I found this info that relates to the mot emissions testing. Looks like pre 1975 motors are exempt after that it starts at 4.5% and gradually reduces until make and model settings take over.
If your motor exceeds these figures then the mot testers will weaken the mixture till the correct reading is obtained – trouble is this will seriously effect the driving and performance of your car.
My car is over 40 years old so would rather have a fail if it only fails on emissions and still be able to legally drive it on the roads.
MGB tips mods and maintenance 