Jet Sizes & Part Numbers, Mercury WH WMH WMV Carbs
- Mike Hill
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- Nov 27, 2024
- 5 min read
Updated: Jul 21
Mercury Carburetor Jet Selection and Tuning
Mercury calibrated carburetor jets for WH, WMH, and WMV-series carburetors are used across a broad range of Mercury and Mariner two-stroke outboards. Depending on the carburetor model and calibration, these jet styles can be found on select 2.0L, 2.4L, 2.5L, and 3.0L engines rated from approximately 135 through 250 HP.
Common applications include the Mercury Black Max, XR2, XR4, XR6, 150, 175, 200, 220, 225, 250, SST-120, SST-140, and other carbureted Mercury V6 performance outboards.
Correct jetting is essential because a carbureted two-stroke depends on fuel flow for both combustion and internal lubrication. A mixture that is too lean can increase combustion temperature and detonation risk. A mixture that is unnecessarily rich may reduce power, foul spark plugs, increase smoke, and leave heavy carbon deposits.
Main Fuel Jets and Air-Circuit Jets Are Not Tuned the Same Way
Although these parts are often collectively called Mercury main jets, the same style of calibrated brass jet may perform different jobs depending on where it is installed.
In many WMH and WMV carburetors, the main fuel jet meters fuel from the float bowl into the high-speed circuit. A larger main fuel jet normally allows more fuel to pass, creating a richer wide-open-throttle mixture. A smaller main fuel jet reduces fuel flow and creates a leaner mixture.
On many WH-series carburetors, similar calibrated jets may also be used in idle, intermediate, vent, back-draft, or air-bleed circuits. An air jet behaves opposite to a main fuel jet: increasing the air-jet opening generally admits more air and leans that circuit, while reducing the opening generally richens it.
For that reason, never assume that installing a larger-numbered jet will always richen the engine. First identify the carburetor model, jet location, and whether the jet is metering fuel or air.
Mercury Jet Size and Part Number Chart
Marked jet size | Nominal opening | Mercury OEM part number |
40 | 0.040" | 1399-7570 |
42 | 0.042" | 1395-5315 |
44 | 0.044" | 1395-7394 |
46 | 0.046" | 1399-5317 |
48 | 0.048" | 1395-6246 |
50 | 0.050" | 1395-6028 |
52 | 0.052" | 1395-6359 |
54 | 0.054" | 1399-5225 |
56 | 0.056" | 1399-5213 |
58 | 0.058" | 1395-7831 |
60 | 0.060" | 1395-6487 |
62 | 0.062" | 1399-4217 |
64 | 0.064" | 1399-4216 |
66 | 0.066" | 1399-4215 |
68 | 0.068" | 1395-6029 |
70 | 0.070" | 1395-6030 |
72 | 0.072" | 1395-6207 |
74 | 0.074" | 1399-3794 |
76 | 0.076" | 1399-3796 |
78 | 0.078" | 1395-6680 |
80 | 0.080" | 1395-6201 |
82 | 0.082" | 1399-3518 |
84 | 0.084" | 1399-3517 |
86 | 0.086" | 1395-5815 |
88 | 0.088" | 1395-6202 |
90 | 0.090" | 1395-6247 |
92 | 0.092" | 1395-5733 |
94 | 0.094" | 1395-8423 |
96 | 0.096" | 1399-6249 |
98 | 0.098" | 1395-7335 |
100 | 0.100" | Buckshot Racing #77 100-77 |
The stamped number generally represents the nominal calibrated opening in thousandths of an inch. However, the number should be treated as the jet’s calibrated identification—not an invitation to resize it with a drill bit.
WH-Series Carburetors
Mercury WH carburetors were used on many early and mid-generation V6 two-stroke outboards, including numerous 2.0L and 2.4L Black Max, 150, 175, 200, 220, XR2, XR4, and Mercury Hi-Performance applications.
A WH carburetor may contain several replaceable calibrated jets serving different portions of the fuel and air-metering system. Depending on the exact WH number, the carburetor can use main fuel jets along with idle or back-draft air jets.
The specific carburetor identification—such as WH-20, WH-22, WH-31, WH-46, or another calibration—is important. Two WH carburetors that look nearly identical may require different jetting because of changes in venturi size, booster design, bleed circuits, compression, porting, exhaust configuration, or intended horsepower.
