Tech guide for Mercury WH slosh tube carburetors used on 2.0L, 2.4L, and 2.5L V6 two-stroke outboards. Learn how slosh tubes work, common WH carb problems, rebuild tips, jetting notes, and Buckshot Racing #77 carb parts, fittings, gaskets, linkage, and rebuild kits. Mercury WH slosh tube carburetors are a proven performance carburetor setup used on many classic Mercury V6 two-stroke outboards, including 2.0 Liter, 2.4 Liter, and 2.5 Liter applications. For racers, builders, restorers, and high-performance outboard owners, the WH carburetor remains popular because it is simple, tunable, serviceable, and capable of strong fuel delivery when properly cleaned, rebuilt, jetted, and synchronized. Buckshot Racing #77 builds, services, and supplies parts for Mercury WH slosh tube carbs, including rebuild parts, jetting components, gaskets, fittings, linkage pieces, slosh tube parts, washers, screws, carb mounting hardware, and race-ready carburetor support parts to keep these classic Mercury V6 carburetors running in top form. What Makes Mercury WH Carbs Different Mercury WH carburetors are commonly used as a three-carburetor set on V6 two-stroke outboards, with top, center, and bottom carburetors feeding the engine through the intake system. These carburetors rely on clean fuel passages, correct float height, proper jetting, tight gaskets, accurate linkage adjustment, and consistent fuel supply to maintain reliable performance. Unlike modern electronic fuel injection, the WH carburetor system is fully mechanical. That makes it easier to inspect, tune, rebuild, and service, but it also means every passage, jet, gasket, float, needle, seat, fitting, and throttle linkage adjustment matters. A small fuel restriction, air leak, incorrect jet, or uneven float level can create poor idle quality, hesitation, bogging, lean running, or piston-damaging fuel starvation. What Slosh Tubes Do Slosh tubes are used to help stabilize fuel behavior inside Mercury WH carburetors during hard acceleration, rough water, sharp turns, drag racing launches, and high-performance operation. When fuel moves violently inside the bowl, it can uncover fuel circuits, disturb metering, or create inconsistent delivery. Slosh tubes and related bowl control parts help manage that movement so the carburetor can maintain more consistent fuel metering. For Mercury Racing and high-performance lake use, this matters because a two-stroke engine depends on proper fuel delivery for both power and piston cooling. A lean condition at high RPM can quickly damage pistons, rings, and cylinders. Properly built WH slosh tube carburetors help reduce fuel delivery problems caused by bowl movement and can improve throttle response, consistency, and reliability in demanding conditions. Compatible Mercury V6 Applications Mercury WH slosh tube carb setups are commonly associated with classic Mercury 2.0L, 2.4L, and 2.5L V6 two-stroke outboard engines. They are used by builders working on Mercury Racing engines, lake racers, Mod-VP style boats, drag boats, tunnel hulls, bass boats, and restored high-performance Mercury outboards. Because Mercury produced many WH carb variations with different calibrations, stamped numbers, jet sizes, and application-specific setups, carburetors should always be identified before ordering parts or changing jetting. A WH carb body, throttle linkage, gasket, plate, or jet should be matched to the exact engine combination, fuel system, compression, exhaust, intake, reeds, timing, and intended RPM range. Common WH Carb Problems Most Mercury WH carburetors are decades old, and many have seen ethanol fuel, storage varnish, corrosion, worn gaskets, damaged fittings, plugged jets, loose linkage, or previous repairs. Common problems include hard starting, poor idle, fuel leaks, bogging, hesitation, uneven cylinder fueling, dirty idle circuits, incorrect float height, worn needle and seat assemblies, clogged main jets, cracked or hardened tubing, missing slosh tube parts, and air leaks at the carburetor gaskets or mounting plates. A proper WH carb rebuild is more than just cleaning the outside. The carburetors should be disassembled, inspected, cleaned internally, rebuilt with correct gaskets and parts, checked for flat sealing surfaces, set for proper float level, fitted with the correct jets, and synchronized so all three carburetors open evenly. Buckshot Racing #77 WH Carb Parts and Kits Buckshot Racing #77 offers parts and rebuild support for Mercury WH slosh tube carburetors to help owners keep their carb sets complete, clean, and reliable. Available parts include carburetor gaskets, slosh tube components, Tygon tubing, elbow fittings, tee fittings, adapter fittings, plugs, tie straps, hoses, linkage kits, throttle levers, rollers, screws, lockwashers, stainless washers, carburetor plate parts, carb mounting screws, and related hardware used in Mercury WH carb service. Buckshot Racing #77 also supports WH carb tuning with main and idle jet components for builders who need to restore factory-style operation or fine-tune a modified Mercury V6 setup. Correct jetting is critical on any