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LOP Ops Periodic Roughness IO-550


Txbyker

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On a recent trip I ran -25 LOP at 8000.  I have Tempest fine wires and GAMI's .1 gph separation.  The engine runs smooth for the most part however every once in a while I feel a shutter of sorts.  Nothing long, nothing hard but just a noticeable stumble about every 4 minutes or so. I have data galore.  The EGTs are all consistent +/- a few degrees with no swings of 30-50 that might indicate exhaust valve rotation.  Fuel flow is consistent + or - tenths.  I did an on-ground mag check yesterday looking for plug issues and ran the engine on each mag.  Left mag #2,4,6 temps all rise together.  Right mag #1,3,5 rise pretty consistently with the exception of #1 rising about half that of 3 and 5.  Any other ideas where to look?  FYI, I was at the Mooney PPP and two different instructors commented on the stumble although not concerned.

Russ

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It is probably a spark plug or mag. Lean mixtures are harder to light. That is why your engine starts to stumble when you get it too lean. 

A ground check is normally done at a rich mixture and a low power setting. Nothing like cruise power LOP. I could also be in the mag and altitude related. I would look for a slight rise in one of the EGTs a few seconds after the stumble happens.

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Russ,

The first test I would do is to slightly enriched the mixture when that happens - just a bit at a time - to see if it clears up by getting just a tad more fuel. A GAMI spread .1GPH is excellent, be happy, this should allow you to get very aggressive LOP if you desire as long as all of the other systems are in good shape: plugs, mags, plug wires (don’t overlook them).

Also, another thing you can try is to take away an inch of MP and see if that changes anything.

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4 minutes ago, DVA said:

Russ,

The first test I would do is to slightly enriched the mixture when that happens - just a bit at a time - to see if it clears up by getting just a tad more fuel. A GAMI spread .1GPH is excellent, be happy, this should allow you to get very aggressive LOP if you desire as long as all of the other systems are in good shape: plugs, mags, plug wires (don’t overlook them).

Also, another thing you can try is to take away an inch of MP and see if that changes anything.

Thanks.  I have 100hrs on these mags and new Continental plug wires.  I will try the test next time I fly.

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I had a periodic stumble one time which could not be reproduced on the ground or on test flights. After a while I realized it would only happen when I had been flying at altitude for a bit. The air is an insulator and at alttitude as the air is thinner the ignition components if starting to breakdown will sometimes fail at higher altitudes due to the thinner atmosphere. A mag check at altitude will sometimes show things not seen at run up or at lower altitudes.

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Does the #1 rising half while the other plugs on the same bank rise more during the run-up single mag test mean something about #1?  (This seems to be a hint)

if they rise equally this is because the same amount of excess fuel is being delivered to the exhaust pipe. Sounds like #1 might not have the same FF as the others fuel injectors.  (This might be a possibility)

Thinking this, I would want to test the theory... doing a mag check at altitude.  Stay on one mag long enough to see the rise then...

Do it ROP and LOP for comparison.

See if you can get FF to #1 to stand out as the problem.  If it IS running slightly lower FF it will turn off before the others while going deep LOP... lean slowly until one plug stops firing...  at lower altitudes a well balanced IO550 can go deep LOP to about 90°F LOP.

If altitude is the issue for a mag, it could be unmasked doing this as well. At altitude....

with this info in hand, check with your mechanic to see if it is wise to just pull the injector for cleaning and inspection?

PP thoughts, not a mechanic.

Best regards,

-a-

Edited by carusoam
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Remember the good old days, when the engine ran rough because it was too lean we would just wind the red knob in until it ran smooth?  Those were the days!  At 8000 feet or higher in your O you can set the mixture anywhere you want and not hurt a thing.  Just enrichen it until it's smooth.  But, as others have already pointed out, it's most likely your ignition system.  

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5 hours ago, carusoam said:

Does the #1 rising half while the other plugs on the same bank rise more during the run-up single mag test mean something about #1?  (This seems to be a hint)

if they rise equally this is because the same amount of excess fuel is being delivered to the exhaust pipe. Sounds like #1 might not have the same FF as the others fuel injectors.  (This might be a possibility)

Thinking this, I would want to test the theory... doing a mag check at altitude.  Stay on one mag long enough to see the rise then...

Do it ROP and LOP for comparison.

See if you can get FF to #1 to stand out as the problem.  If it IS running slightly lower FF it will turn off before the others while going deep LOP... lean slowly until one plug stops firing...  at lower altitudes a well balanced IO550 can go deep LOP to about 90°F LOP.

If altitude is the issue for a mag, it could be unmasked doing this as well. At altitude....

with this info in hand, check with your mechanic to see if it is wise to just pull the injector for cleaning and inspection?

PP thoughts, not a mechanic.

Best regards,

-a-

Thanks.  We are pulling that injector.

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I think I recall advice from John Deakins that at higher altitudes you should go closer to peak. I think the idea is that thinner air makes the fuel/air mixture harder to light.  What was your manifold pressure? At lower power settings I just go for peak. I'm trying to keep the CHT's from dropping too low after all. It also makes it simpler.

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7 hours ago, triple8s said:

I had a periodic stumble one time which could not be reproduced on the ground or on test flights. After a while I realized it would only happen when I had been flying at altitude for a bit. The air is an insulator and at alttitude as the air is thinner the ignition components if starting to breakdown will sometimes fail at higher altitudes due to the thinner atmosphere. A mag check at altitude will sometimes show things not seen at run up or at lower altitudes.

High altitude miss.  It can actually be fairly violent and hurt the engine if you don't get on top of it quickly.  The lack of "air insulation" allows the current to track across the magneto cap, so the wrong spark plug ignites.  But you need to be fairly high to see it.  In my experience, definitely over 18k, and actually I have only seen it in the 20's.  Power back (pull the MP) to cure it, and then descend to a lower altitude.  Talk to ATC if you need to of course.  

Typically it means that there is a leak of some kind in the pressurized mags.  But I don't think that is what our OP is experiencing, not at low alt.  I do believe it is an ignition weakness somewhere.

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1 hour ago, pinerunner said:

I think I recall advice from John Deakins that at higher altitudes you should go closer to peak. I think the idea is that thinner air makes the fuel/air mixture harder to light.  What was your manifold pressure? At lower power settings I just go for peak. I'm trying to keep the CHT's from dropping too low after all. It also makes it simpler.

MP=22.4" @ 8000

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There is something called cycle to cycle variability when running LOP.  People used to call this "lean misfire", but that wasn't correct.  When LOP, the flame front progresses slower than when ROP.  The slow propagation of the flame front causes a slight occasional vibration or roughness.  My guess, without seeing any engine data, is this is what you are experiencing.

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1 hour ago, Greg_D said:

There is something called cycle to cycle variability when running LOP.  People used to call this "lean misfire", but that wasn't correct.  When LOP, the flame front progresses slower than when ROP.  The slow propagation of the flame front causes a slight occasional vibration or roughness.  My guess, without seeing any engine data, is this is what you are experiencing.

That could be.  I have reviewed the data dump and all appears in line.

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Russ,

you might want to post a video of the run-up you described earlier.  If the EGT rise you described is constantly occurring that could get some brain cells working together on the idea...

I think there may be something with a single mag showing different rises across the cylinders...

Best regards,

-a-

Edited by carusoam
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