I'll probably get shot down for this so It'll be my only post on the subject.
Ethanol in the fuel.
It isn't designed for our engines and I believe it is a contributing factor in the problems with the big ends in Rotax engines because it is hydroscopic and the oxygen encourages rust to form on the rollers and crank pin when the engine is used infrequently.
This is made worse because the seal prevents you from directly oiling the big end through the reed block to prevent the corrosive when the engine is left.
The 2% oil mix is too low to protect the engine when not in use.
I believe ethanol is only added when the fuel is in the tanker because it is so corrosive.
It affects the fuel/air ratio so your jetting may not be what you think it is or should be if you are using jetting software.
When it's added to the tanker, could it only make a short journey to the filling station, resulting in an incomplete mixing.
It doesn't t matter so much with car engines with ECU's, but you have a 'race' engine (cough) with no such compensation.
So in answer to your question, yes it could be fuel, most of us have chased a problem and suspected the fuel.
I believe Texaco are the only company to not use ethanol in certain parts of the country, they should be able to provide you with a list of filling stations that are e thanol free.
Super unleaded doesn't usually contain ethanol but the Rotax isn't a high compression engine so needs the lower octane fuel.
Google 'ethanol in petrol' and you will probably find a more detailed explanation of just how crap the stuff is for your type of engine.
On the other hand I could be talking tripe.
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