Carbs, Etc.
Sounds like you have a buyer!
E9's exhaust is on the opposite side, so warping from heat generally isn't an issue. Over/ uneven tightening is though.
A rough or lumpy idle can be many things, including intentional as in a camshaft change or worn parts non related such as motor mounts that have long since given up but look normal.
A quick check is to cover the small idle air bleed hole next to the top jets/ vent while running. What you're doing is seeing if the mixture changes and the engine does nothing, increases speed, or decreases speed and in this case smooths out. If it smooths out appreciably, air is getting in somewhere it shouldn't and the hunt begins. The average mechanic would spray carb cleaner on it and pronounce AHA, warped! as the Weber legend goes.
But the guy who takes his time usually finds it something else like the secondary vacumn diaphram, choke body gasket, cold start valve we're discussing earlier, or simply a linkage adjustment; all very close to one another and act just like worn throttle shafts and warped bodies/ plates.
Balance- easy check with a vacumn guage first; if you don't have a fancy motometer/ manometer handy. Plug it in to each manifold, in turn. Are they pretty even? Believe it or not you can tune pretty close to the best burn ( (fuel/air ratio) with a simple guage. Tune for the best vacumn on each bank/ carb, then do the small adjustments together ( at the same time like the manual.)
Good vacumn means healthy engine and better fuel economy. Mo power too!. Then go find the fancy meter and smile because you just outsmarted the factory.