Touring the North West Coast of Scotland in two Austin Sevens, as a family

[Stuart Giles]

The spark plugs were not sealing in the head, this allowed air into the combustion chamber, Stuart.

Would have thought that a plug leak would have a lot more effect on power than mixture; because of the pressure differential between the partial vacuum on the inlet stroke versus the 'squeeze' effect of the compression stroke and also the huge pressure rise at the beginning of the power stroke. Whatever, a leak at the plug seat isn't good, might be worth using some PTFE tape on the threads/plug seating to stop/reduce the leakage until you can get home and sort the plug seat spotfacing issue sorted permanently.

Do you think this would weaken the mixture and cause the issues, we experienced? FYI, both cars were running identical plugs (D16) from the exactly the same source

IIRC, Champion D16 is equivalent to the NGK A6 plug, so not especially a soft/hot plug for an A7. So it does come back to weak mixture. Assuming an SU carb, is the fuel level in the carb visible just below the jet? float fuel level is probably the biggest governing factor for mixture on these carbs, this and the piston spring or piston weight has much more effect on mixture than the carburettor needle. But stopping/reducing the plug leak must be worth doing even if it isn'the root cause of this issue.

All the best for the rest of your family holiday.