At 40:1 you're gonna be fouling plugs like crazy if the sled is idling or just putting around alot. If you're gonna ride the balls off it, then I would mix 40:1, but if you're just going to be putting around, then mix 50:1.
50:1 is normally fine if you are using a quality oil. However, if you are using standard 2-stroke oil, you are better off running a richer mixture, simply to be on the safe side. You can always look into a jetting guide if it is a major concern, but if I were you...I would run between 32:1 and 40:1, replacing spark plugs every couple hundred miles or so if they foul. That's what I do...
For the record, I run around 32:1, and I don't foul plugs like crazy. I use NGK, maybe plug manufacturer has an affect?
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