The throttle body (TB), manufactured by Bing, measures 29mm in diameter at the butterfly. It weighs only 122 grams. Compared with a traditional Dellorto trials carb with a 26mm venturi, the Bing TB yields about 24% more airflow area (but the intrusion of the butterfly reduces that somewhat). When the butterfly is completely closed some air can still get through a 1.5mm bypass hole in the butterfly.
Unlike an automotive system with a manifold air flow (MAF) sensor (and oxygen-sensor feedback), the TR280i assumes a particular airflow at a particular throttle opening and engine speed. With this type of EFI system, when the air filter gets dirty it causes the engine to run with excess fuel near idle. With a dirty air filter, the airflow is reduced but the standard amount of fuel is injected resulting in a rich condition. The same thing happens when the TB gets dirty with soot that occludes the small area opened by the butterfly near idle. That rich running is a self-reinforcing problem because more soot is then deposited on the inside of the TB.
Initially, I did not like how light the return spring is on the throttle body but have grown accustomed to it.
All the photos below are repeated in the Image Carousel, so you can get a better look (at least on a computer).
Cable attachment on left, TPS on right
Idle stop adjuster is adjacent to spring
Butterfly partly open, note 1.5mm bypass hole