Engine

• 1: ECU limitations
• 2: ECU tuning
• 3: Intake
• 4: Exhaust
• 5: Big Bore
• 6: Camshafts

1 - ECU limitations

ECU on a motorcycle is a mini-computer which has many functions. In a context of engine performance ECU collects and process data from different sensors and controls fuel injection and ignition systems.

Apart from ignition timing and amount of fuel KLX’s ECU also can limit amount of air comning to cylinder using subthrottle (“secondary throttle plate”). Subthrottle is a useful concept on big powerful bikes which might help ECU to not get you into trouble when you twist you hand too quick :-) But it’s not so useful on a 300cc dual sport bike.

How does ECU limits power on KLX250/300? There are two main limitations.

1. Airflow restriction with subthrottle
2. Soft rev limit

But don’t get upset! We will discuss how to defeat ECU limitations!

Subthrottle

For some reason (emissions?) Kawasaki engineers decided to limit subthrottle opening even if main throttle fully open. Subthrottle operated exlusively by ECU and has no mechanical connection to motorcyle throttle tube.

On KLX300 this results in power drop at 5000 following spike at 7000 rpm when subthottle finally getting fully open:

There is no such dramatic midrange torque & power drop on KLX250. But secondary throttle still limits the airflow. Please see red line on the dyno chart below (stock KLX250 without single modification):

Rev limiter

A lesser know fact is that Kawasaki implemented a soft rev limiter at 7500 rpm on KLX250 (most of the markets) and at 8000 rpm on KLX300. The detailed mechanism is not fully studied, but likely following things happen :

1. ECU limits airflow with subthrottle
2. ECU reduce voltage to ignition coil
3. ECU retards ignition

As a result we see a torque and hp cliff after 7500 (KLX250) and 8000 (KLX300) rpm:

Please read ECU tuning to learn how to mitigate this.

2 - ECU tuning

The KLX250/KLX300 platform offers significant power increase potential with minimal (or no) investment in performance parts.

The basic principle is simple:

1. Improve airflow through the intake, head, and exhaust by removing bottlenecks.
2. Adjust fuel supply to match the increased airflow.

The first step involves hardware upgrades (see the Intake and Exhaust sections).

The second step requires ECU tuning which consists two things:

1. Fuel management (common approach)
2. Advanced ECU tuning altering ignition timing

Please read through the entire article below for better understanding.

Fuel Injection Basics

Let’s take a step back to understand how basic fuel injection works.

The bike’s ECU (computer) receives signals from various sensors and determines when and how much fuel to inject into the airflow for complete combustion in the cylinder.

How does the ECU know how much fuel is needed? The KLX250/KLX300 has an O2 sensor in the exhaust that measures oxygen levels. The ECU operates in two modes:

1. Closed loop: Uses O2 sensor readings to adjust fuel delivery.
2. Open loop: Ignores O2 sensor readings and relies on a pre-programmed fuel map.

Open loop

Open loop is simpler. The ECU uses a fuel map (lookup table) based on sensor values to determine fuel injection amount. For example: For example: Inject 1g of gasoline if:

• Air temperature is 20 degrees
• Air pressure is 101 kPa
• RPM is 4000
• Manifold pressure is 50 kPa
• Throttle position is 50% open

Open loop operation is limited and less efficient than closed loop, as it can’t adapt to varying conditions. It’s primarily used at wide-open throttle (WOT) and idle.

Closed Loop

In closed loop, the ECU combines sensor readings (including the O2 sensor) with the fuel map to make real-time adjustments. It can increase or decrease fuel delivery by up to 20-30% based on the O2 sensor’s feedback.

Ideally, the ECU would always operate in closed loop, eliminating the need for additional tuning. However, there are limitations:

• The O2 sensor needs time to warm up.
• The engine must be in a stable operating condition.
• WOT conditions use a dedicated fuel map, bypassing the O2 sensor.
• Sensor failures require fallback options.

Fuel Tuning

Increasing intake, head, or exhaust airflow requires a corresponding increase in fuel. While the ECU can compensate to some extent in closed loop, it’s insufficient for open loop conditions. Therefore, additional fuel control is essential.

There are two primary methods to control fuel on the KLX250/KLX300 with standard OEM ECU:

Replace injector

The stock injector has a flow rate of 220cc/min. A common upgrade is to replace it with a 270cc/min injector from a Suzuki GSX-R1000 (part number 15710-21H00). This provides a 22.7% increase in fuel flow, which is roughly equivalent to the displacement increase from a KLX250 to a KLX300.

• Pros: Reliable, inexpensive (~$20).
• Cons: Limited control, adds a fixed amount of fuel across the entire RPM range.

Piggyback Fuel Controllers

Piggyback controllers intercept the signal between the ECU and injector to adjust fuel delivery. They offer more precise control than simply replacing the injector, allowing for different fuel adjustments across different RPM ranges. Popular options include EJK and PowerCommander Fuel Controller (PCFC).

