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RPM drop at Run-up Conundrum

WingedFrog

Well Known Member
Last winter I started facing an unusual drop at run-up after starting the engine. It was with Circuit B off which I interpret as affecting circuit A. As the drop was about 300 accompanied with roughness and fluctuations I did not take-off and taxied back to my hangar. When I repeated the run-off at the hangar, the drop had disappeared and I went on and had a normal flight. The same problem reoccurred through winter months. When Spring flying came and during summer I had little trouble but still noticed some roughness after the engine started although it always disappeared at the run-up after taxiing. Came the colder months and I am again in the last winter situation so I decided to troubleshoot the ignition packs by switching the input from the generator between modules A and B. The roughness and drop appeared with A off this time! I concluded that the module A was faulty and decided to separate myself of $900 by ordering a new ignition module. Once the new module installed I had the same problem on the new module in position of A (i.e. with B off)!!!
I then did what i should have done first: looking at the schematics in the Heavy Maintenance manual (Fig 74-18) I concluded that I had misdiagnosed the module A and that the problem was likely coming from the generator, more specifically from one of the two trigger coils for ignition circuit A referenced as #2 on Fig 74-18.
I would appreciate some advice on this issue as my area does not have Rotax 912 specialists. My main concern now is how to access and troubleshoot these two coils. I am not sure the access is possible without removing the engine from its mount.
...In the mean time I have got an extra ignition pack for future failures!
 
Couple of questions...

  • How many hours on engine?
  • Are you checking RPM drop at 4000 RPM?

I had same problem and was convinced one ignition circuit was faulty. Engine has 500TT. I rebuilt both carbs and problem went away.
 
Jean-Pierre

I found an email from Lockwood Aviation that describes a series of steps to diagnose carb Vs. ignition...

Test 1: Put the engine where it’s unhappy – say, 3500 rpm on ignition circuit B – and pull the choke full ON. You are likely to get an rpm rise. Ignore that. What you are looking for is, does the roughness get better or worse, or does it stay the same. If it gets better or worse, the problem is likely to be carburetion. If the roughness stays the same, the problem is likely to be ignition.

Test 2: Swap the two horizontally mounted 6-pin connectors. Do the mag check again and see if the problem moves from one side to the other. Write down the results. Put the connectors back where they belong.

Test 3: Swap the two vertically mounted 6-pin connectors. Do the mag check again and see if the problem moves from one side to the other. Write down the results. Put the connectors back where they belong.
 
Thank you Jim, your advice is appreciated and I will follow it and report on the results. The engine has 258 hours on Hobbs and I check the RPM at 4,000. I have swapped the two connectors coming from the generator that feed the ignition modules as I reported in my post (I cannot remember if they are vertical or horizontals!). The RPM drop switched from module A to B (i.e. it happened initially with B off and now with A off). Outside the 4000 RPM test, I also noticed some roughness when the engine is started cold and runs in the range <2,500 RPM as recommended by Rotax. When I shut the good side in this condition the roughness increases and the range varies widely with excursions over several hundred RPMs. All this disappears when the engine is warm well beyond the 122 F recommended for run-up.
 
Sounds like what I had going on back in the spring and early summer. Check carb floats to be sure each pair weighs less than 7 grams. As you can probably tell, I'm leaning toward carburation...
 
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