Not sure if it's relevant, but I have had a similar issue with my MGB on twin SU's.
Can't remember the exact science, but it goes something along the line of the hot underbonnet air is different to cool air, and it make the fuel mixture richer. Leaning the fuel mixture corrects it.
MG had been running fine for ages, but was taking a while to warm up in the morning so thought I'd richen the mixture slightly. Straight away the hot running problem started again, so reverted back.
