912ULS occasional engine surging

Try some silicone baffle material and zip ties to wrap ignition wiring that passes through the intake manifold. The heat there can degrade the insulation enough to arc through to the manifold. 

Jack,

Do your plug wires have the protective sleeves on them? The sleeves are a silicone impregnated fiberglass strand material that protect the wires from physical damage, heat and arching.  Because the wires are routed parallel and close together in some areas, these sleeves are important.  Rotax used to only sell these as part of a complete set with the wires, but in the latest parts manual it looks like they now sell these by the piece. There are also other silcone/fiberglass plug wire sleeves sold at performance auto parts stores. That being said, I like the Rotax OEM sleeves.  

I believe the problem was pre-ignition. I enlarged the cowling air inlet and the problem went away.

Thanks for all the possible solutions.

Guys,

Thanks for all the good ideas. It turns out the problem was pre-ignition when the engine got hot. I cut two holes in the cowling and the problem went away!