Engine stumbles in cruise

I have been fighting an issue on my Lycoming O360-A4M for about 5 months now. Every once in a while, but only in cruise, the engine stumbles. I was finally able to catch it on my EMS. All 4 EGTs, the RPMs, and Fuel Flow drop while the MP and Carb Temp climb. It all happens for about 1-1/2 seconds then everything comes back to normal. The Fuel pressure appears to remains normal during the stumble.



I have replaced the magneto's (old ones had over 600hrs so they were due anyways), ignition wires, spark plugs, Carb, Electric Boost Pump and the Mechanical pump but the problem continues. We did an induction leak check, ignition stress test and fuel distribution test and they all passed passed with flying colors. Worst of all it never happens on the ground, even with an aggressive run-ups. Any ideas are welcome since we are baffled by this.

Float valve sticking?

Just thinking out loud, but if an intake valve stuck, the exhaust is a lot higher pressure and pushing exhaust into the intake would drive mixture way lean, and cause an increase in intake temp. 2400 RPM is 40 rev/sec or 80 firings per sec. If the valve was open very far then it would kiss the piston too.

If it was an exhaust valve the same thing would happen.

1.5 sec is 20 firings of a single cylinder. Did one head have more of a temperature excursion that another? It might be subtle. Combustion destruction on all cylinders may happen quick enough that they all go together.

You might check the piston tops to see if there are any lasting impressions, if you have some kind of scope to see that. When were your valve stem clearances checked? Wobble test, I mean.

Maybe others (RocketBob, Mahlon, rv6ejguy, Ross F) will have a better idea. Intriguing.

Can you open up the graphs in time to see what falls/rises first?

Fuel should see Fuel Pressure do something, then EGTs rise then fall maybe (as it leans before cuts off).

Ignition issue (P-Leads?) would see EGTs fall v quickly, Fuel Pressure late on.

My instinct (well guess) is the Fuel Pressure. Your RPM only drops a few 000 RPM it seems (?), so why does Fuel Pressure fall? The EGTs were rising it seems prior to the 1st event (?) which seems like it is getting leaner?

As I say, only a guess...

Ignition Wire Routing

Check the routing of ignition wires (& wiring to mags). I have seen interference from other wiring when routed with ignition wires. If overlap, separate & analyze results.

Since all four cylinders are affected that makes it pretty obvious (at least to me) that its not a valve train problem.

Perplexing is carb temp...those are typically thermistors and I'm surprised that they can react that quickly to a temperature change considering the thermal mass of the carburetor. Which seems suspect.

Do you have a keyswitch for the mags?

THANKS for these ideas. Gives me more things to consider.

aerhed said:

Float valve sticking?

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This was one of my original thoughts so I replaced the carb with a newly overhauled unit. The problem continued.

Andy Hill said:

Can you open up the graphs in time to see what falls/rises first?

Fuel should see Fuel Pressure do something, then EGTs rise then fall maybe (as it leans before cuts off).

Ignition issue (P-Leads?) would see EGTs fall v quickly, Fuel Pressure late on.

My instinct (well guess) is the Fuel Pressure. Your RPM only drops a few 000 RPM it seems (?), so why does Fuel Pressure fall? The EGTs were rising it seems prior to the 1st event (?) which seems like it is getting leaner?

As I say, only a guess...

Click to expand...

The chart is already open up a LOT showing only a few minutes of the flight when the stumble occurred. The entire flight was 1.2hrs and uneventful. I had just done an ignition stress test so the engine was aggressively leaned. This makes no effect on when the engine stumbles. The numbers shown on the chart are just to show the color coding of which line corresponds to which value. It isn't showing when the engine is stumbling.

If one p-lead shorts out then the EGTs would climb not drop since the other magneto would still be firing. If both p-leads short out then all these parameters would happen as shown. The only place both p-leads are anywhere close to each other is in the ACS switch. I am going to replace it and see what happens.

RPMs actually drop a little over 600rpms so the stumble is very noticeable. All 4 EGTs climb about 60F just before they fall off about 200F then come back. Fuel pressure stays relatively constant and does not vary with the rest of the parameters.

