I sure do not know about Series 3's but have suffered with Stomberg floats leaking over the years causing poor running engine. I also had vacuum leak at a gasket that stumped me for a long time. I also think I remember the needle valve on the later Strombergs being adjustable with a special allen wrench that reaches down to adjust at the bottom of the damper oil reservoir. Just some thoughts on things to look at or play around with if you are sure it is some sort of carb or vacuum leak issue.
Recently on one of my 70's Chevys I fixated on a poor running engine being a carb problem. Turned out to be an ignition coil that was breaking down internally and going bad especially when hot and at low RPM. I could have saved hours of work trying to figure things out if I was not so sure early on that I had a carb problem instead of an ignition problem....but many times I am not the sharpest tool in the shed trouble shooting-wise.
David Barnes
68 E-Type FHC