The 1988 kawasaki js550 mikuni bn38 high/low settings was one of the most popular stand-up jet skis of its era, known for its reliability, responsive handling, and easy-to-service two-stroke engine. At the heart of this machine’s fueling system is the Mikuni BN38 carburetor, a simple yet precise unit designed to deliver the correct fuel-to-air ratio across the entire throttle range.
Two of the most important adjustments on the BN38 are the low-speed screw and the high-speed screw. These control fuel delivery at different points in the throttle and must be set correctly to achieve smooth idle, crisp acceleration, and strong top-end performance without risking engine damage. This guide will walk you through the factory settings, adjustment process, and common pitfalls to avoid when tuning your JS550 carburetor.
Understanding the Factory High and Low Settings for the 1988 Kawasaki JS550 Mikuni BN38 Carburetor
The Mikuni BN38 is a diaphragm-style carburetor with two external fuel mixture screws:
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Low-speed screw: Controls the fuel mixture from idle to about 1/4 throttle.
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High-speed screw: Controls the fuel mixture from roughly 1/2 throttle to wide-open throttle.
The factory baseline settings for the 1988 Kawasaki JS550 BN38 are generally:
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Low-speed screw: 1-1/8 to 1-1/4 turns out from lightly seated.
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High-speed screw: 7/8 to 1 turn out from lightly seated.
These values are intended as a starting point for fine-tuning. Water temperature, elevation, fuel type, and wear on internal engine components can all require small adjustments from these stock settings.
Step-by-Step Process to Adjust the Low-Speed Screw for Optimal Idle and Acceleration
The low-speed circuit is the foundation of smooth performance. If it’s set too lean, you’ll get hesitation, poor idle quality, or even engine stalling. If it’s set too rich, you’ll notice a burbling exhaust note, sluggish throttle response, and excessive smoke.
Locating the Low-Speed Screw on the Mikuni BN38
The low-speed screw on the BN38 is typically on the side of the carburetor near the intake manifold. It’s the smaller of the two adjustment screws and can often be accessed without removing the carb, depending on your hull space.
Recommended Initial Low-Speed Setting from Factory Specifications
From the factory, the 1988 Kawasaki JS550 BN38 was set to 1-1/8 to 1-1/4 turns out from lightly seated. “Lightly seated” means turning the screw clockwise until it gently stops—without forcing it—then turning it counterclockwise the specified amount.
Adjusting the Screw for Best Idle Quality
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Warm up the engine in the water for at least 5 minutes.
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Set idle speed to around 1,200 RPM in the water.
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Slowly turn the low-speed screw in (leaner) until idle begins to stumble.
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Then slowly turn it out (richer) until idle smooths out and throttle response is crisp.
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Fine-tune until you get the highest, smoothest idle without bogging on acceleration.
Dialing in the High-Speed Screw for Top-End Performance and Engine Safety
The high-speed screw is critical for preventing a lean condition at wide-open throttle. Running too lean at high speeds can overheat and seize a two-stroke engine.
Finding the High-Speed Screw on the BN38 Carburetor
The high-speed screw is located on the same side as the low-speed screw but closer to the fuel inlet and often larger in size. Some models include a limiter cap to prevent over-adjustment, which can be removed for precise tuning.
Stock High-Speed Screw Position for the 1988 Kawasaki JS550
The stock setting is 7/8 to 1 turn out from lightly seated. This setting typically provides a safe, slightly rich mixture for break-in or general riding.
Fine-Tuning for Water Temperature, Altitude, and Load
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Colder water → More oxygen density → May require slightly richer settings (turning out).
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Higher altitude → Less oxygen density → May require leaner settings (turning in).
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Heavier rider or load → Engine works harder → Slightly richer settings help prevent overheating.
When adjusting, make changes in 1/8-turn increments and always test under load in the water.
Balancing High and Low Settings for Smooth Throttle Transition and Reliability
Once each screw is individually tuned, you may need to make small adjustments to ensure a smooth transition from idle to top speed.
Symptoms of Incorrect Balance Between High and Low Circuits
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Bog on acceleration → Low-speed too lean or high-speed too rich.
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Hesitation at midrange → Possible overlap misadjustment between circuits.
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Surging idle → Low-speed too lean.
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Excessive smoke or sputter → Low-speed too rich.
Best Practices for Final Carburetor Synchronization
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Always tune in-water, under load—tuning on the trailer is inaccurate.
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Use fresh, properly gapped spark plugs before tuning.
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Make small, incremental adjustments and keep notes of each change.
Common Mistakes to Avoid When Setting the Mikuni BN38 High and Low Screws
Even experienced tuners sometimes make errors when adjusting a BN38 carb. Avoid the following:
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Over-tightening screws – This can damage the needle tips and carb body.
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Tuning without load – The engine behaves differently under load in the water.
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Ignoring weather changes – Temperature and humidity can change fuel requirements.
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Making big adjustments – Large changes can overshoot the optimal setting.
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Running too lean – Lean mixtures might feel faster briefly but risk engine seizure.
Final Thoughts
The 1988 Kawasaki JS550 with its Mikuni BN38 carburetor is a simple yet highly tunable setup. By starting with the factory baseline settings—low-speed at 1-1/8 to 1-1/4 turns out and high-speed at 7/8 to 1 turn out—and making small, careful adjustments under load, you can unlock smooth idle, instant throttle response, and reliable top-end performance.
With patience and attention to detail, your JS550 will run as strong as it did the day it left the factory—if not better.
