misfire at 4K

[Colin Thom <colinthom@shaw.ca>]

Hi Gang, 
 
I've got a misfire at almost exactly 4000 rpm on my 1975. The engine is
a symphony of pistons up to that point, and fine once I get past it, but
at around 4K it stumbles. No backfires from either the tailpipes or the
carbs. I've already replaced the condensor. Any ideas? Thanks as always
for the guidance colinthom@shaw.ca.
Thanks. Colin CF31098UO