Changing the idle jet for a richer one and a lot of fiddling made a big improvement and I almost wanted to keep the shiny Weber, but for some reason, after winter storage, the mixture seemed all wrong, so the search was on for the usual culprits like ignition faults and air leaks. What else could cause this sudden lack of performance? It must be something trivial. The interior of the Weber looked clean enough, but to be sure I filled the tank with fresh fuel. No improvement, so the search was on, culminating in a nice old SU HS.
Here you can see some shiny new bolts in place While rooting around down there I found the flange from the cat to the exhaust was loose and leaking. in principle that could cause trouble too, so I inserted some new nuts and bolts to reattach the exhaust.