Backfiring on Deceleration...

[Rev]

70, with 27 degrees initial advance, something is definitely off. Could be you're 1 tooth off on the distributer, but I don't think so. What I really think is that your centrifigal advance mechanism in the distributor is failing to give you any advance at RPM, leaving you with 27 degrees at all RPM's. This would leave you with an inadequate advance at the higher RPM's resulting in the phenomenon that you describe. Put a timing light on it at higher RPM's and see where you're at. I may be totally off my gourd on this, but it's worth a look.

Rev

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'66 Coupe, 306, windage
tray, TRW flat tops, 351W
heads,pocket ported, 268
CC hydraulic cam, R tip
R's, Edelbrock Performer,
600 CFM Holley vac. secs.,
Mallory Unilite, coil, fuel
pump, Tri-Y headers, 2 1/2 "
custom pipes, 3 chamber
Flows, stock C-4, 3.25
Currie built 8" rear,
March under drive pulleys,
13.97 e.t., 100.1 mph
1/4 mi.