Troubleshooting Ignition issues 912UL

Hi all

Just a comment.  The cable manufacturer would normally specify if you lube or not.  As noted some may be lubrication free and this should be called out by the OEM or cable supplier I would think.

As for balance, carb sync, normally the mechanical is fine to start and once we get into pneumatic balancing generally you should start at 3000 on a warm engine.  This is to be sure you are off the idle stops and into the cable ranges where there is tension on both sides.  If in balance at this point they need to track equally from that point on.  After this go back into the idle range to tweak idle stops.  So work on the cables with the adjusters then go back to verify idle stops are correct as in mechanical balance.  This is the fastest way I have found over the years.  

Cheers

Ok, I heard back from Pipistrel and they think that’s carb sync. They asked me to re-do the carb sync at 3500rpm. I can only work on my plane on weekends, so will do that this next one. I hope it is, that would be the easiest explanation and solution.

If it does not solve the problem, then my next suspect is a bad boot. In that case, I will run the engine in the bad “mag” and use a thermal camera to find which exhaust has the lowest temperature — if I have an inconsistent firing (or no firing) in a cylinder while running in a single lane, it will show up as cold exhaust gas. If that works out, I will try then to find which of the two boots in that cylinder is problematic by using an AC current detector… not sure if that works, but will try. If I can’t find which boot is not working, then my plan is to remove both boots and reinstall them swapped, to see if the problem changes sides. If it does, then will replace both by new. If doesn’t, then… well, boots will be ruled out.

PS: Before doing all that, I will inspect all cables for damaged, as suggested in this thread

" Before doing all that, I will inspect all cables for damaged, as suggested in this thread"

External inspection is unlikly to acheive anything much, as a worn cable may appear just fine, on the outside.

If all other systems are operating as expected, the carbs have been balanced, cables correctly seated & secured and you still have uneven balance indication, as you SMOOTHLY increase/decrease rpm from, idle to flying power and back, you likly have worn/damaged throttle cables.

If lubrication works, great! Be aware the improvement may be short lived. If short lived  - replace cables.😈

Hi Jeff,

Jeff Quote,

"I know you do a lot of balancing and I only work a few aircraft. I think I read a comment from you where you said you balance at 3500 RPM. I know that what sean says is true that the balance makes the biggest difference at low to idle RPM, but on the other hand you don’t spend much time at those RPM.  I find the most noticeable vibration on the carbureted engines is when you throttle back to 3500-4000 RPM so I pay attention to that range when balancing. What is your opinion?"

Hi Jeff,

It isn't that we are going to spend time at that rpm, but to make sure the carbs stay synced at all rpm high and low. I do a lot of carb syncs every year. One of the most misunderstood thing that mechanics and owners do. I cover this in major detail in my class. Once you know what the gauges are telling you and which adjustment to go after a carb sync takes 5 - 15 minutes. First I quit using 2500 rpm like the book said because when you do that you are so far down in the idle circuit that if you advance the throttle more to 4K rpm or more the needles split again. I have used the 3300 - 3500 rpm for the high rpm sync because when I do and then advance the throttle the needles on the gauges don't split again. The 3500 rpm is a little better are representing more air and fuel flow and is more out of the lower idle jet circuit.  I wouldn't call it perfect, but it works well and when I do a carb sync my needles on the gauges don't separate when you go to higher rpms. Then I reduce throttle to the idle rpm and then adjust a carb to either bring the idle rpm up or down and then make sure they are equal. Plus once you know what the gauges are telling you you can diagnose and troubleshoot. Can't really diagnose with something like a CarbMate. 

Note *** Always set the high rpm sync first and not the idle rpm. If you do the idle rpm first then the high rpm you'll have to do the idle sync part again.

Even though you may have a great sync on the ground then there may be a spot somewhere in the rpm (usually lower) where it isn't perfect when flying. Many experience this. That said it just isn't carbs causing these vibrations. It can be wind, prop out of pitch on the blades, prop not balanced or the gearbox needs attention. So other things are in play.

An example,

I just had a Flight Design CTSW come into the shop. It sounded odd to me so we did a carb sync. The owner said a mechanic just did the sync. I put the gauges on and ran it up to that 3300 - 3500 range and they were really close on the high rpm sync so I tweaked them, but that wasn't where a heard and felt the vibration. Once the owner went to idle the carbs were off by 6" of vacuum. The I reached over and pressed on the throttle arms to make sure they were all the way back and found the left side still had a ways to go back before even touching the idle stop screw so it wasn't adjustable. The left carb was getting more fuel than the right. So I had to shorten the left side cable so there was no more slack throttle at idle. Once the cable was shortened then I had idle stop screw adjustment I had to redo the high rpm and  then could sync the carbs at idle and pick the rpm I wanted to set it at.

Just to close the loop here, problem appears to be solved. Yesterday I unplugged the connectors at the modules A/B to measure the trigger coil resistance (which came all in range). I put it back, made sure the pins and connectors were well secured, made sure the grounds were well secured, and ran the engine to check ops and was not able to replicate the problem. Tried multiple times, including a test flight, the max drop I am getting now is 150rpm, and difference between mags around 50. All temperatures looking good and engine running smooth. I believe it must have been either a connection that became lose, or whatever contaminant in the spark plug area burning out. I will continue monitoring during run-ups. Thank you all for the help!