No leaks - no pong of petrol nor marks and stains. I do know a bit about Amal Concentrics, although am no expert, but spent a long time fettling the Enfield until needle, slide and jets were spot on (with a lot of help from Hitchcocks). What is odd about the Beezer is that the carb seems to set up fine by the builder, and it was checked by the bloke who sold it on (Eddy Wright of TT sidecar fame). It all looks new and I would be surprised if the needle and jets were not new. It was a nut and bolt rebuild and ticks over perfectly. Yes, it's full advance on the lever while riding. So I'm not entirely sure that it is running rich. There isn't an air filter to get blocked, and it's only done 150 miles since engine rebuild. Guessed (probably wrongly) that the light soot was due to running in and excess oil making it through the piston rings before they are properly bedded in. The centre core looks healthy enough - sort of chocolate brown, but there is a hint of soot. Because it is running so well I thought I would try hotter plugs, which have now arrived and I'll fit them tomorrow - Champion N5Cs.
If I am barking up the wrong tree and the bike really is running rich, I'll have to start thinking about the carb, perhaps raising the needle clip a notch as a starter. Apart from sooty plugs and dreadful petrol consumption, what are the indicators of really rich running? And could rich running really halve the fuel consumption?
I will also drain the tank and have a look at the levels of the tap tubes.
Many thanks for the suggestions.