912 ULS engine runs rough on one ignition

[Anticept]

The secondary side of the modules is still relatively low voltage until after it passes the coil packs. You could get yourself some analog to digital converters to analyze on a computer to compare the two and try to locate where the issue lies.

One more thing that got me thinking... what if you have a chipped flywheel hub or something else is wrong with it? The system is pretty sensitive, where things such as RPM will change where firing events are triggered on the trigger cam positioning (leading vs trailing edges). Since the ignition system has trigger cams for A and B, one of those being damaged will manifest as an ignition timing issue on one side only.

[Tom Baker]

I had to troubleshoot a rough running 912 a couple month ago. It had a 800 rpm drop. I know my issue is not what you have going on, I had a broken coil wire, but I used an old school timing light on the sparkplug leads to track it down. You might be able to check the leads and see if one has an irregular spark.

I know you said the carburetors have been gone through, but have you tried pulling on the choke and checking to see if the drop changes? A drop that is only 100 RPM greater than normal would more likely be a carb issue in my mind. I know that the issue changing sides when you are troubleshooting makes you think ignition, but one side running a little lean can cause the same issue due to the different burn patterns in the cylinders running on one ignition.

[Madhatter]

32 minutes ago, Tom Baker said:

I had to troubleshoot a rough running 912 a couple month ago. It had a 800 rpm drop. I know my issue is not what you have going on, I had a broken coil wire, but I used an old school timing light on the sparkplug leads to track it down. You might be able to check the leads and see if one has an irregular spark.

I know you said the carburetors have been gone through, but have you tried pulling on the choke and checking to see if the drop changes? A drop that is only 100 RPM greater than normal would more likely be a carb issue in my mind. I know that the issue changing sides when you are troubleshooting makes you think ignition, but one side running a little lean can cause the same issue due to the different burn patterns in the cylinders running on one ignition.

You are correct. I was thinking earlier that a 4 probe egt would help. Also you and I know that an ohm meter can not test a high voltage ignition wire,  it requires a high voltage cable tester.

[FredG]

Tom, we used an inductive timing light to check for voltage in each of the eight spark plug leads.  All triggered the timing light.  

We are going to try a second set of carburetors on the engine to see if that affects the problem (or eliminates it completely).

[Tip]

I had this issue on a 2008 LS. The smaller white wire is soldiered to the braid on the larger wire. The braid was frayed and the wickers poked out through the heat shrink sleeve grounding out that side intermittently. I had to wiggle the modules wires while the engine was running to find the problem.

[FredG]

Tip, thanks, we will check for that.

[Gabriel]

After all the tests, including the resistance of the spark plug caps ( 5K ohms), we have changed all the spark plug caps (with an old set) and It has significantly improved the performance and the drop in r.pm.

We are awaiting the arrival of a new set of spark plugs and caps for testing.

Regards

[FredG]

Gabriel, do you think that the connection between the copper strands in the spark plugs wires were not making good connection with the post in the spare plug cap or do you think the caps were defective in some way despite showing 5K ohms when tested?  

I tested the eight caps on this engine I wrote about and all were within 5% of 5K ohms.

Thanks, Fred

[Anticept]

One of the things I noticed with plug caps is that some of them get real gunked up. Ill spray them with electronics cleaner.

They're also not *that* expensive if you still have the old ngks. Ive seen copper corrosion where they screw into the cables too.

That said, you said the mag it happens on changes when you change the plugs, so its very unlikely to be anything after the modules.

[Gabriel]

10 hours ago, FredG said:

Gabriel, ¿crees que la conexión entre los hilos de cobre de los cables de las bujías no estaba haciendo una buena conexión con el poste en la tapa de la bujía de repuesto o crees que las tapas estaban defectuosas de alguna manera a pesar de mostrar 5K ohmios cuando se probaron?  

Probé las ocho tapas de este motor sobre el que escribí y todas estaban dentro del 5% de los 5K ohmios.

gracias fred

We tested all 'boots' and all were Ok.
But ohm meter works with low voltage DC and 'boot' really works with high voltage and vibrations. 
I'm not 100% sure about this, but after all testing is the only thing that improves behavior.

Anticepto, when we changed module connectors, first horizontal connectors (from trigger coils) and then vertical connectors (to HV coils), results indicate that the problem was in high voltage coil 1 or 4, those connected to module B, in the HV cables, 'boots' or spark plugs of the coils 1-4.
So we checked the resistance of the pipes and considered them good.
There was nothing left to change and we tried other used 'boots' with very good results.
Today we will install new spark plugs and NEW 'boots'.

