I have actually made some progress this morning. Av8tor is on the money here it seems. I also have been suspecting that the pop off pressure might be the trouble, but did not know enough about it to tackle it.
I have two WJ57 carbs. So I pulled the fuel metering lever springs out of both of them and compared them. The new aftermarket one is noticeably weaker. This would cause a low pop off pressure and would explain why no matter where I bent the spring to, the engine could not be leaned out.
I now have the engine starting easily using the choke!
It will run ok and is good at mid-range and high rpm.
It will not idle and cuts out.
One of the suggestion in the Walbro chart for this, is sticking throttle butterfly. I will investigate after lunch.