Rotax 912 ULS, intermittent rough running during engine run up and in flight.

[GDM]

For owners of Rotax 912 ULS engines my recent experience of intermittent rough running on engine run up and once in flight

may be of value solving this issue if it ever arises.

on a recent flight with a friend there was an intermittent vibration of an otherwise very smooth engine. The Rotax 912 ULS has just over 400 hours flight time and the annual and 5 year rubber replacements had been just completed by an experienced LAME.

Runups to 4000 rpm before flight were excellent, drops of 30 or 40 rpm on both L and R.
We flew down to the surf coast and the Port Phillip Heads uneventfully. On the return my passenger and I felt a shudder, the passenger thought we hit a bird, I felt otherwise. The shudder returned a number of times intermittently. I though I may need to check if it was a capacitive box problem. So In flight I switched from BOTH to LEFT and the engine wanted to stop. I quickly went back to BOTH.

on landing I did some runups and got some rough running on the LEFT switch.

Over the next few weeks and flights run ups were smooth , so I flew and sometimes the runups were out of spec. So I tacied back to the hangar and jiggled wires and spark plug leads. This often lead to smooth, problem free runups and flights.

the problem escalated and the intermittent rough runups became more common. This issue needed to be solved!

i watched a Rotax training doco on YouTube and followed the methodology, change one variable at a time.

this involved switching the connectors coming from the stator to the Capacitive boxes A and B. 
Result, Left switch still causing rough running.

return that connector to its original position then swap the connectors from the boxes. Same result, Left switch , rough running.

Add starting carb at 4000 rpm on right switch, resulted in more revs, smooth running, but adding start carb on left switch resulted in greater rough running.

at this stage I realised that the issue was from the left male connector back to the stator.

I tested the resistance of the coils on the stator, all came out at the book 250 ohms, continuity was fine, no breaks.

I tested the red leads which power the Capacitive boxes by cranking the engine under load by slipping a few stands of wire into the connectors. They gave good values 7.6 volts ac at 500 rpm.

the final test solved the problem. The earth wire…..I set up the multimeter, one probe on the earth wire on Box A, the LEFT, white(the heavy maint. Diag. Shows brown) and the other lead to a ground on the engine. With the ignition switch on OFF, the multimeter set on continuity screamed as it should, on Right it screamed  as it should, but…when  on LEFT it should be quiet..

it screamed. Box A, the LEFT was earthing! A quick inspection of the wire found the break. There was a failure in the plastic coating of the wire and it was touching on a metal braided loom. 
An easy fix. 
I hope my experience is of potential help to somebody else.

 

 

 

 

 

[RFguy]

this wire that failed- did it fail due to insulation cracking (maybe due to continuoue expose to solvents) ?

did it fail because it was rubbing / chafing  on something else?

Was it subject to repeated bending, even very small amounts????

 

[GDM]

It was a straight section of wire, so I believe it failed due to rubbing.

 

[IBob]

Something similar happened with colleague's aircraft.

I believe the braided Rotax loom was pinched between the engine and the ringmount, resulting in a short and the intermittent loss of one ignition system.

[skippydiesel]

Very well done GDM - Following a well considered steeped diagnostic process is THE way but unfortunately we (me included) all too often, leap to conclusions - usually the most expensive ones.

[GDM]

Rotax Tech Talk:

Troubleshooting the 912/914 Ignition system

[RFguy]

I'd like to see rotax wiring and looms built with Tefzel and Mil27500 series shielded wire....etc  You can get multi colours in tefzel, also.  it's not all just white. 

[perrynz]

Hi GDM. I had similar if not identical problem. I posted the following 21 June 2019:

 

Hi Friarpuk. I had on-going ignition problems with my 912 after about 20hrs from new. When carrying out the pre take-off ignition checks, I would regularly experience RPM drops of 400 to 1000RPM, even to the point that the engine would basically quit but then spring back to life again. I tested and suspected lots of things. Checked earth connections. Changed all of the spark plugs etc etc. Eventually, in order to isolate a CDI unit failure/problem, I swapped the CDI unit (6 pin) plugs around. (The ones that come out of each CDI) This immediately fixed the problem, but I didn’t know why. (proved it wasn’t a CDI fault anyhow)

 

Alas, after probably another 20hrs the problem reoccurred. Long story short, after pulling most of my hair out, and thinking life without an aeroplane would be much simpler, I stumbled on THE fault. One of the two brown wires (that go to the ignition switches) had been fretting itself on the bracket that holds the CDI units. When I swapped the 6 pin plugs over, it had temporarily re-routed the wire so it sat off that bracket, and the engine ran perfectly for a while.

 

I could see an area of slight discolouration on the suspect wire, and double checked my suspicion with a multi meter. Sure enough, there was bare wire earthing itself onto the bracket. Effectively, each time the wire touched the bracket due to vibration, it had the same effect as switching off one of the ignitions.

 

Anyway Friarpuk, it may not be your engine’s problem, but it’s worth a look. Here’s a pic. If you follow the brown wire down and enlarge the pic, you can see the fretted area.

 

Cheers. Perry

 

 

Edited November 16, 2023 by perrynz

[GDM]

I wish I had found your article weeks ago. Those little wires are very fragile.

[RFguy]

yep. should be wired in Tefzel...... and restrained regularly.

 

AND the CDI packs should NOT be sandwiched together- the bottom one will run as hot as hell (and they fail)

 

put some washers on the screws between them so they are spaces at least 3mm

 

Rotax is good with mechanical stuff but they know F.A. about good electrical installs. 

-glen

Edited November 16, 2023 by RFguy