BillL said:

Just thinking out loud, but if an intake valve stuck, the exhaust is a lot higher pressure and pushing exhaust into the intake would drive mixture way lean, and cause an increase in intake temp. 2400 RPM is 40 rev/sec or 80 firings per sec. If the valve was open very far then it would kiss the piston too.

If it was an exhaust valve the same thing would happen.

1.5 sec is 20 firings of a single cylinder. Did one head have more of a temperature excursion that another? It might be subtle. Combustion destruction on all cylinders may happen quick enough that they all go together.

You might check the piston tops to see if there are any lasting impressions, if you have some kind of scope to see that. When were your valve stem clearances checked? Wobble test, I mean.

Maybe others (RocketBob, Mahlon, rv6ejguy, Ross F) will have a better idea. Intriguing.

Click to expand...

I haven't checked that possibility and will bore scope it next. Once again THANKS for these ideas. Gives me more things to consider. This is baffling.

rocketbob said:

Since all four cylinders are affected that makes it pretty obvious (at least to me) that its not a valve train problem.

Perplexing is carb temp...those are typically thermistors and I'm surprised that they can react that quickly to a temperature change considering the thermal mass of the carburetor. Which seems suspect.

Do you have a keyswitch for the mags?

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Yes I have an ACS A-510-2 switch.

The carb temp climbs 10C then back down. Fuel flowing through the carb helps cool it, so if the fuel stops then the temp would climb. Fuel pressure stays constant but the flow drops in sync with the RPMs dropping. In this case which came 1st the chicken or the egg?

GalinHdz said:

Yes I have an ACS A-510-2 switch.

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I suggest you temporarily wire up some toggle switches, easy to do. I've seen those things ground the mags if you pushed sideways on the key.

If both mags get turned off then everything on your graphs makes sense.

rocketbob said:

I suggest you temporarily wire up some toggle switches, easy to do. I've seen those things ground the mags if you pushed sideways on the key.

If both mags get turned off then everything on your graphs makes sense.

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Hmmmmmmmmmmmm. Makes total sense.

rocketbob said:

Since all four cylinders are affected that makes it pretty obvious (at least to me) that its not a valve train problem.

Perplexing is carb temp...those are typically thermistors and I'm surprised that they can react that quickly to a temperature change considering the thermal mass of the carburetor. Which seems suspect.

Do you have a keyswitch for the mags?

Click to expand...

I certainly could not disagree with this - - if the engine just shuts off like a switch. I suppose you could ground check it too with a test light and wiggling the switch around. A grounding wire to chassis would not seem to just bang on for a second.

How about a water slug? Even that would seem to not just stop, but to rumble down then recover. I get more understanding in how the event feels than limited data.

Bob, does an intermittently sticking valve just drop off one cylinder and run really rough? Is that what you were thinking?

Keep looking, thinking . . . fly defensively (obvious, but there - - I said it)

If both mags stop firing, even for a few RPM's, the engine will pump raw fuel into the exhaust and when the mags start firing again that raw fuel will ignite with a BANG. However, if just one ignition is shut down, you will see an RPM drop and you won't pump raw fuel into the exhaust to be burned with a bang later.

What P/N carb do you have? Sounds like you may need an M kit modification, if it is a 10-3878.
Good Luck,
Mahlon

That does not look like a stumble to me. Thats a cut off. Either fuel or ignition. My bet is only ignition could produce those results. Carb temp change. An electrical anomaly could cause this.
To test ignition, assuming the failure can be reliably produced, Id disconnect p-leads and go fly. All normal safety precautions should be applied of course.

N941WR said:

If both mags stop firing, even for a few RPM's, the engine will pump raw fuel into the exhaust and when the mags start firing again that raw fuel will ignite with a BANG. However, if just one ignition is shut down, you will see an RPM drop and you won't pump raw fuel into the exhaust to be burned with a bang later.

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Good point on both mags cutting off at once. The engine never backfires just runs rough for a second then goes back to running smooth.

mahlon_r said:

What P/N carb do you have? Sounds like you may need an M kit modification, if it is a 10-3878.
Good Luck,
Mahlon

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It is a recently rebuilt (2hrs) Marvel-Schebler MA-4-5, 10-3878. I will check the paperwork if the “M” kit modification was done. What does this “M” modification do?

