Go to (shop.evinrude.com) there you will find parts list for that model. The twin carb setup has no adjustment on the carbs, all idle adjust is made with the linkage.

OK, that is the idle air bleed, and not adjustable. Loosening it will just created more of an air leak, causing it to run lean, the air bleed will fall out if left loose, and may be swallowed into the engine! Tighten up those air bleeds!
If you are concerned that the engine is running lean at idle, plugging the air bleeds with a tooth pick(s) is a quick way of richening up the idle circuit slightly. But, before getting in too deep on idle problems, the sync/link should be checked.