Intake

Work in progress

Subthrottle

The electronically controlled secondary throttle plate, as mentioned in the ECU section, is one of the first restrictions on engine performance. Fortunately, removing it’s quite simple.

Removal Process

Access the throttle body. This may require removing some bodywork for better clearance.
Using a JIS screwdriver, remove the screws holding the secondary throttle plate in place.
Many instructional videos can be found on YouTube for visual guidance.

A lesser-known fact is that the ECU might control idle speed with a small rocker arm connected to the throttle plate. This mechanism is somehow linked to sub-throttle control. Removing the sub-throttle plate can often lead to unstable idle.

Recommendation If you remove the sub-throttle plate, it’s recommended to also remove the spring inside the throttle body. This will help prevent high or erratic idle RPM.

Performance Gains

Removing the sub-throttle plate alone can offer a noticeable improvement in throttle response and a power increase of around 4hp in the midrange on a KLX300 (see dyno chart below). This modification will also benefit a KLX250.

Airbox & Snorkel

The next significant restriction on airflow is the airbox opening, also known as the snorkel. The stock snorkel has a limited cross-sectional area. Here are two main options for increasing airbox airflow:

KDX Snorkel

The most popular option is the “KDX snorkel” (OEM part number14073-1577) from a Kawasaki KDX220 2-stroke dirt bike. Its popularity stems from its bolt-on compatibility and doubled airflow capacity compared to the stock KLX snorkel. Importantly, it doesn’t create excessive intake noise (“roaring”).

Benefits:

Noticeable (but not dramatic) power increase in the midrange RPM range (see dyno chart below).
Easy bolt-on installation.
No additional noise compared to open airbox (“roaring”)

Please also notice that air-fuel ratio changed from ~13:1 to 14:1.

Warning!

The dyno chart above shows a KLX250 with the sub-throttle plate still in place. Replacing the snorkel with a KDX unit and removing the sub-throttle plate will lean out the air-fuel mixture significantly (potentially reaching 16:1) and can lead to detonation and engine damage. These modifications should not be done together without proper fuel adjustments with piggyback.

Removing airbox cover

Many riders believe that replacing the snorkel with a KDX unit provides sufficient airflow, and removing the airbox cover won’t offer any additional benefit. However, dyno results (at least for bikes with performance exhaust) suggest otherwise.

The blue line (25 hp) represents the KDX snorkel with a modified stock exhaust (end tip replaced with a larger 25mm pipe).
The red line (26.5 hp) represents the KDX snorkel with a full performance exhaust (header, midpipe, and muffler).
The pink and green lines (28.5 hp) represent an open airbox (“lidless”) with a performance exhaust.

As you can see open airbox provide +2hp and +2-3Nm of torque across all RPM range! The disadvantage of open airbox is that it significantly increase intake noise (“roaring” sound).