Kahuna said:

That does not look like a stumble to me. Thats a cut off. Either fuel or ignition. My bet is only ignition could produce those results. Carb temp change. An electrical anomaly could cause this.
To test ignition, assuming the failure can be reliably produced, Id disconnect p-leads and go fly. All normal safety precautions should be applied of course.

Click to expand...

This is my next test to do. Could an intermittent Mag timing problem cause these indications? I have dual P-Mags (114) but this actually started before I installed them thinking the original mags (over 600hrs on them) were the cause. The problem continued and is getting more pronounced.

mahlon_r said:

What P/N carb do you have? Sounds like you may need an M kit modification, if it is a 10-3878.
Good Luck,
Mahlon

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Its a 10-3878 and was overhauled by Kelly Aerospace Power Systems on Jan 02, 2014. The paperwork says:

Overhauled in accordance with Precision Airmotive Corporation FSM-OH2 dated March 30, 1993. Complied with AVCO SB 366 Rev A, AVCO SB 330B, MS SB 297A, MS SB A1-76, MS SB A1-84A, MS SB A2-67, MS SB A1-69, A69-24-3, AD69-22-3, AD72-6-5, AD89-4-2.

We installed it just 2hrs ago.

GalinHdz said:

This is my next test to do. Could an intermittent Mag timing problem cause these indications? I have dual P-Mags (114) but this actually started before I installed them thinking the original mags (over 600hrs on them) were the cause. The problem continued and is getting more pronounced.

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No it really cant. Even if they were getting some anomalous MP sense or some other thing, getting both to die is not a timing problem. BUT, you raise an interesting question now that I know they are P-mags. The P-mags generate their own power above a certain RPM, and if ships power is CUT, then they run on their own. What I do not know is how the unit behaves when this happens. Is there a brief kill during cut over? Or is there in fact no cutover at all and its running entirely on its own power above at its min threshold RPM. Im not sure of this. I have installed a number of these on the team and as I recall there is no noticeable change in behavior when ships power is cut above the threshold. I would not expect the behavior to be a cut like this on loss of power. Voltage sensing devices are very fast. And that you had this BEFORE the p-mags, well then im leaning towards the ground supply giving you intermittent kill the pmag signals. Im still of the notion, pull the kill wire and go fly. And while your up there, kill the power and observe the behavior.

GalinHdz said:

...

This is my next test to do. Could an intermittent Mag timing problem cause these indications? I have dual P-Mags (114) but this actually started before I installed them thinking the original mags (over 600hrs on them) were the cause. The problem continued and is getting more pronounced.

Click to expand...

Sounds like a good reason to buy one of our EICommanders. Just kidding, sort of. They have helped customers figure out their ignition issues.

However, in your case, since this was happening prior to your switch to P-mags it may well be related to your P-lead, if you are using the same P-leads with both ignition setups.

One other thing to look at, now that you have P-mags. Are they grounded independently to the engine case or are they grounded to your forest of tabs?

N941WR said:

Sounds like a good reason to buy one of our EICommanders. Just kidding, sort of. They have helped customers figure out their ignition issues.

However, in your case, since this was happening prior to your switch to P-mags it may well be related to your P-lead, if you are using the same P-leads with both ignition setups.

One other thing to look at, now that you have P-mags. Are they grounded independently to the engine case or are they grounded to your forest of tabs?

Click to expand...

I am using the same P-leads I was using with the old magnetos. I have 3 ground wires for each magneto. 1 to the engine block, 1 to the "forest of tabs" and 1 going to the ACS A-510-2 switch.

I also monitor and log the ship's power to the P-Mags and they stay perfectly on voltage even during the stumble.

The more I hear, the more I lean towards Rocketbobs sticking valve. It sure is cheaper to check the valves than to keep replacing accessories.

When the stumble as depicted by the graphs you posted happened, what was the OAT?

