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You could try some green bearing retainer and let it dry. Should stay in place w/you put the bowl on though, I thought.
This pin will not come out once the float bowl is fitted; the protruding grooved end should almost touch the bowl to prevent it moving.Surprised yours has come out. Put it in place minus the float bowl and see how far the plain end enters its locating hole. Maybe it's not quite long enough for some reason.Easy to see on this photo (which was taken prior to strip down and deep clean during my rebuild).