We recommend our own PlanetKLX2 piggyback controller, which features:

• Precise fuel management in three independent RPM ranges.
• Easy operation via Bluetooth mobile app.
• Rugged waterproof design.
• Plug-and-play installation.
• Low cost - $140 shipped worldwide. If you need one - please see Shop section.

With correct air-fuel mixture(controlled with piggyback), removed subthrottle plate, free flowing full exhaust (with header) and open airbox (removed airbox cover and snorkel) KLX300 can produce about 28.5hp @ 7500rpm (pink line on the chart below).

Advanced ECU tuning

While fuel tuning is essential it’s not sufficient for really high performance KLX. As we discussed in ECU limitations Kawasaki put “soft rev limiter” into ECU which limits top end power.

We are aware of 3 ways to mitigate soft rev limiter

Unrestricted KLX250 OEM ECU

Different countries impose different emission regulations. If we investigate KLX250 Service Manual we will discover that on the most of the markets:

1. Bike produce 22hp@7500 rpm
2. Ignition is BTDC 10° @1300 (rpm) to ∼ BTDC 34° @11000 rpm

There are however 2 exceptions: Indonesia and Japan. In these countries service manual states following:

1. Bike produce 24hp@9000 rpm
2. Ignition is BTDC 10° @1300 (rpm) to ∼ BTDC 39° @11000 rpm

Things however are even more complicated. On a fresh (2016+?) Indonesian motorcycles ECU also have restricted ignition (tested on our own KLX250 2022).

We’ve been able to purchase several unrestricted Japanese & Indonesian ECUs and test on the dyno. Results are impressive - 30.93hp@8200rpm (open airbox + full exhaust + removed subthrottle plate) and stable overrev power up to 10000rpm.

If you need one - please see Shop section.

Compare to the pink line which is standard KLX250 ECU (same mods - open airbox + full exhaust + removed subthrottle plate).

ECU reflash

Best Dual Sport Bikes (BDSB) offers ECU reflash for KLX300 for $349. According to their dyno it shows nice power boost (~10%) over entire RPM range. With modified OEM echaust and additional holes in the airbox reflash offers about 25 peak hp. Early customers feedback is very positive.

See below KLX300 dyno charts from BDSB:

1. Red - factory stock KLX300 with secondary throttle (“subthrottle”) plate in place
2. Yellow - factory stock KLX300 with secondary throttle (“subthrottle”) out
3. Orange - ECU reflash + modified exhaust + drilled airbox

Aftermarket ECU

Thai company Apitech offers aftermarket ECU replacement for KLX250/KLX300. This is probably the most advanced solution as their standalone ECU provides complete engine management control including fuel management, ignition curves, throttle control, manifold pressure, etc. The downside is that the software is not very intuitive and skilled engineers exist only in South East Asia. Also according to Keng Engine (one of the “big bore” kit suppliers from Thailand) they stopped offer Apitech ECU to their customers due to poor reliability of their boards.

3 - Intake

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

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

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.

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).

Planetklx2 custom 3D-printed snorkel

We’ve developed and tested a custom airbox lid with the integrated snorkel which is available for purchase now.

The lid provides even better airflow than an open airbox (see detailes in the review below):

• Adds 2.5hp and 2-3Nm of torque over standard snorkel

• Adds 2hp and 2-2.5Nm of torque over KDX snorkel

Ultimate intake and snorkel review part 1

Ultimate intake and snorkel review part 2

4 - Exhaust

5 - Big Bore

AHL “299” kit

The most cost efficient option to upgrade KLX250 to KLX300 is AHL “299” kit which is very similar to OEM 292cc kit. Kit consists of everything needed for upgrade:

• cylinder
• piston 78mm
• piston pin
• piston rings
• piston circlips
• gaskets

Piston weights:

• OEM 72mm piston (249cc cylinder) is 235.5g (including pin and piston rings)
• AHL 78mm piston (292cc cylinder) is 272.7g (including pin and piston rings)

And the best part is that in most of the countries you can get it under $200 and sometimes even under $100. For the entire kit! Here is a video review:

6 - Camshafts

KLX250/KLX300 cams with KACR release mechanism

All KLX250, KLX300 (including 2021+ models) have same camshafts. This is cross-tracked using part numbers and camshaft specs in service manuals.

Ninja Z250SL / KLX300R (2020+)

Updated Kawasaki OEM camchafts with better top-end power could be found in Ninja Z250SL and KLX300R 2020+

• intake 12044-0786 / 12044-1505
• exhaust 12044-0787 / 12044-1506
• 32T timing gear/sprocket - 12046-0576 (identical for intake and exhaust and different from klx250/klx300 - see video!)

Keng Engine

Keng Engine produce own camshafts for his engine builds.