Did you install a fuel pressure regulator (if you have weber carbs)?

Also, did you check for a vac leak (I remember this happened to me; turns out the gasket that sits between the manifold and the carb's had a hair line fracture: when car was cold and metal sort of shrank it would be problematic; but when car warmed up and metal expanded the problem would go away).

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Current:
1971 Mercedes Benz 250 (Euro Spec)
1972 Mercedes Benz 250 (US Spec)
Past:
1972 Mercedes Benz 280