That definitely looks like a Weber conversion, i'm wondering if what had happened was the "O" ring on the anti-run on valve had perished and was admitting air - essentially a small air leak, just enough to make it run excessively lean.
When you removed the solenoid for the anti-dieseling/run-on valve, does the plunger in it have a conical tip?
If so, is it gummed up around the edge where it seals the airways inside the carb off?
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Cheers
Dave
Next Door to Top-Gun with a Honda CR-V & S Type Jag  Volvo gone but not forgotten........
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