deposits on DCPR8E spark plugs

Wayne Lindebaum wrote:

Looking at those plugs I'd say you have failed valve stem seals that does that  that buildup is oil ash you most likely not using Rotax oil but a different brand of similar oil.the extra carbon in the cylinders will cause pre-ignition that causes the melting of the carbon look the pre-ignition also fretts the crankcase  is have the engine check out and services repaired by a good rotas service agent. I saw a motor destroyed in 650 hrs as he saved about 500 in servicing costs but that intern cost him a new motor  so he threw away 20k to save 500 

Hmmm.  I have always used Aeroshell Sport Plus 4 which I understand is the recommended oil for the 912ULS, so if failed valve seals are my problem, its probably not the oil at fault.

Here is a better photo of the lumpy deposits.  They scrape off easily with the lightest pressure from a knife.

Looks like lead fouled,try unleaded fuel.