Acciaccatura distoring spacing

[David Sumbler]

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\version "2.19.82"

\paper { line-width = 150\mm }
#(set-global-staff-size 16)
<<
  {
    \new Staff \time 5/4
    r2 \tuplet 7/4 { g'16 a' b' c'' d'' e'' f'' } g''8 r r4 |
  }
  {
    \new Staff
    \tuplet 3/2 4 { \repeat unfold 3 { r8 \repeat unfold 3 {
\acciaccatura a'' g'' } }
                      r8 \repeat unfold 2 { \acciaccatura a''8 g'' } }
|
  }
>>
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

When the above is compiled, there is an unwanted gap between the 3rd
and 4th septuplets in the upper staff.  This undoubtedly has something
to do with the acciaccatura in the other staff.

(There are 10 other staves in the original passage this is taken from,
some of which also have triplet quavers but not acciaccature.  Removing
the one line with the grace-notes gives an evenly spaced septuplet.)

Is there any way of getting the septuplet evenly spaced?

David