I can only imagine that the problem could be that with the TYGA seat unit the reg rec doesnt get as much air as normal or the position of it on the TYGA sub frame isnt as solid as the OE one.

Either could be helped by bolting the reg rec onto a fair sized peice of alloy then fitting that to the subframe to act as a larger heat sink.
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If I have to take the carbs off once more...