this week and report back.  I haven't pulled the plugs since the prepurchase inspection 11.5 months ago, so I'm hoping that the issue will be solved when we clean/gap them during the annual next week.

Oil consumption is very good (quart every ~8 hours), and I don't really notice any premature darkening.  The oil is 40 hours old and is dark but not excessively so.  I'll send it in for analysis at the annual.

1 hour ago, Raptor05121 said:

Could it be a prop imbalance?

What are the symptoms of a prop imbalance?  How is it diagnosed?  How many AMU's to rebalance?

[Hank]

It may help to clean and gap the plugs every oil change. Won't add much time, and you'll have something to do while waiting for the oil to all drain out. I used to hang out and talk to whoever was around, or go grab a cold drink or a burger while I waited.

[Marauder]

It doesn't sound like you have an engine analyzer on board. What you are describing sounds like a number of factors could be involved. Plugs, valves & mags would be high on my list.

Sent from my iPad using Tapatalk

[goterminal120]

If you are running champion plugs, fine wire or massive, replace them. I replaced all 12 on my 172 for the same reason, runs way better. Changed 4 fine wire champions on the Mooney. Cht's are closer and no split on mag check. Champion plugs are junk.

[Yetti]

This works OK for cleaning.    I lean aggressive.  On rollout, engine will stumble if throttle added.  After 75 hours found 2 plugs with lead balls

http://www.harborfreight.com/pneumatic-spark-plug-cleaner-32860.html

[3914N]

Did an in-flight mag check today.  Running on a single mag made the roughness worse, but not to the point where there was a significant power loss.  No perceptible difference between L/R.  

I think I'll get through the annual next weekend, including a good clean/gap on the plugs, and see how the results are.  I'm also highly considering replacing the bottom four with fine wire, regardless of condition.  Good idea?

[Mooneymite]

Be sure to check your plugs' resistence.  I had a bad Champion plug that caused very similar symptoms.  Finally found burn marks around one of the plug's resistor where the spark was jumping internally.  All mag checks had been normal.  After replacing the Champion massives with Tempest fine wires, the roughness went away and hasn't come back.

[Yetti]

When were mags rebuilt last?  May be time.

[3914N]

Coming back with an update.  

We are about 10 hours into annual.  The spark plugs looked pretty good after 90 hours since cleaning, except for one that was worn out of tolerance.  None of the plugs show evidence of uneven fuel distribution.  As the new owner, I elected to replace all 8 plugs.  Probably conservative, but I'd like to make everything consistent.  They're 8 years old anyways.

We discovered that the mag timing was significantly off (20 and 22 degrees TDC), which we fixed back to 25 & 25.  I'm a bit surprised that the mag timing was that far off.  They were rebuilt 160 hours / 8 years ago.  I don't know if they were re-timed at the last annual as there is no explicit record of it.  Maybe that type of drift is normal?

Compressions are all between 76 and 79.

It will be another week before we get everything back together and do an engine run.  I'll report back.  Hopefully the mag timing and plug replacement solves the issue.

Edited August 2, 2016 by 3914N

[Yetti]

May want to have the mags rebuilt.  My understanding is the E gap is off due to wear which causes the timing to drift. 

 

Just a PP that watched a good AP do things properly

[Andy95W]

4 hours ago, Yetti said:

May want to have the mags rebuilt.  My understanding is the E gap is off due to wear which causes the timing to drift. 

Actually, this is a commonly misunderstood concept.  The E-gap (Efficiency gap) is the point at which the magnetic field stress is greatest, which means that the change in flux is also greatest, so that when the points open and the magnetic field collapses there is the greatest voltage induced into the secondary winding of the coil.

What this means basically is that E-gap controls the intensity of the spark.  Engine timing is controlled by the points opening as the rotor is aligned with the proper cylinder in the distributor.  

It is possible for the timing to drift due to wear in the cam follower portion of the points or erosion of the point faces themselves while the E-gap remains virtually the same.

Truthfully, I didn't really understand this idea when I got my A&P license.  It wasn't until later when I started working on magnetos that I finally decided to satisfy my curiosity into what exactly the E-gap is and why it matters.

[Vance Harral]

11 hours ago, 3914N said:

We discovered that the mag timing was significantly off (20 and 22 degrees TDC), which we fixed back to 25 & 25.  I'm a bit surprised that the mag timing was that far off.  They were rebuilt 160 hours / 8 years ago.  I don't know if they were re-timed at the last annual as there is no explicit record of it.  Maybe that type of drift is normal?

There is a Lycoming Service Instruction which recommends changing the timing on the Lycoming IO-360 series from 25 BTDC to 20 BTDC.

There's a lot of debate about whether it's a good idea to follow the service instruction.  That's up to you.  But the most likely explanation for your timing being "significantly off" is that it was deliberately set for 20 BTDC the last time it was adjusted.

[3914N]

16 minutes ago, Vance Harral said:

There is a Lycoming Service Instruction which recommends changing the timing on the Lycoming IO-360 series from 25 BTDC to 20 BTDC.

There's a lot of debate about whether it's a good idea to follow the service instruction.  That's up to you.  But the most likely explanation for your timing being "significantly off" is that it was deliberately set for 20 BTDC the last time it was adjusted.

Vance, does that service instruction apply to both the O-360 and IO-360?  I have the former in my 'G. 

[Vance Harral]

7 hours ago, 3914N said:

Vance, does that service instruction apply to both the O-360 and IO-360?  I have the former in my 'G. 

That's a good question, and I'm sorry to say I don't know the answer.  I'm fixated on the IO-360 series since that's what's in my airplane, and forgot C and G models are carbureted.  The service instruction relates to detonation margin, which perhaps is only a concern in the 200hp IO-360s in the E/F/J.

[Andy95W]

O-360 timing is 25° BTDC.  

I've  been digging through different references but can't find it, sorry.  Maybe Clarence can find the proper document. 

[JimL]

Check the mag switch.  We had a similar problem with my neighbor's Swift with an IO-360.  If we pulled the p-leads off the mags, it ran smooth as glass.  Put them back on and the roughness reappeared.  After installing a new mag switch, I disassembled the old one and found carbon tracking between the contacts.