WMH and WMV Carburetors
The later WMH and WMV carburetor families were used on many Mercury 2.0L, 2.4L, 2.5L, and select 3.0L two-stroke outboards. Applications include various 135, 150, 175, 200, 225, 250, XR6, and performance V6 models.
In these carburetors, the high-speed main fuel jets strongly influence mixture at elevated throttle openings. However, transition quality also depends on the complete calibration package, including idle jets, progression circuits, air bleeds, emulsion passages, float level, needle-and-seat condition, fuel pressure, throttle synchronization, and ignition timing.
Changing only the main jets will not correct every hesitation, bog, rich idle, lean sneeze, or high-speed problem.
When Jetting Changes May Be Required
A Mercury outboard should normally begin with the factory jetting specified for its exact carburetor and engine serial range. Jet changes may become necessary after substantial changes such as:
Increased compression or modified cylinder heads
Ported blocks or altered exhaust chest work
Aftermarket reeds or modified intake systems
Different carburetors or larger venturis
Tuned exhaust or racing exhaust adapters
Changes in fuel formulation or octane
Operation at significantly different altitude
Competition tuning for changing air density
Temperature, humidity, barometric pressure, and altitude affect the amount of oxygen entering the engine. Cooler, denser air may require more fuel, while high altitude usually requires less fuel because the air contains less oxygen per intake charge.
These corrections should be made from a known safe baseline and in small steps.
Reading the Engine During Tuning
A two-stroke Mercury should not be tuned by sound alone. A combination of measurements gives a much better picture of the fuel mixture:
Spark plugs and piston tops: Useful when examined after a controlled test, although modern fuel and oil can make color interpretation less obvious than it once was.
Exhaust-gas temperature: Helpful for comparing cylinders and identifying a rapid move toward a leaner or hotter condition, but it must be interpreted for the specific probe location and engine combination.
Cylinder-head or water temperature: Confirms whether the cooling system and thermostatic components are operating correctly.
Fuel pressure and delivery volume: A restricted filter, weak pump, blocked tank vent, deteriorated hose, sticking needle and seat, or incorrect float level can imitate improper jetting.
RPM and boat performance: Repeatable acceleration, top-end RPM, and speed data help identify whether a change improved the complete setup.
Buckshot Racing #77 Tuning Procedure
Begin by recording the carburetor identification, current jet sizes, engine configuration, compression, ignition timing, fuel type, propeller load, weather, and test results.
For a main fuel jet, begin rich enough to protect the engine and reduce the opening gradually only after confirming that fuel pressure, cooling, timing, and mechanical condition are correct. Change one size range at a time and inspect all six cylinders for differences.
Never assume that every cylinder must use the same jet. Some Mercury factory calibrations use different jet sizes by cylinder because individual cylinders may run hotter or receive a different airflow distribution.
When working with an air-bleed or idle-circuit jet, remember that the tuning direction may be opposite: a larger air opening can make the circuit leaner. Confirm the circuit before making any adjustment.
Verify the Exact Engine and Carburetor
Mercury produced many carburetor calibrations under the same horsepower and displacement families. A 200 HP 2.4L, 200 HP 2.5L, and 200 HP 3.0L may all use different carburetors and jet packages. Even engines of the same displacement and horsepower can differ by model year and serial range.
Record the carburetor number stamped or cast into the body and verify the engine serial number before selecting jets. Mercury likewise directs owners to use the engine serial number when locating the correct service and parts information.
Final Thoughts
The correct Mercury carburetor jet is determined by more than horsepower or displacement. The WH, WMH, and WMV carburetor model, jet location, engine configuration, fuel system, operating conditions, and intended use must all be considered.
On a main fuel circuit, larger jets generally richen the mixture. In an air-bleed circuit, a larger jet may lean it. Understanding that difference is critical when tuning a Mercury 2.0L, 2.4L, 2.5L, or 3.0L V6 two-stroke outboard.
For a stock engine, begin with the Mercury-specified calibration. For a modified or racing engine, make conservative changes, document every step, and confirm the results with temperature, fuel-pressure, plug, piston, RPM, and performance data.
Good jetting is not simply about producing more power—it is about delivering the correct fuel mixture while protecting the powerhead.
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