two-stroke outboard, especially on high-performance 2.4L and 2.5L engines where lean running can cause expensive powerhead damage. Rebuild and Setup Recommendations A quality WH carb rebuild should begin with clean fuel, clean tanks, fresh fuel lines, a good primer bulb, proper fuel pump function, and clean filters. Rebuilding carburetors without correcting fuel supply problems can lead to repeated contamination and poor performance. During assembly, every gasket surface should seal correctly, all fittings should be tight, float height should be checked, jets should be verified, and the throttle linkage should be synchronized. After installation, the engine should be checked for proper idle, clean transition, strong acceleration, fuel leaks, and consistent cylinder temperature or plug readings. On modified engines, jetting should be approached carefully and verified under load. Why Builders Still Use WH Carbs Mercury WH carburetors remain popular because they are simple, strong, and highly serviceable. For many racers and Mercury V6 builders, a properly prepared WH carb set offers the mechanical feel, throttle response, and tunability that made classic Mercury performance outboards famous. With the right parts, careful setup, and regular maintenance, WH slosh tube carbs can continue to perform well on 2.0L, 2.4L, and 2.5L Mercury V6 engines. Buckshot Racing #77 Whether you are restoring a classic Mercury V6, freshening a lake racer, building a 2.5L performance motor, or maintaining a Mercury Racing carb setup, Buckshot Racing #77 can help keep your WH slosh tube carburetors complete and ready to run. From rebuild parts and fittings to jets, gaskets, linkage components, slosh tube parts, and carb service support, Buckshot Racing #77 is committed to keeping these proven Mercury WH carburetors alive, tuned, and race-ready.

Mercury 12415 Temperature Sender Installation Guide | 12415A2 Outboard Temp Sender Kit The Mercury 12415 temperature sender is used to send engine temperature information to an analog water temperature gauge. The related Mercury 12415A2 temperature sender kit is commonly used when adding a water temperature gauge to Mercury and Mariner outboards, especially 75 HP, 90 HP, 100 HP, 115 HP, and 125 HP two-cycle engines from 1993 and newer that were equipped with a factory coastal flush system. This is a temperature sender for a gauge, not a simple overheat warning switch. A sender changes resistance as engine temperature changes, allowing the gauge needle to move. A warning switch is normally an on/off alarm device. Before installation, make sure you are wiring the Mercury 12415 sender to a compatible analog temperature gauge, not trying to replace an alarm switch or SmartCraft temperature sensor. How the Mercury 12415 Sender Works The Mercury 12415 temperature sender installs into the engine’s water jacket or sender mounting location and reads cylinder head or cooling system temperature. The sender terminal connects to the tan temperature gauge wire, while the sender body grounds through the engine mounting location. For the gauge to read correctly, the sender must have clean metal-to-metal contact with the engine. Heavy thread tape, corrosion, paint, or loose mounting hardware can prevent a proper ground and cause inaccurate readings. Basic Wiring The Mercury analog temperature gauge uses a simple three-wire layout. The tan wire from the Mercury 12415 temperature sender connects to the “S” terminal on the gauge. The purple wire supplies switched 12-volt ignition power to the gauge. The black wire provides gauge ground. If the gauge has a lamp terminal, that terminal may be connected to the instrument lighting circuit or another proper switched 12-volt lighting source. The tan sender wire is the signal wire. It should be routed away from hot exhaust parts, moving linkage, sharp edges, and ignition wiring where possible. Keep the wire protected, secured, and clean from the sender to the engine harness or dashboard harness. Installation Location On many carbureted and EFI Mercury outboards, the sender installs at the port cylinder head using the proper sender cover, plug, and mounting hardware. The mounting hole should be cleaned before installation so the sender assembly seats properly and grounds correctly. Mercury’s gauge instructions list the temperature sender as part number 12415, with the sender cover and plug ordered separately in some installations. On Direct Fuel Injection models, Mercury notes that the existing temperature sender in the starboard cylinder head provides gauge-compatible temperature information through the tan lead in the remote control or ignition/choke harness. In that case, a separate temperature sender may not be required. Always verify your exact engine model and serial number before adding or replacing sender hardware. Torque and Assembly Notes Proper tightening is important because the sender must seal correctly and maintain a good ground path. Mercury’s installation instructions list the sender unit mounting screws at 150 lb-in and the temperature sender at 70 lb-in. The gauge terminal nuts and retaining bracket