[boxhead]

Have you got a good quality inline filter fitted?
When I had my datsun 180B SSS & my dune buggy it was very important to use a good inline filter.
I honestly never had any troubles I believe because when I put it all together (includeing rebuilding the carbs) I had no troubles at all, everything was clean and no dust, no rags, only solvent and compressed air was used to clean them.
My carbs in both cases where side draft so allthough not the same they where very similar, 45mm side drafts.
If the carbs are clean and the lines are clean then a good filter should stop everything.
Try a new filter if you allready have 1, maybe it is where the crap is commeing from.