1988 Kawasaki JS550 Mikuni BN38 High/Low Settings: A Comprehensive Guide
The 1988 Kawasaki JS550, a legendary personal watercraft, utilizes Mikuni BN38 carburetors. Optimizing these carbs is crucial for peak performance, fuel efficiency, and a smooth riding experience. However, finding the perfect high and low settings often involves trial and error, specific to your machine's condition and riding style. This guide will explore the intricacies of BN38 carburetor adjustment, providing insights into understanding the high and low speed settings and troubleshooting common issues.
What are High and Low Speed Settings?
The Mikuni BN38 carburetors on your JS550 have separate adjustments for high and low speed circuits. These circuits control the fuel mixture at different throttle positions:
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Low-speed circuit: This controls the fuel mixture at idle and partial throttle openings. Proper adjustment here ensures smooth idling, good throttle response at low speeds, and avoids bogging down.
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High-speed circuit: This governs the fuel mixture at wide-open throttle (WOT) and high RPMs. Correct adjustment maximizes power and performance at higher speeds.
Understanding the Adjustment Screws
Both the high and low speed circuits are adjusted via screws on the carburetor. These are typically labeled:
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Pilot screw (Low Speed): This screw controls the air/fuel mixture at low throttle openings. Turning it clockwise leans out the mixture (less fuel), while counter-clockwise enriches it (more fuel).
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Main jet (High Speed): This jet determines the amount of fuel delivered at high throttle openings. While not directly adjustable like the pilot screw, its size is crucial for high-speed performance. It usually requires replacing the jet with one of a different size to make adjustments.
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Air screw (Low Speed): This screw, often found near the pilot screw, adjusts the air intake to the low-speed circuit. Similar to the pilot screw, turning it clockwise leans the mixture, and counter-clockwise enriches it.
How to Adjust the Mikuni BN38 Carburetors on your 1988 Kawasaki JS550
There's no single "perfect" setting. Optimal settings depend on factors like altitude, temperature, fuel quality, and the overall condition of your engine. However, here's a general procedure and troubleshooting:
1. Start with the Basics: Ensure your engine is clean, your fuel is fresh, and your air filter is clean. A dirty air filter can significantly impact carburetor settings.
2. Warm-up the Engine: Allow the engine to reach operating temperature before making adjustments.
3. Adjust the Low-Speed Circuit: This is often done using the pilot and air screws. Start by turning them to the factory settings (consult your service manual for specifics). Then, make small adjustments (1/4 to 1/2 turn at a time), testing the engine after each adjustment. Look for a smooth idle and good throttle response at low speeds.
4. Adjust the High-Speed Circuit: This usually requires jetting changes. Poor high-speed performance might indicate the need for a richer or leaner main jet. This is typically done by swapping out the main jet for one with a different number. Consult a service manual or jetting chart for guidance.
Troubleshooting Common Problems:
H2: My JS550 is bogging down at low speeds: This often indicates a lean low-speed mixture. Try slightly enriching the mixture by turning the pilot and/or air screw counter-clockwise.
H2: My JS550 is sputtering or hesitating at high speeds: This could indicate a lean high-speed mixture. You might need to try a richer main jet.
H2: My JS550 is hard to start or runs rough at idle: This may point to a problem with the idle speed screw or a vacuum leak somewhere in the system. Consult a service manual and troubleshoot accordingly.
H2: What tools do I need to adjust my Mikuni BN38 carburetors? You'll primarily need a small screwdriver (possibly a specialty screwdriver depending on your carburetor) to adjust the pilot and air screws. You may also need a jetting kit and various tools to change jets.
H2: Where can I find a service manual for my 1988 Kawasaki JS550? Online retailers such as eBay or Amazon often sell service manuals for older personal watercraft models. Checking online forums dedicated to Kawasaki Jet Skis might also lead you to helpful resources.
Remember: Carburetor adjustment requires patience and attention to detail. Make small, incremental adjustments and test the engine frequently to avoid damaging your engine. If you are uncomfortable working on your carburetors yourself, it is always best to consult with a qualified mechanic or jet ski specialist. This guide provides general information and should not substitute for consulting a qualified professional or a factory service manual.
