Ive got the same problem with my TR250. No air leaks, but runs lean. This is the first set of strombergs I have had this problem with. A real shame since I think the stromberg is a fairly decent design compared to some other carbs. I tried playing with the amount and consistency of oil in the slides, which helped the driveability a bit, I tried closing off the idle bypasses, and the temp compensators, also blocked the decel thingies, and the vacuum retard. Now with the needles raised way up and the jets lowered down it runs well enough to idle sometimes and move without the choke, but it could run a lot better. Some jackass converted these carbs to fixed jet at some point but forgot to convert the slides to externally adjustable needles so that is my next step in the process, along with flatness checking the starter box mountings and some (hopefully) better starter boxes, but considering costs and the amount of manhours spent already I am seeing webers in this cars future....
Maybe someone out there has isolated this problem?