Steve,
I think you mean 18's and 31's.
I have done this and swapped all over from big valve spec from the 31's to the 151's. You would think there would be no difference but there was. Big fat stumble/hesitation that, for the life of me, felt electrical. It was not. I started lifting idle jet sizes and was getting better. I lost some confidence when a prat supplier told me I was doing the wrong thing. I should have carried on and may have sorted it out (Des Hammill says so). I was up at 2.5 turns on the mixture screws which tells you the idle jets are not big enough.
I did the Lotus thing and dropped the chokes from 33 to 30mm and also went down with the main jets and altered the air correctors (per advice). So good was the result I have never touched it since. One day rolling road maybe or may put the 33mm back etc and buy a handful of idle jets and try it out. I think it fair to say that we underestimate the amount of time we spend on idle jet fuel and progression holes and not on main jet and butterfly. I know little about all this, others so much more, but that is my 151 carb tale.
Mike