[Anticept]

I was speaking in FredG's direction on the issue (or are you and he both involved with the same airplane?)

FredG, I have a question:
 

Quote

 

When the six-pin plugs from the trigger coils to the ignition modules are swapped (B trigger coils to A module and A trigger coils to B module), the roughness changes side (ie, the single ignition switch position that was rough became smooth and the formerly smooth side becomes rough).  

Given the above, it should be no surprise that swapping the six-pin plugs from the ignition modules to the ignition (high voltage) coils also changed the side of the roughness.

 

When you say this, did you swap the low voltage side connectors back first, then changed the high voltage side? Or did you have them BOTH reversed by the time you completed your testing?

[FredG]

Corey, I did both.  First, the six pin connectors from the trigger coils to the ignition modules were swapped and the problem changed sides.  Then, after restoring the A connector to the A module and the B connector to the B module, the six pin connectors were swapped again, but this time the power (red) and p-lead (to the ignition switch) were kept with their original modules.  The problem again switched sides.

 

[Anticept]

The reason I ask is because lets say cylinder 1 top is attached to module A.

If the problem is cylinder 1 top, running on module A would run rough.

If you swap the connectors on the airplane side, it will change key positions, following module A.

WITHOUT swapping the connectors back, if you then swap the coil side, it will switch to module B, changing key positions again, because now the cylinder 1 top is connected to B.

But, if the problem is with a trigger coil attached to A and NOT cyl 1 top, swapping the airplane side will cause the key position to change because now the coil with an issue is now attached to module B.

WITHOUT changing the airplane side back, you swap the ignition side, but this time the issue would REMAIN on module B and the key position where the roughness occurs would NOT change. That's because theoretically if nothing is wrong with the equipment after the modules, they will perform the same, thereby showing that the issue is not after the modules.

You can then prove if the modules are the issue or not by substituting another known good set or putting them on a known good engine, leaving you with only the stuff behind the modules.

[FredG]

Corey, your logic is correct.  At this point, the only part not tested by replacement are the ignition coils and wires.  The trigger coils were replaced, the ignition modules were replaced, the spark plugs were replaced and the key switch for grounding p-leads was removed from the circuit.  

We also rebuilt a set of carburetors from another airplane and installed them with pneumatic balancing.  The roughness seemed to decrease substantially, but the airplane has not yet been flown.  So, I am not quite ready to declare victory.  

Also, although the propeller has been dynamically balanced, we may check the balance again and will also ensure that the prop blades are set to equal pitch.

 

[FredG]

The engine is running much smoother now.

To recap, the following did not help:
Replace all spark plugs
Replace ignition modules with known good units
Replace trigger coils with known good units
Test resistance of plug caps and high tension (plug) wires
 

Taking a set of carburetors from another engine, rebuilding them fully (including adjusting the idle mixture screw), resulted in a much smoother running engine.  At this point, no additional repair or troubleshooting will be attempted.

As noted by Tom Baker, above, when the problem changed sides with swapping ignition component plugs we inferred that the problem was ignition and not induction.  It is confusing that induction can result in poor running that changes sides with changing ignition components, but that appears to have been the case here.

Thanks to all who offered suggestions.  

Fred

[Anticept]

One of the things I have seen that have an effect, is how the carb is "tilted" in the socket.

More than a few degrees, and it starts to change how the engine runs, but not always. Temperature, fuel type, etc all have been a factor.

I'll tilt the carb ever so slightly if I am getting one set of plugs that are way more dirty than the other at inspections to try and make the front/rear run a little richer/leaner. It doesn't take much!

[Tom Baker]

I want to take a moment and expand on why a carburation issue presents as an ignition issue when troubleshooting. Let me also preface this with the fact that I am not an engineer, but have been a pilot and mechanic for over 40 years. What I am sharing are things I have learned over that time.

My first introduction to this came as a pilot. I was flying with my boss at the time in a Cessna 172. The said boss was a cheapskate, and tried to pinch pennies where ever he could. He wanted me to lean the airplane when we were flying, even at lower altitudes. The problem was there was no EGT. He taught me to simply do a mag check. If you are to lean it will run rough on one mag, while it is still smooth on both.

