\version "2.8.2"

\layout { ragged-right = ##t }

% manual setting, this works:
\relative c' {
  \set tupletSpannerDuration = #(ly:make-moment 1 8)
  \times 2/3 {
    \repeat unfold 4 { f16 [ g a bes a g ] }
  }
}

\relative c' {
  \set tupletSpannerDuration = #(ly:make-moment 1 8)
% not manual, setting the same to quarters doesn't work:
  \set Voice.beatLength = #(ly:make-moment 1 4)
  \times 2/3 {
    \repeat unfold 4 { f16 g a bes a g }
  }
}

\relative c' {
  \set tupletSpannerDuration = #(ly:make-moment 1 8)
% not manual, setting the same to eight's does work:
  \set Voice.beatLength = #(ly:make-moment 1 8)
  \times 2/3 {
    \repeat unfold 4 { f16 g a bes a g }
  }
}

\relative c' {
% normal sixteenths do work for quarters:
  \set Voice.beatLength = #(ly:make-moment 1 4)
    \repeat unfold 4 { f16 g a bes }
}

\relative c' {
% normal 32's don't work for quarters:
  \set Voice.beatLength = #(ly:make-moment 1 4)
    \repeat unfold 4 { f32 g a bes }
}

% normal 32's do work for halfs:
\relative c' {
  \set Voice.beatLength = #(ly:make-moment 1 2)
    \repeat unfold 4 { f32 g a bes f g a bes }
}