John,
While I'm not totally familiar with the '76 TR6's emissions equipment (I'm
not planning to use the '76 engine in my rebuild), I do recall a similar
event with my '70 MGB years ago. The problem turned out to be a faulty "gulp
valve" which controls air injection from the air pump into the exhaust
manifold mostly during high vac throttle closure like between shifts or
deceleration.
If the air flow is not present during high vac, the exhausted mixture of
gases is not "leaned out" in the manifold and pops (detonates) somewhere down
in the hot exhaust system.
Just a hunch...good luck.
Chip Krout
'76 TR6 CF57822U (Being Reborn For Y2K)
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