The only thing I can think of that would cause the carb temp to rise - in conjunction with a loss of power - is the lack of vaporizing fuel. If the temp rose to the prevailing OAT it would tend to confirm this. If it went higher than that, then you have a weird problem indeed.

Vaporizing gasoline can drop the carb temp by 20 degrees C.

Kahuna said:

No it really cant. Even if they were getting some anomalous MP sense or some other thing, getting both to die is not a timing problem. BUT, you raise an interesting question now that I know they are P-mags. The P-mags generate their own power above a certain RPM, and if ships power is CUT, then they run on their own. What I do not know is how the unit behaves when this happens. Is there a brief kill during cut over? Or is there in fact no cutover at all and its running entirely on its own power above at its min threshold RPM. Im not sure of this. I have installed a number of these on the team and as I recall there is no noticeable change in behavior when ships power is cut above the threshold. I would not expect the behavior to be a cut like this on loss of power. Voltage sensing devices are very fast. And that you had this BEFORE the p-mags, well then im leaning towards the ground supply giving you intermittent kill the pmag signals. Im still of the notion, pull the kill wire and go fly. And while your up there, kill the power and observe the behavior.

Click to expand...

The 114 series P-mags cut over to their internal power around 800 RPM +/- and the switch over is seamless. They never miss a beat.

However, if he continued to use the same P lead with the P-mags that he did with his standard mags AND there is a short issue with one of the original P leads, then one ognition could be cutting out.

One other thought, do the stumbles occur on the same tank? Is it possible thst you might have a particularly blocked fuel line, similar to what Jamed Clark experienced with the RV-1?

N941WR said:

One other thought, do the stumbles occur on the same tank? Is it possible thst you might have a particularly blocked fuel line, similar to what Jamed Clark experienced with the RV-1?

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This has been my thought, more or less, while reading this thread. A sticking valve would cause one EGT to drop, but not all 4, right?

A p-lead grounding would do this, maybe, but not necessarily a change in carb temp or fuel pressure. However, this is a very easy thing to test by disconnecting the p-lead wires and testing.

It seems to me that a fuel supply problem would do exactly this. Maybe the engine-driven pump is on it's way out. Try cruise with the boost pump on and see if this changes anything. I can't think of anything that would cause a drop in fuel pressure except fuel supply or fuel pump.

Jesse said:

It seems to me that a fuel supply problem would do exactly this. Maybe the engine-driven pump is on it's way out. Try cruise with the boost pump on and see if this changes anything. I can't think of anything that would cause a drop in fuel pressure except fuel supply or fuel pump.

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The fuel pressure stays constant but the fuel flow drops in sequence with RPMs and EGTs. We replaced both the electric and mechanical fuel pumps with no change in the problem.

Jesse said:

A p-lead grounding would do this, maybe, but not necessarily a change in carb temp or fuel pressure. However, this is a very easy thing to test by disconnecting the p-lead wires and testing.

Click to expand...

If one p-lead would ground out then the EGTs would climb instead of drop since one magneto would still be working. If both of them ground out then all 4 EGTs drop. I am going to check this possibility next.

N941WR said:

One other thought, do the stumbles occur on the same tank? Is it possible thst you might have a particularly blocked fuel line, similar to what Jamed Clark experienced with the RV-1?

Click to expand...

I checked this possibility and it happens with either tank.

My mistake. I saw that fuel flow dropped and read it as fuel pressure dropped.

I have seen an instance where the suspect was a short of both P leads possibly by a fibre of wire, either in your EMS connector shell (tacho inputs), or at the key switch.

Isolate the tacho wire and a P Lead on the right mag. You will have one live mag for a while during this test period, so be careful but it is quite possible. I have seen it with my own eyes and it could only be found in the data. In the case I saw it was very quick.

