Do you mean 40IDA vs 40IDS ? (The IDF is a 2-barrel carb whereas the IDA/IDS/IDT's are 3-barrel).
Essentially the IDS was a modified IDA which incorporated an extra high-speed enrichment circuit. This apparently gave no increase in performance but was meant to be a safety feature to prevent destructive detonation at high revs.
Andy Boyle(UK)
'69 911E
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