I am delighted to report that the engine is starting and running nicely now. Three problems were found. First of all, the fuel metering pop off pressure was far too low! It was around 10 psi. The recommendation for this model Walbro is 15 - 25 psi. I adjusted it up to around 20.
Second problem was the spark gap was too big. Late last night I measured it a bit more carefully and it was close to .68mm. My pal Barry was here who has just built a race car and described the same problem with his modified Zephyr engine. Spark failure/flame out with throttle. He adusted his plugs down to around .020 (approx .5 mm). I did the same to mine, and now it take fully throttle without any miss-firing. Easy starting too.
The third problem was a leaking connection of the rubber manifold to carburettor connection. It slipped out slightly after I remounted it and ran it for a while also. I will machine a thin walled, flanged spigot to go inside it on Monday when I go into the Mens Shed. That will located it internally and externally positively.
We had the engine running nicely at all throttle levels. It was idling well, until the manifold rubber started to slip again and leak.
This problem will be an easy fix.
Many thanks to Av8tor for the pop off pressure test tip. And to Al for the intake leak. To Barry for the spark plug gap suggestion. And of course to Zagnut for his endless patient technical descriptions and corrections. To Bogbeagle and to all the others who have hung in there on this thread throughout this tiresome and sometimes boring saga!
And a special thanks to Gizmo for boasting that he could fix it if he was here!