When I first got my Westy seven years ago I ran right out to the FLAPS
and replaced every vacuum line with new vacuum line.
I fixed all the leaks but it didn't run as well with the new lines.
I went back to the FLAPS and bought the rigid small ID line like the
original lines and installed them using the new rubber lines as
connection and it ran better.
I can only surmise that the smaller ID acts as a regulator device and in
some situations is more desirable.
I still use the smaller ID rigid lines and haven't done any swapping of
the lines to either prove or disprove my supposition.
However I always use only the smaller ID rigid lines on any Westy I
restore and it often improves engine starting and performance. I always
find a wide variety of leaks, fuel, vacuum, oil etc on these old *near
dead* Westies and the rigid lines seem to help.

Stan Wilder
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