Most carbs/ jets / float -valves get blocked by contaminated fuel either from filling dodgy tanks , old fuel pipes breaking down -perishing . 2stroke oil cans usually have grit stuck around the neck if left about ,i keep my filling funnel in a plaggy bag until i need it ,, though i have know problems with some near brand new engines where overzealous tank fitting sealant has dissolved , gone through the filter then left a residue in the bottom of the float . Insects can also get into carbs if left stored for long periods ,,, you only want a spider having a crap on top of a jet : )
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