After 5 posting on various Mercury forums, and getting no reply, I have discovered that if you have to ask what a plunger rod is and the rubber bellows that go on it, you know nothing of Mercury, Tohatsu or Nissan carbs. These motors all take the same carb kit and these same carbs are used on many 9.9, 20, 30 hp motors. One local "certified" Mercury mechanic .had to look on computer to see what a plunger rod is. Another said he would rebuild carbs for 350 dollars but I must get an intake manifold gasket. This 50 4 stroke does not have a gasket only O rings . If anyone , novice or professional ever rebuilt this carb, you would not forget what a plunger rod is, the rubber bellows that go on it, and the function they serve.
Upon disassembly, for the "professional" mechanics and novices, the plunger rod, about 1 1/2 long by 3/32 D, is activated by throttle advance, goes through a brass fitting about 1/2d with 7/64 hole in center {very loose plunger rod fit and not sealed}. Rod goes to bottom of bowl to depress about 1/2 inch plunger that forces more fuel into carb. To my best assumption, the rubber bellows on rod at outside top of carb, serves to protect from debris and to protect from fuel linkage.
As someone who does not have great knowledge, it was inconceivable that no one answered this after all those postings. Maybe this will help someone else who may need an answer. Thanks for your forum.