I had a similar problem in my 85 Westy....After trying every combination of
lighting in the world I accidently discovered that if you turn the
temperature down to min from max before you attempt to light you will get
the flame to light.  And then after it lights wait a few seconds and then
slowly turn to max.  My therory is that there is too much gas flowing and it
is blowing out the flame before it gets a chance.

Malibu Tony.......... 85 Westy Wolfburg