A reed valve draws fresh air from the airbox and feeds it directly into the exhaust port.

During deceleration and light throttle, some unburned fuel always makes it into the exhaust. Adding fresh oxygen at the port lets that fuel finish burning in the pipe instead of exiting as raw hydrocarbons, which lowers the bike's emissions.

Because it behaves like a deliberate exhaust leak near the head, it is the leading cause of the deceleration popping these bikes are known for. That popping usually gets worse once you fit a freer-flowing exhaust.

A charcoal canister traps fuel vapor that would otherwise evaporate out of the tank. The ECU then opens a purge valve to feed those vapors into the intake, where they get burned.

It started as a California (CARB) requirement and is now standard on US bikes. The goal is keeping fuel vapor out of the atmosphere.

It is bulky plumbing to have on a dirt bike. If the bike tips over, raw fuel can flood the canister, which disrupts tank venting and can starve the fuel pump. Off-road, it is a genuine failure and maintenance point.

An oxygen sensor in the header measures exhaust oxygen and lets the ECU adjust the fuel mixture in real time.

It keeps the bike within its certified emissions numbers by holding the mixture lean at idle, cruise, and light throttle.

A lean mixture passes the test but does not make the bike feel good. On pre-2017 models especially, it is a major reason they ran hot, felt abrupt off idle, and stalled easily. KTM improved this significantly on the 2017-and-newer generation, but the sensor and the lean calibration are still there.

Vents the pressure and blow-by gases that build up inside the engine cases.

Emissions rules require that oily vapor to be routed back and burned rather than vented to the atmosphere, so the stock hose loops it back toward the intake.

Minor on its own, but it is one more line feeding oily vapor back into the intake, which you would rather avoid once the rest of the gear is gone.

The tank's breather line. From the factory it runs through a check and rollover valve that ties into the EVAP system.

As part of the sealed EVAP system, it controls fuel vapor and prevents fuel from spilling if the bike tips over.

Once the canister is gone, the tank still needs to breathe, or it will vapor-lock and starve for fuel. That calls for a simple vented line rather than the restrictive factory setup.

The stock ECU is calibrated to hit the specific fuel mixture targets required to pass the emissions test.

Those targets are what keep the bike compliant, so the factory calibration is deliberately lean and conservative, tuned to pass rather than for power or throttle response.

Because it is locked and runs lean, a reflash or aftermarket ECU is the first upgrade for almost everyone. The emissions delete and a proper tune go hand in hand.