DR650 Carb Upgrade: Unleash the Power

Suzuki DR650: Unlocking Performance with the Keihin FCR-MX Carburettor

The Suzuki DR650SE is a renowned dual-sport motorcycle, celebrated for its ruggedness and versatility. However, like many stock machines, there's always room for improvement, particularly in the area of carburetion. The original Mikuni BST40 CV (Constant Velocity) carb, while functional, often leaves riders yearning for more responsiveness and outright power. This article delves into a popular and highly effective modification: replacing the stock BST40 with a Keihin FCR-MX series carburettor, specifically the 39mm variant. This upgrade promises to transform the DR650 from a capable workhorse into a veritable fire-breather, significantly enhancing its low-end grunt and overall throttle performance.

Understanding the Limitations of the Stock BST40

The stock BST40 carburettor, typical of many CV carbs, employs a vacuum-operated slide. While this design offers smooth throttle application and can contribute to fuel economy, it often suffers from several drawbacks. These include a tendency for surging, particularly at lower RPMs, an anemic throttle response that can feel sluggish, and a general limitation in airflow capacity. The DR650 is inherently a torque engine; it's designed to pull strongly from lower revs, rather than screaming at high RPMs like a modern sportbike. The ignition system's rev limiter further restricts the bike's top-end potential. Therefore, optimising the carburettor to capitalise on the DR's existing torque characteristics makes sound mechanical sense.

Why the Keihin FCR-MX 39mm?

The Keihin FCR-MX series is a highly regarded performance carburettor, known for its superior design and flow characteristics compared to many stock CV units. The choice of the 39mm FCR-MX over a larger 41mm unit is a deliberate one, based on dyno testing and an understanding of the DR650's powerband. The 39mm FCR-MX offers several key advantages:

• Improved Airflow: Unlike the BST40, the FCR-MX does not have a butterfly valve and shaft obstructing the main airflow path after the slide. This results in a much more direct and less restrictive intake.
• Enhanced Low-End Performance: The 39mm diameter increases air/fuel velocity at lower engine speeds, directly benefiting the DR's strongest part of the power band.
• Floating Slide Seal: The FCR-MX features a floating slide seal, which further refines low-end carburetion and throttle response.
• Superior Technology: The FCR-MX incorporates numerous advanced features that contribute to its overall performance and reliability.

The decision to opt for the 39mm over the 41mm is crucial. Dyno testing suggests that a 41mm carb on a stock or mildly modified DR650 would likely intrude upon the most productive part of the engine's power band, offering little to no additional top-end power while potentially sacrificing low-end response. The 39mm FCR-MX, conversely, provides ample airflow for the DR's capabilities without compromising its inherent torque characteristics.

Key Features of the Keihin FCR-MX

The FCR-MX is packed with features that set it apart from the stock BST40:

• Multiple Venting Points: Optimised for consistent fuel delivery under various conditions.
• Accelerator Pump: A crucial feature that squirts a small amount of fuel directly into the intake tract when the throttle is rapidly opened. This eliminates hesitation and bogging, providing an immediate surge of power. The sealed actuator shaft ensures longevity and prevents contamination.
• Large Capacity Float Bowl: Features jet baffling and a larger volume, ensuring a stable fuel supply, especially during aggressive riding.
• Enclosed Throttle Cam System: Protects the throttle mechanism from dirt and debris, ensuring smooth and reliable operation.
• Air Cut Valve: Reduces lean decel popping (backfiring) by momentarily enriching the mixture during deceleration.

Sourcing and Adapting the FCR-MX

A common and cost-effective approach is to source a used OEM Keihin FCR-MX carburettor from another application. Yamaha's YFZ450 four-wheeler (models from 2004 to 2007) is a popular donor, as these often come with the desired 39mm bore and the essential air cut valve. These carbs also typically feature a TPS (Throttle Position Sensor) block, which can be beneficial for certain electronic tuning or diagnostics, although it's generally not required for a basic DR650 installation and can be retained. The idle speed adjustment is usually conveniently located on the back of the TPS block.

The primary challenge in fitting an FCR-MX from a different model is ensuring it mates correctly with the DR650's intake manifold and airbox boot. This typically requires a custom adapter sleeve:

Adapter Sleeve Fabrication:

The sleeve is typically machined to specific dimensions (e.g., 54.5mm ID x 58.5mm OD x 26mm long) and often features a chamfer on the inner diameter at the rear for a smoother transition. JB Weld is a common and effective adhesive for securing the sleeve to the FCR-MX air horn.

Throttle Cable Adaptation

Adapting the stock DR650 throttle cables to the FCR-MX is another critical step. The FCR-MX uses a different throttle cam system. For the push cable (which operates the slide), the blank mounting casting on the FCR-MX needs to be drilled and tapped to accommodate the DR's cable end. A common method involves drilling a hole (e.g., a #10 drill) and tapping it to 6mm x 1 thread, then cutting a slot for the inner cable. Care must be taken not to create a gap that allows dirt or water ingress into the sealed cam system.