I do not believe this problem is caused by an ignition malfunction because. Before the first stumble occurs, fuel flow is decreasing and EGT?s are increasing, at the first stumble carb temp suddenly increases. This pattern repeats on the second deeper stumble. Fuel flow after increasing and recovering from the first stumble decreases, EGT?s rise during this second decrease of fuel flow until the fuel flow is below combustible levels, EGT?s rapidly drop off. Carb air temp rises at the same time. This is an indication that fuel is no longer flowing and being vaporized inside the carb.
Because you replaced the carb it kind of rules out a sticky float. If it was a fuel supply problem you would probably see a change in fuel pressure which you are not observing. If it was water moving thru the system fuel flow would not go down when EGT?s went up. Ignition system faults, would not have effects on fuel flow like this .This engine leaned out until it would not run, recovered then repeated the process
I believe the fuel line between the pump and carb or the carb body itself is getting too hot and is forming vapor bubbles.

Titan-Xpert said:

I believe the fuel line between the pump and carb or the carb body itself is getting too hot and is forming vapor bubbles.

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Quite possible however one would most likely see that sort of problem right after takeoff, which logically goes against when this kind of problem typically manifests itself...low air mass thru the cowling, heat soaked fuel line from ground operation.

Vapor bubbles typically cause fuel flow readings to be erratic also.

RPM Surges

What's causing those RPM surges that follow the stumble? Are they related to the fluctuating fuel flow?

Have you tried intentionally cutting your engine while circling over an airport to see what the graphs look like for comparison? Just briefly cut your mags, and then briefly cut mixture (and/or maybe fuel selector) to see if either produces a similar plot?

My money is on a fuel issue due to the rising carb temp, to me that says it isn't vaporizing fuel during that time period. My guess is if you try the diagnosis above, your carb temps won't rise when you cut the mags... Just be ready for a BOOM when you light it back up

Good luck, and above all be safe while testing!

Galin, sorry you are fighting this strange problem!

Question - generally where is your mixture set during these events? I.e., peak, ROP, LOP?

I'm wondering if some play in the mixture control cable could be the culprit. The hysteresis inherent to cables running through curved sheaths means that one can always move the lever (on the carb) back and forth some while the red knob is locked at the panel. If you approach leaning from the rich side, the lever could move additionally lean at some point after you've locked the knob (perhaps vibration induced). If instead, after you've leaned, you were to move the knob slightly rich, the lever on the carb can not then subsequently move additionally towards the lean side (indeed, it could only move towards the rich side).

Sorry this is not easy to follow (try writing it!), but it might be worth seeing how much play the mixture lever has next time you have the cowl off. Put the mixture knob at about the cruise position, and go around and move the lever on the carb back and forth.

GalinHdz said:

Its a 10-3878 and was overhauled by Kelly Aerospace Power Systems on Jan 02, 2014. The paperwork says:

Overhauled in accordance with Precision Airmotive Corporation FSM-OH2 dated March 30, 1993. Complied with AVCO SB 366 Rev A, AVCO SB 330B, MS SB 297A, MS SB A1-76, MS SB A1-84A, MS SB A2-67, MS SB A1-69, A69-24-3, AD69-22-3, AD72-6-5, AD89-4-2.

We installed it just 2hrs ago.

Click to expand...

The M kit stops the behavior you are experiencing in that carb P/N. It involves installing a kit P/N A666-660 which helps vaporize and distribute the fuel from the carb better. It will fix the stumble issue, make the engine richer at full rich setting and generally improve the way it runs. Although there is the possibility of another unrelated cause, I am 90% sure that installing the kit will fix your problem. Your engine is exhibiting the classic symptoms of needing it.
Good Luck,
Mahlon

Great info from Mahlon -

Page 27 here -

http://hhh.gavilan.edu/hspenner/Main/Manuals/marvel-scheblerservicebulletinsyears69-59.pdf

Since the SB M mod is for Mooneys and Cherokees only, it seems reasonable that the carb. overhaul folks didn't perform it.

mahlon_r said:

The M kit stops the behavior you are experiencing in that carb P/N. It involves installing a kit P/N A666-660 which helps vaporize and distribute the fuel from the carb better. It will fix the stumble issue, make the engine richer at full rich setting and generally improve the way it runs. Although there is the possibility of another unrelated cause, I am 90% sure that installing the kit will fix your problem. Your engine is exhibiting the classic symptoms of needing it.
Good Luck,
Mahlon

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I just read the SB so I will check to see if the "M" is stamped on the carb.

az_gila said:

Great info from Mahlon -

Page 27 here -

http://hhh.gavilan.edu/hspenner/Main/Manuals/marvel-scheblerservicebulletinsyears69-59.pdf

Since the SB M mod is for Mooneys and Cherokees only, it seems reasonable that the carb. overhaul folks didn't perform it.

