912 ULS rough running at 4500 rpm

I have a 912 ULS that runs rough after the engine is warmed up and flying - turning the ignition switch it appears on both A and B systems but is more pronounced on B .
I have done the following:

- replaced the carb floats and also sent the carbs to Lockwwood, they gave them a clean bill of health
- replaced the ignition module and tried on both sides
- replaced the plugs
- replaced two double ignition coils ... one showed ohm variation with heat
- gaped the trigger coils
- installed carb heat

I'm out of ideas --- any suggestions?

Karl

Stupid question: did you go through a proper carburettors synchronisation?


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Ignition wires?

Check leads where they attach to plug cap if that centre screw isn’t correctly installed may be your issue hard too. Find probably just have to check them all, may get an idea from plug colour

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Originally Posted by umberto.orlandi

Stupid question: did you go through a proper carburettors synchronisation?


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Hi Umberto - the answer is no, I suppose I assumed that the factory looked at that when I sent them back, but i could be wrong... that will definitely be my next step... thanks

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Originally Posted by jrevens

Ignition wires?

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Hi John, the new coils came with new wires already attached, but thanks for the suggestion

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Originally Posted by sturdee

Check leads where they attach to plug cap if that centre screw isn’t correctly installed may be your issue hard too. Find probably just have to check them all, may get an idea from plug colour

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Thanks, I plan to examine the plugs tomorrow

In the heavy maintenance manual there is a resistance value for the plug caps. I'd suggest checking these before attaching them to the new coil-wire assemblies. Replace them if necessary (mine are snowmobile parts of the same resistance value at a fraction of the cost).

I also have found that gaping the plugs to .5-.6mm (.020-.024") works better with these ignitions.

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Originally Posted by hallschuch

Hi Umberto - the answer is no, I suppose I assumed that the factory looked at that when I sent them back, but i could be wrong... that will definitely be my next step... thanks

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Synchronizing has to be done with the carbs installed on the engine and running. However, at or near wide open throttle, synchronizing is much less of a factor in rough running.

agree with everyone above regarding the sycronization being key - I wouldnt really dive down a rabbit hole on any other inspections or checks until that is done first. Once the carbs are synced - if the issue persists, then dive into plug cap resistance and electrical connections. While it could very well be an electrical issue, attempting to troubleshoot an engine that isnt tuned to the base levels will only have you pulling your hair out chasing gremlins.

Any updates?!?!