Vacuum Petcock Integration

The DR650 features an OEM vacuum-operated petcock, which requires a vacuum source. The FCR-MX usually has a staked brass plug in a suitable location. This plug can be carefully drilled out and tapped to accept a small fitting (e.g., a 1/4" stainless steel tube threaded with 1/4"-28 thread and secured with red Loctite) to provide the necessary vacuum signal for the petcock to function correctly.

Installation and Airbox Modifications

Once adapted, the FCR-MX should fit into the space previously occupied by the BST40. Careful routing of cables and hoses is essential. To take full advantage of the increased airflow capability of the FCR-MX, modifications to the stock airbox are highly recommended. This might involve removing baffles or creating additional openings. Alternatively, some riders opt to remove the airbox side cover entirely. However, this is generally discouraged due to the significant increase in intake noise, which can be extremely loud and annoying, and the increased vulnerability of the air filter to dirt and water ingress.

Another small but significant modification is the removal of the "little jewel" – a restrictive insert often found in the head pipe flange where the carburettor connects to the exhaust header. Removing this further enhances exhaust flow.

Tuning and Fine-Tuning

A performance carburettor like the FCR-MX requires proper tuning to achieve optimal performance. This involves adjusting various jetting components:

• Main Jet: Controls fuel delivery at wide-open throttle.
• Pilot Jet: Manages fuel delivery at idle and low throttle openings.
• Jet Needle: Affects the fuel mixture in the mid-throttle range.
• Leak Jet: Plays a crucial role in the accelerator pump circuit, influencing the initial fuel delivery during throttle snaps.
• Air Cut Valve: (As mentioned) Helps prevent deceleration popping.
• Accelerator Pump Spring: Increasing spring pressure can improve the accelerator pump's response. The "Merge Racing APS Spring" is a popular upgrade over the "o-ring mod" for more reliable performance.

Accelerator Pump Timing: After changing the leak jet and APS spring, it's vital to check and adjust the accelerator pump timing. This is done by:

1. Connecting a fuel tank to the carb.
2. Priming the carb until fuel fills it.
3. Twisting the throttle cam a few times to bleed air from the AP pump.
4. Observing where the fuel squirts onto the back of the slide. The goal is for the fuel to just miss the slide cut-away arc. Adjust the timing screw on the throttle cam as needed.

Jetting Recommendations (Example)

Specific jetting will vary depending on exhaust system, airbox modifications, altitude, and fuel. However, a common starting point for sea level with certain aftermarket exhausts might include:

• Main Jet: 155
• Jet Needle: EMP-5
• Pilot Jet: 40
• DeZine Extended Pilot Screw: 2 turns out (+/-)
• OEM Pilot Air Jet: 100
• Main Air Jet: 200 (fixed on YFZ450 FCR-MX)
• Leak Jet: #35
• Accelerator Pump Spring: Merge Racing APS Spring

Conclusion

Upgrading the Suzuki DR650SE's carburettor from the stock BST40 to a 39mm Keihin FCR-MX is a significant performance enhancement. While it requires some mechanical aptitude and potentially custom fabrication for adapters, the rewards are substantial. The improved throttle response, increased torque, and elimination of stock carb limitations transform the riding experience, making the DR650 an even more capable and enjoyable machine for both on-road and off-road adventures. This modification truly allows riders to harness the full potential of the DR's torquey engine, offering a visceral connection to the machine that the stock setup simply cannot match.

Frequently Asked Questions (FAQ)

Q1: What type of engine is the Suzuki DR650?
The Suzuki DR650SE is equipped with a 644cc, air-cooled, four-stroke, SOHC, single-cylinder engine.

Q2: How many miles of fuel economy data has the Suzuki DR650 provided?
According to available data, 44 Suzuki DR650s have contributed approximately 180,000 miles of real-world fuel economy and MPG data.

Q3: Is the Keihin FCR-MX 39mm the best choice for a DR650?
For most DR650 riders seeking improved torque and throttle response without sacrificing low-end performance, the 39mm FCR-MX is widely considered an excellent choice. Larger carbs may not offer significant benefits and could even hinder performance on this specific engine.

Q4: What are the main advantages of the FCR-MX over the stock BST40?
The FCR-MX offers superior airflow, a direct slide design (no butterfly valve obstruction), an accelerator pump for improved throttle response, and generally more robust construction and adjustability.

Q5: Will I need to modify my airbox for the FCR-MX upgrade?
Yes, to fully benefit from the increased airflow of the FCR-MX, airbox modifications are highly recommended. Simply removing the airbox side cover is an option but leads to excessive noise and intake vulnerability.

Q6: Can I use a used FCR-MX from another bike?
Yes, used FCR-MX carbs from bikes like the Yamaha YFZ450 are commonly used. However, you may need to fabricate adapter sleeves for the intake and potentially adapt throttle cables and vacuum lines.

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