Click to expand...

THANKS!

Any resolution to your problem?

Michael Henning said:

Any resolution to your problem?

Click to expand...

Not yet but after my last test flight I got some new information and data pointing to a situation that explains everything. However, I don't want to claim victory until I am 1000% sure. Once we get the "fix" completely tested (hopefully by next weekend) I will post the results.

EUREKA!

After months of fighting it looks like we got the problem fixed. Just like most similar problems, this one is actually quite simple and explains other issues I thought were unrelated.

After changing the ignition switch to eliminate the possibility of an internal ignition P lead short I went on a test flight. Doubting this was the problem, I decided that if the problem continued I would do several other in flight tests. I coordinated with the tower to stay over the approach end of the runway during the flight.

With all the ground tests completed and passed, I took off and climbed without issue to 1,500ft just as the tower had cleared me. A few minutes after reaching 1,500ft the engine started to run rough again. Knowing I was within easy gliding distance of the runway I got up the guts to do the “in-flight” single magneto check. I switched to left mag only and, as I expected, there was absolutely no change in the engine performance. It continued to run a bit rough. When I switched to right mag only, complete and absolute silence. THE ENGINE HAD QUIT!

With “pucker factor” at a maximum, I switched to both magnetos and the engine came back to life but still running a bit rough. With this new information I decided to land and as I descended, the engine started running smooth as silk again. Once on the ground I tried everything I could to reproduce the problem with no avail. The engine continued to run smooth. But now I know without a doubt I had a magneto problem.

I called Brad (E-Mags) who has been working with me on this problem from day one, and told him what happened on this flight. With this information he speculated both magnetos have separate problems, one problem masking the other. My left magneto has a problem causing it to run slightly rough. The right magneto compensates when it is working correctly and covers the problem. When the right magneto cuts off for that brief time frame, then the left magneto’s problem manifests itself until the right magneto comes back on line and the engine runs smooth again. I downloaded the data from my EMS and we could see it happening in perfect harmony with the symptoms. Brad told me to ship him both magnetos (making sure I identified which was which) and he would abuse them in his shop to figure out if this is what is actually happening.

In addition to explaining why the engine ran a bit rough only once in a while it explained two other situations which, until then, I thought were completely unrelated to the intermittent stumbling issue. 20/20 hindsight.

About a week after installing the magnetos (Oct 2013) I was going to take a short local flight when, while doing the engine run up, the RPM indications on my DYNON SkyView EMS dropped to zero even though the engine continued to run smoothly. No matter what I did the RPMs stayed at zero with the engine still running. I aborted the flight and checked the wiring. Finding nothing wrong I called DYNON and we realized I had configured the computer to read only from the right magneto. Since I had not installed the 33K resistor on the magneto tach sensor line we figured the computer had disconnected the signal which explained why I had no RPM indication even though the engine was still running smooth. We configured the computer to read from both magneto’s and have not had the problem since. With 20/20 hindsight this might have been when the right magneto computer board started having its intermittent problem but the left magneto, which was operating fine back then, compensated and covered the problem. BTW: With the Skyview EMS you do not install a 33K resistor with the E-Mags only the 5.1v diode on the tach line.

Then, back in January 2014, I was doing an engine run up prior to a flight when I felt the engine running different. I couldn’t pinpoint why, but it just felt different to me. But no matter what I did during the run up everything looked fine so I continued and had an uneventful flight. A few flights after this the intermittent roughness during cruise flight started. I am speculating that this is when the left magneto started acting up and I have been chasing two ghosts ever since. Again, hindsight is 20/20.

Brad received the mags but didn’t find any “smoking gun” when he tested them. However, since the right mag had stopped sending a tach signal before and had stopped operating in flight he decided to change the PC board in it. He also sent me a new wire harness for the left mag and told me to replace the spark plugs with new ones. The only thing Brad charged me for all of this work was shipping the mags back to me.