The second came with an old Aeronca running rough at lower power settings, and really rough on one ignition. After going through the ignition with a fine tooth comb, we took a look at the carburetor. It had a blocked idle passage causing it to run lean at lower power settings.

Third was a CTSW. Outside air temperature was around 20°f. Ignition check was greater drop than I was used to, about 250 drop, and also running rough. Both ignitions were the same. I had been through the carbs, and put everything back just like it was. I went back and took another look at the carbs, and the circlip was in the #2 slot instead of the #3 slot causing it to run lean. That on top of the cold air was causing the issue.

So why does the carburation cause an issue that appears to be ignition. Under normal operation both sparkplugs are firing to give the desired controlled burn pattern of the fuel air mixture in the combustion chamber. This controlled burn pattern can make it so a slightly lean engine still feels smooth. When you shut off one ignition it changes the burn pattern in the combustion chamber. Due to the design of the cylinder head the swirl pattern of the fuel air mixture is different between the front and rear cylinders. Between these two things if the fuel air mixture is on the lean side it will cause the burning of the fuel air mixture to not be smooth hence the roughness tied to the ignition. If it is just one carburetor that is lean, you will feel the roughness, but the typical RPM drop will normally be 50-100 more than usual. Most ignition issues will cause a greater loss of RPM. 

[FredG]

Tom, thanks for the explanation.  When I discussed the rough engine with Bret Lawton from Leading Edge Airfoils (a ROTAX distribution center), he emphasized that I should not do any diagnostics on the electrical system until I was absolutely positive the entire induction system (including carbs, of course) was completely correct and problem-free, no matter what changed when switching from one ignition to the other.

To those of experiencing (for the first time) a carb problem manifesting differently on ignition A versus ignition B, it can be misleading.  Lesson learned!

Corey, thanks for the mention of how the carbs are tilted.  It is interesting that the socket has no "keyway" or other designation that permits greater precision regarding tilt (rotation in the socket).

[Anticept]

The technique I mentioned is still useful to at least eliminate the issues on the back of the engine, and only takes a moment to do.

Usually when I troubleshoot, I go after the fast, low hanging fruit first, and swapping the pins is pretty fast if nothing is obvious.

[Jim Meade]

If you swap pins does that eliminate an induction problem?  If not, then one could chase one's tail.  Does low hanging fruit mean something easy to check, even though it may lead to a misleading symptom, or is low-hanging fruit an exercise that has a high probability of finding the problem quickly?  And if it finds an ignition problem, does that rule out an induction problem?

[Anticept]

Jim, please see this post and worth through the logic. Doing this will eliminate a large part of the ignition system as the culprit. That's what I am referring to.

 

 

 

[Tom Baker]

Low hanging fruit equals the easiest to check items.

Swapping pins does not eliminate an induction problem as Fred found out. With the induction problem the fuel air mixture is out of whack as it goes into the combustion chamber. With both sparkplugs firing you will not notice the problem, but with only one firing like during an ignition check the fuel air mixture doesn't burn smoothly, and you get the roughness and drop in RPM. The problem shows when you check the ignition, but is due to the fuel air mixture.

Troubleshooting can be tricky. If you take away one cylinder on an ignition check you would expect a drop of around 400 RPM, if you take away two the drop is around 800 RPM. With this kind of drop I would immediately expect ignition, or one carburetor completely blocked from fuel. Had that happen once. If the drop is smaller and consistent rough I would expect induction. A small drop that has an erratic roughness, more like occasional bumps, back to ignition. 

[Anticept]

I didn't say it would eliminate an induction problem.

I said it would eliminate the back of the engine as being the culprit and narrow down the issue. You would do this towards the beginning of troubleshooting when you see roughness between the mags.

If it's a module/back of the engine problem, swapping the plugs to the front of the engine won't change the side the issue is on.

If it's a front of the engine problem, swapping EITHER set of plugs will cause the issue to flip sides, but if you just swap the ones going to the front of the engine is the definitive one.

[procharger]

A timing light will tell you more than anything if its intermittent. You can see

if it's solid blinking or not. You rev engine up to high RPM and see if light changes

under power. I have used this method a few times.

[Gabriel]

Solved !

Looks like two mixed problems.

Firstly, some kind of "boots/ HV cables" problem.
Then we have detected a "Throttle Valve" deviation in one carburator. 
A slight deviation in the piece caused by a poorly adjusted screw.

When the carburetors were changed there was the problem of high tension (boots/cables) and it made us think that the carburetors were fine.

Regards and good luck.