#28 is an air jet. It functions exactly like #29, the pilot air screw except that it provides a fixed amount of air depending on the jet size instead of being a variable metering needle. Stock RD carbs do indeed have this jet, but it is permanently made into the carb body. It is not behind a ball on the 350 carbs. It can be changed by drilling and tapping the bore it is in and using standard Mikuni air jets. The ball at 7:00 0-clock is just to block off the bore for the pilot air after it is drilled at the factory. I have never had to remove it to adequately clean this air passageway - it only has air traveling through it so stays quite clean - of course you have to check it anyway with a jet of carb cleaner. It is very important to understand the function of these two parts - the fixed size air jet and the needle if you want to be successful at properly cleaning the carb and getting it to perform correctly. The air jet supplies air to the main fuel system and interacts with the main jet and emulsion tube (#11). It functions like the pilot air screw/needle in that is supplies air to the main fuel supply like the needle supplies air to the pilot fuel supply. If you look at the carbs construction, you will see that the horizontal bore for the air jet intersects the vertical bore for the emulsion tube. Fuel gets sucked up into center of the emulsion tube through the main fuel jet #36, but before it gets to the top and enters the main bore of the carb, it gets mixed with the air supplied by the air jet inside the emulsion tube. So if the jet is incorrectly sized or any part of the emulsion tube, its bore, the passageway for the air jet or the jet itself is dirty or damaged, the fuel mixture will be wrong. The pilot system is the same. Air is supplied by the pilot metering needle. The pilot system gets an adjustable air supply because the fuel/air needed at idly is so tiny that just a hair inaccuracy will screw up the idle and it would be too fussy to fit a fixed jet. Anyway, the air from the needle goes to the pilot emulsion tube where it gets mixed with the fuel supplied by the pilot jet. The pilot system and the main system are completely separate from each other and share no internal passageways or components. You do not see a pilot emulsion tube in the exploded view because the tube is actually part of the pilot jet itself. The pilot jet is just the size of the hole bored into the end of the jet. The tubular part with all the little holes bored in it is the pilot emulsion tube. Fuel gets sucked up through the jet in the end, and air from the needle gets sucked into all those little holes and mixes with the fuel before it ever goes into the main bore of the carb. So when you take the carb apart to clean it, you have to remove the pilot jet with its made on emulsion tube and make sure all the little holes are clean, that the bore or well that it goes into is clean and that the passageway from the needle to the well is clean. If you look at the pilot jet, you will see that the end that goes into the carb body first has a taper. That taper seats to a mating taper in the body, so air in the well surrounding the pilot jet emulsion tube can't leak past the joint - all the air must go through all the little holes and mix with the fuel. Once you understand this, it is easy to follow the paths of the air, fuel, and air/fuel mix and make sure the passageways are clean. On most Mikuni VM carbs, the air for the pilot system is supplied by the bore at 8:00 o-clock. Often you can see the tip of the needle through this bore. The air for the main system is at 6:00 o-clock. If the carb is clean and disassembled, you should be able to shine a bright light into the emulsion tube bore - straight up from the bottom - and see the light through the main air jet - it is a tiny hole!