We installed the magnetos, the new wiring harness and replaced all the spark plugs on Saturday. I did multiple ground tests and everything passed with flying colors. By the time we had everything closed back up it was night so I decided to do the flight test on Monday.

For testing I made the flight with the left magneto on ship power but the right magneto on internal power only. This insured that if there were something wrong with my electrical system at least one magneto would not be affected. I climbed to 1,700ft and stayed over the approach end of the runway for 45 minutes. Everything went perfectly smooth. Not one hiccup, stumble or any other abnormality. I was one very happy camper when I landed.

Being the belt and suspenders kind of guy, I am still a bit apprehensive that the problem is fixed but it really looks like we got it. The next few flights will just be just hops around the pattern until I have flown about 5hrs without a stumble of any kind. Only then will I start going further away as I get more confident this has completely eliminated the problem.

I want to thank everyone here that has helped during these baffling times especially Bill (N941WR), Kahuna and Brad at E-Mags. Brad and I talked multiple times during the ordeal and he never tried to push the blame somewhere else just kept a logical troubleshooting sequence until he nailed the problem. His customer support is superb and I am glad I went with his magnetos.

I also want to thank Tom (g_zero) for letting me vent frustrations during our multiple hangar sessions.

2nd test flight completed today without a hiccup, stumble or any other abnormality. I had planned to fly for about 1.5hrs but nasty weather moved in so I landed after just over 45min.

Now have flown just under 1.5hrs total without any issues. Looking better every day.

Makes no sense to me that you replaced magnetos with pmags to solve the problem, it persisted, then you sent the pmags in, put them back on with new plugs, and the problem went away...?

Sounds like you had the same plugs in all along..? Were they automotive plugs? I have had spark plug tips on NGK plugs come loose which caused an occasional miss.

rocketbob said:

Makes no sense to me that you replaced magnetos with pmags to solve the problem, it persisted, then you sent the pmags in, put them back on with new plugs, and the problem went away...?

Sounds like you had the same plugs in all along..? Were they automotive plugs? I have had spark plug tips on NGK plugs come loose which caused an occasional miss.

Click to expand...

I had Tempest Aviation plugs with the original mags and went with NGK auto plugs for the Pmag installation. The original mags had almost 600hrs when I replaced them with Pmags. The NGK plugs were in perfect shape when I pulled them but since they are so cheap I replaced them anyways.

The symptoms went away with the Pmags / NGK plugs until just over 40hrs of use. I thought the original problem had returned, which wasn't the case. The problem was different, it just manifested itself in the same way. Murphy is alive and well in aviation. When I finally did the airborne mag check that is when we found that one Pmag was intermittent which explained everything.

Who would have Thunk it?

GalinHdz said:

Who would have Thunk it?

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A bloke who I know that suffered 2 x PMAG failures like you.....at the same time climbing out of YBSU. Fortunately like Piere he has spent his whole life flying ag work and has very good skills close to the ground and just made the airport.

Who would have thunk it.

RV10inOz said:

A bloke who I know that suffered 2 x PMAG failures like you.....at the same time climbing out of YBSU. Fortunately like Piere he has spent his whole life flying ag work and has very good skills close to the ground and just made the airport.

Who would have thunk it.

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Fortunately for me the engine never quit during the entire time.

Even when one mag would stop for those few seconds on a very intermittent basis, the other one never stopped working. It was the similarity to the problem with my original mags that pointed me in the wrong direction. Once I got the "guts" to shut down one mag at a time when the problem manifested itself while flying did I find the problem. THEN everything made sense, especially the other symptoms which I thought were unrelated. 20/20 hindsight.

Once I got the "guts" to shut down one mag at a time

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This is never a gutsy test. It is SOP for some, and should be for all.

Gutsy? It's mandatory...

A mag check when an engine is running rough is absolutely mandatory. An ignition source, whether mechanical mags or electronic ignition, can fail in such a way that it could cause serious damage to the engine unless it is switched off. Turning off the offending ignition source can make the difference between a smoother running engine and a subsequent safe landing OR a not-so-good outcome.