hardware are listed at 12 lb-in. Do not overtighten the gauge terminals, sender terminal, or mounting hardware, because small electrical studs and sender parts can be damaged. Quick Gauge Test Before blaming the sender, verify the gauge wiring. With the key on, grounding the tan sender lead momentarily should make the temperature gauge move toward hot. If the gauge responds, the gauge, power, ground, and sender lead are likely functioning. If the gauge does not move, inspect the purple switched power wire, black ground wire, tan sender wire, gauge terminals, and harness connections before replacing parts. Common Problems Most temperature gauge problems are caused by poor grounds, loose terminals, corroded tan wire connections, incorrect sender type, damaged gauge wiring, or using a warning switch instead of a gauge sender. A sender that is installed with too much thread tape or mounted into a dirty, painted, or corroded surface may not ground correctly. A gauge that pegs hot, stays cold, or moves erratically should be diagnosed before replacing parts. Buckshot Racing #77 Installation Tip All electrical connections should be clean, tight, and dry. Use dielectric grease only inside rubber boots or weather seals to help block moisture. Do not pack dielectric grease onto the metal sender stud, ring terminal, gauge terminals, or exposed electrical contact surfaces. The metal terminals need clean contact, while the sealant or grease should only help protect the connection from moisture after proper contact is made. Final Check After installation, reconnect the battery, turn the key on, and verify gauge operation. Start the engine on a proper water supply and watch the temperature gauge as the engine warms up. Confirm that the sender does not leak, the gauge responds normally, and the harness is secured away from heat and moving parts. For saltwater use, inspect the sender connection and gauge wiring regularly because Mercury considers saltwater operation severe service. Whether you are adding a water temperature gauge to a Mercury 75 HP, 90 HP, 100 HP, 115 HP, or 125 HP two-stroke outboard, replacing a damaged sender, or cleaning up old wiring, the Mercury 12415 temperature sender and 12415A2 sender kit provide a reliable way to monitor engine temperature with an analog gauge.

Complete Mercury 98866A25 Laser EFI and XRI ECU injector harness wiring guide for Mercury 2.5L EFI outboards. This Buckshot Racing #77 tech article explains the 16-pin ECU connector, wire colors, injector circuits, fuel pump wiring, switchbox signals, primer circuit, intake air temp wiring, warning circuit, and factory TPS wiring. Includes Race Harness vs. Stock OEM Harness notes, including why Buckshot Racing #77 Race Harnesses omit Pins 13, 14, and 15 for Brucato ACU, PCU, Digital ACU, MAD EFI, and other programmable racing EFI systems that do not use the factory TPS circuit. The Mercury 2.5L Laser EFI and XRI ECU injector harness is the main electrical link between the Electronic Control Unit and the engine management system. Every injector pulse, switchbox signal, fuel pump command, sensor input, warning circuit, primer circuit, and ground path depends on this harness. A damaged wire, weak ground, corroded terminal, or loose connector can cause hard starting, poor idle quality, intermittent misfires, injector drop-out, fuel pump problems, or complete engine shutdown. This guide covers the Mercury ECU and injector harness 98866A25, commonly used on Mercury Laser EFI, XRI, Pro Max, and related 2.5L EFI outboards using the 16-pin ECU connector. Understanding the pinout and wire colors makes troubleshooting faster and helps prevent unnecessary replacement of ECUs, injectors, switchboxes, sensors, or fuel system parts. ECU Pinout and Wire Color Functions Pin 1 uses a white wire for injectors 1 and 2. This circuit controls the injector pair for cylinders number 1 and number 2. Clean terminals and proper continuity are critical because any resistance or loose connection can affect fuel delivery to both cylinders. Pin 2 uses a red wire for the fuel pump circuit. This circuit is listed on older reference sheets as fuel pump, and should be shown as red on the updated Buckshot Racing #77 pinout graphic. Fuel pump wiring should always be checked carefully because poor voltage or a weak connection can create lean running, hesitation, or no-start problems. Pin 3 uses a blue wire for injectors 3 and 4. This circuit controls the injector pair for cylinders number 3 and number 4. A poor connection on this wire can create a two-cylinder fuel delivery problem that may feel like ignition failure. Pin 4 uses a black wire for the idle box ground. On engines equipped with the factory idle stabilizer or idle box system, this provides the ground path for that circuit. Poor grounds are one of the most common causes of erratic EFI operation. Pin 5 uses a yellow wire for injectors 5 and 6. This circuit controls the injector pair for cylinders number 5 and number 6. Like the other injector circuits, it must have clean, tight, low-resistance connections. Pin 6 uses a red wire for injector power, fuel pump