A clever bugger like you will be able to sort it out. Mechanical fuel pump pressure is dictated by the spring under the diaphragm. probably about 2 1/2 psi your electric pump might be about 4+ psi Maybe too high. Volume is another issue. A hard diaphragm or lost travel, due to wear reduces the volume, but this wont be the cause of the running rich.It will make it starve under power.
Pull the cold start bit off the carb, and clean it up.
If its running really rich, it will soot up the plugs, and make it run like a sack of......You know!
It's British, so everything possible can go wrong. It exists to make you think everything else is easy. I looked into the engine bay of a ferret once.... That was enough for me. Then I did up a trans, and a few bevel boxes. I always associate the two words "Ferret, and "Masochistic" together.
