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50/50 mix


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Today while surfing for any article I could find on fuels, I came across this interesting article from another forum.

Someone had the same questions we often talk about on this forum, re; 912 ULS fuels.


I don't remember the name of the mechanic that has disassembled many of the 912 engines but according to him, engines that used the mix of mogas and 100LL, 50/50, were extraodinarily clean when disambled, straight mogas came in second as "clean" and 100LL exclusively came in last with the word "dirty." No word on oils used but I presume they followed the Rotax reccomendations.


Roger or others care to comment on his findings? It would be appreciated, as a few of our type tanks have developed leaks, even though the factory said they were suitable for 10%E.

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My questions would be, first, what kind of "unclean" did they find with mogas? Second, why? (Ethanol is not "dirty" lead usually is.) Third, is there something about the Rotax that makes it not work as well with auto fuel? (If so, Rotax needs to know so they can fix the problem on all their recreational products.)

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Ethanol fuel is just fine for the Rotax engine. It is okay for the FD aircraft. If you own another type aircraft you just need to make sure your fuel tanks are okay with the ethanol.


Fuel preference in order would be:

straight 91+ oct. auto fuel ethanol free,

91 oct auto fuel with 10% or less of ethanol,

a 50/50 blend of auto fuel and 100LL

and last is straight 100LL with a lead scavenger like decalin.


The main items that may be considered dirty on disassembly would be lead deposits and or excessive carbon build up.

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