power, and main power in. This is one of the most important power circuits in the harness because it feeds the EFI system. Voltage drop, corrosion, or overheated wiring on this circuit can cause hard starting, weak injector operation, fuel pump issues, or intermittent ECU problems. Pin 7 uses a purple wire for the warning circuit. The updated graphic should show Pin 7 as purple, not red. This circuit connects to the warning system used for engine protection and alarm functions. Pin 8 uses a green/white wire for the cylinder number 3 switchbox signal. This circuit provides an ignition reference signal used by the ECU. Damage or corrosion here can create confusing ignition and fuel delivery symptoms. Pin 9 uses a green/red wire for the cylinder number 5 switchbox signal. This is another ignition reference circuit. If this wire has poor continuity, the ECU may not receive the correct signal from the ignition system. Pin 10 uses a solid green wire for the cylinder number 1 switchbox signal. This circuit provides the ECU with another key ignition reference. These green switchbox wires should be kept clean, routed correctly, and protected from chafing. Pin 11 uses a yellow/red wire for the primer circuit. This wire activates the electric primer solenoid during starting. If the primer circuit is weak or disconnected, cold starting may become more difficult. Pin 12 uses a brown wire for the intake air temperature sensor. This circuit allows the ECU to read intake air temperature on factory-style EFI systems and adjust fuel delivery as needed. Pin 13 uses an orange wire for the factory TPS circuit. On the stock-style harness, this wire is part of the throttle position sensor wiring and ties into the related sensor circuit shown on the factory-style pinout. Pin 14 uses a tan/black wire for the factory TPS circuit. This wire is also part of the stock throttle position sensor wiring and connects with the related TPS and temperature sensor circuit in the OEM-style harness. Pin 15 uses a light blue wire for the factory TPS circuit. This wire is used with the factory throttle position sensor system. On a stock OEM-style Mercury Laser EFI or XRI harness, Pins 13, 14, and 15 are included because the factory ECU uses TPS input. Pin 16 uses a black wire for the solenoid and intake air temperature ground. This ground circuit supports the primer solenoid and intake air temperature sensor. Clean grounds are essential for stable sensor readings and reliable EFI operation. Race Harness vs. Stock OEM Harness The Buckshot Racing #77 Stock OEM Harness includes all sixteen ECU circuits and is intended for factory Mercury Laser EFI, XRI, Pro Max, and production EFI engines that use the original throttle position sensor. This is the correct style when retaining the factory ECU and factory TPS wiring. The Buckshot Racing #77 Race Harness is built for Mercury Racing Digital ACU, Brucato ACU, Brucato PCU, MAD EFI, and other programmable performance EFI controllers that do not require the factory throttle position sensor circuit. Because those systems do not use the OEM TPS input, ECU Pins 13, 14, and 15 are intentionally omitted from the Race Harness. Pin 13 orange, Pin 14 tan/black, and Pin 15 light blue are the factory TPS circuits. Removing these unused TPS wires makes the Race Harness cleaner, lighter, simpler, and easier to service without affecting compatible racing EFI controllers. Wiring Best Practices Reliable EFI performance starts with clean wiring. Every connector should be fully seated, every terminal should be clean and dry, and every ground should have bright metal-to-metal contact. The harness should be routed away from the flywheel, linkage, steering components, sharp edges, exhaust heat, and any area where vibration can damage insulation. Dielectric grease should be used only inside rubber connector boots and weather seals to help block moisture. Do not apply dielectric grease directly to exposed metal terminals, pins, or contacts. Electrical terminals are designed to make clean metal-to-metal contact when connected, and excess grease on the terminals can interfere with proper connection. Common Harness Problems Most original Mercury EFI harnesses are now decades old. Heat, oil, vibration, moisture, and repeated service can harden insulation, loosen terminals, corrode injector connectors, weaken grounds, damage switchbox signal wires, and create hidden breaks inside the wire. Many problems that appear to be failed ECUs, bad injectors, weak switchboxes, or fuel pump issues are eventually traced back to damaged wiring. Why Buckshot Racing #77 Whether you're restoring an original Mercury XRI, replacing a damaged Pro Max harness, or wiring a Mercury Racing powerhead, the ECU injector harness is one of the most important parts of the EFI system. A properly built harness delivers clean power, accurate sensor signals, reliable injector operation, correct fuel pump control, and easier troubleshooting. Buckshot Racing #77 Race and Stock OEM harnesses are built for racers, engine builders, and high-performance Mercury owners who need dependable wiring and clean installation.